A&P LAB 9

6 steps of digestion

1. ingestion 2. propulsion 3. mechanical breakdown 4. chemical digestion 5. absorption 6. defecation done to reduce all the different kinds of molecules in food into their tiniest and most basic forms

Why do we eat?

1. to obtain the energy we need to stay alive 2. to get the raw materials required for building all of our tissues

calories

A unit used to measure the amount of energy contained in foods

mouth expanded

Also known as the oral cavity, the mouth is the hollow cavity that allows food and air to enter the body. The mouth contains many other organs - such as the teeth, tongue, and the ducts of the salivary glands - that work together to aid in the ingestion and digestion of food. The mouth also plays a major role in the production of speech through the movements of the tongue, lips and cheeks. The mouth is a hollow cavity formed by the space between the lips, cheeks, tongue, hard and soft palates and the throat. Its external opening is located along the body's midline inferior to the nose and superior to the chin. The external opening of the mouth is usually much longer in the horizontal plane, but may be extended through the movement of the jaw to become nearly as wide in the vertical plane as well. The lips are soft, fleshy structures that form the anterior border of the external opening of the mouth. The lips are very flexible and elastic structures and contain many collagen and elastin fibers and adipose tissue covered by a thin layer of stratified squamous epithelium. The exterior of the lips is continuous with the skin and is covered by keratinized epithelium, while the inner surface is continuous with the mucous membrane of the mouth and is covered by nonkeratinized epithelium. Lateral to the lips are the cheeks, which are fleshy structures that form the sides of the mouth. Similar to the lips, the exterior of the cheeks is covered in keratinized stratified squamous epithelium continuous with the skin and the interior is covered in nonkeratinized stratified squamous epithelium continuous with the mucous membrane. Between the epithelium layers are layers of connective tissues, nerves, and muscles. In particular the muscles of the cheeks include the buccinator, orbicularis oris and zygomaticus major, which move the lips and cheeks. The tongue forms the inferior portion of the mouth, but often moves throughout the mouth to occupy almost any region of the hollow cavity. While many people think of the tongue as a muscle, it is actually an organ made of epithelium, several skeletal muscles, nerves, and connective tissues. The tongue contains many small ridges known as papillae that help it to grip and move food around the mouth. Taste buds are hidden in valleys around some of the papillae and produce the sense of taste by detecting chemicals found in food. The tongue also helps to produce speech by altering or stopping the flow of air through the mouth to produce the sounds of many consonants. The hard and soft palates form the roof of the mouth. On the anterior end of the mouth, the hard palate is formed by the inferior surface of the maxillae and palatine bones. These bones are covered with a thin layer of connective tissues and mucous membranes, which form small wrinkles. The roof of the mouth continues posteriorly as the soft palate, a flexible fleshy mass of tissues that ends in the uvula. The hard and soft palates work together to separate the mouth from the nasal cavity. The soft palate moves superiorly during swallowing to cover the nasopharynx of the throat, preventing food from entering the nasal cavity. The throat, or pharynx, is a funnel-shaped tube located in the posterior of the mouth. The pharynx connects the nasal cavity and mouth to the esophagus and larynx in the neck. The region of the throat behind the mouth is known as the oropharynx and forms the posterior wall of the mouth. Food in the mouth is swallowed into the oropharynx and passed on to the esophagus and the rest of the gastrointestinal tract. Air inhaled through the mouth or nose also passes through the pharynx on its way to the larynx, and then passes through the pharynx on its way out of the body during exhalation. Inside the mouth are several structures that aid in the digestion of food. Teeth are hard structures specialized for the biting and grinding of food (known as mastication, or chewing). They form a continuous row in the bottom of the mouth surrounding the tongue on the lateral and anterior sides, as well as another nearly identical row extending from the roof of the mouth. Teeth form deep roots into the bones of the maxillae and the mandible, but grow out through the gums of the mouth to form biting surfaces. The gums, or gingiva, are soft mucous membranes surrounding the teeth, protecting the roots from decay and helping to hold the teeth in place. Finally, many salivary glands surround the mouth and release their secretion, saliva, into the mouth through many tiny ducts. Saliva helps to moisten and chemically digest food in the mouth before it is swallowed. Saliva also protects the teeth from decay by digesting and washing away tiny bits of food that become stuck to the teeth.

Calorie

Amount of energy needed to raise temperature 1 gram of water 1 degree C

physiology of pancreas

Digestion The exocrine portion of the pancreas plays a major role in the digestion of food. The stomach slowly releases partially digested food into the duodenum as a thick, acidic liquid called chyme. The acini of the pancreas secrete pancreatic juice to complete the digestion of chyme in the duodenum. Pancreatic juice is a mixture of water, salts, bicarbonate, and many different digestive enzymes. The bicarbonate ions present in pancreatic juice neutralize the acid in chyme to protect the intestinal wall and to create the proper environment for the functioning of pancreatic enzymes. The pancreatic enzymes each specialize in digesting specific compounds found in chyme. Pancreatic amylase breaks large polysaccharides like starches and glycogen into smaller sugars such as maltose, maltotriose, and glucose. Maltase secreted by the small intestine then breaks maltose into the monosaccharide glucose, which the intestines can directly absorb. Trypsin, chymotrypsin, and carboxypeptidase are protein-digesting enzymes that break proteins down into their amino acid subunits. These amino acids can then be absorbed by the intestines. Pancreatic lipase is a lipid-digesting enzyme that breaks large triglyceride molecules into fatty acids and monoglycerides. Bile released by the gallbladder emulsifies fats to increase the surface area of triglycerides that pancreatic lipase can react with. The fatty acids and monoglycerides produced by pancreatic lipase can be absorbed by the intestines. Ribonuclease and deoxyribonuclease are nucleases, or enzymes that digest nucleic acids. Ribonuclease breaks down molecules of RNA into the sugar ribose and the nitrogenous bases adenine, cytosine, guanine and uracil. Deoxyribonuclease digests DNA molecules into the sugar deoxyribose and the nitrogenous bases adenine, cytosine, guanine, and thymine. Blood Glucose Homeostasis The endocrine portion of the pancreas controls the homeostasis of glucose in the bloodstream. Blood glucose levels must be maintained within certain limits so that there is a constant supply of glucose to feed the cells of the body but not so much that glucose can damage the kidneys and other organs. The pancreas produces 2 antagonistic hormones to control blood sugar: glucagon and insulin. The alpha cells of the pancreas produce glucagon. Glucagon raises blood glucose levels by stimulating the liver to metabolize glycogen into glucose molecules and to release glucose into the blood. Glucagon also stimulates adipose tissue to metabolize triglycerides into glucose and to release glucose into the blood. Insulin is produced by the beta cells of the pancreas. This hormone lowers blood glucose levels after a meal by stimulating the absorption of glucose by liver, muscle, and adipose tissues. Insulin triggers the formation of glycogen in the muscles and liver and triglycerides in adipose to store the absorbed glucose. Regulation of Pancreatic Function The pancreas is controlled by both the autonomic nervous system (ANS) and the endocrine system. The ANS has 2 divisions: the sympathetic and the parasympathetic. Nerves of the sympathetic division become active during stressful situations, emergencies, and exercise. Sympathetic neurons stimulate the alpha cells of the pancreas to release the hormone glucagon into the bloodstream. Glucagon stimulates the liver to begin the breakdown of the energy storage molecule glycogen into smaller glucose molecules. Glucose is then released into the bloodstream for the organs, especially the heart and skeletal muscles, to use as energy. The sympathetic nerves also inhibit the function of beta cells and acini to reduce or prevent the secretion of insulin and pancreatic juice. The inhibition of these functions provides more energy for other parts of the body that are active in dealing with the stressful situation. Nerves of the parasympathetic division of the ANS become active during restful times and during the digestion of a meal. Parasympathetic nerves stimulate the release of insulin and pancreatic juice by the pancreas. Pancreatic juice helps with the digestion of food while insulin stores the glucose released from the digested food in the body's cells. The endocrine system uses 2 hormones to regulate the digestive function of the pancreas: secretin and cholecystokinin (CCK). Cells in the lining of the duodenum produce secretin in response to acidic chyme emerging from the stomach. Secretin stimulates the pancreas to produce and secrete pancreatic juice containing a high concentration of bicarbonate ions. Bicarbonate reacts with and neutralizes hydrochloric acid present in chyme to return the chyme to a neutral pH of around 7. CCK is a hormone produced by cells in the lining of the duodenum in response to the presence of proteins and fats in chyme. CCK travels through the bloodstream and binds to receptor cells in the acini of the pancreas. CCK stimulates these cells to produce and secrete pancreatic juice that has a high concentration of digestive enzymes. The high levels of enzymes in pancreatic juice help to digest large protein and lipid molecules that are more difficult to break down.

physiology of the liver

Digestion The liver plays an active role in the process of digestion through the production of bile. Bile is a mixture of water, bile salts, cholesterol, and the pigment bilirubin. Hepatocytes in the liver produce bile, which then passes through the bile ducts to be stored in the gallbladder. When food containing fats reaches the duodenum, the cells of the duodenum release the hormone cholecystokinin to stimulate the gallbladder to release bile. Bile travels through the bile ducts and is released into the duodenum where it emulsifies large masses of fat. The emulsification of fats by bile turns the large clumps of fat into smaller pieces that have more surface area and are therefore easier for the body to digest. Bilirubin present in bile is a product of the liver's digestion of worn out red blood cells. Kupffer cells in the liver catch and destroy old, worn out red blood cells and pass their components on to hepatocytes. Hepatocytes metabolize hemoglobin, the red oxygen-carrying pigment of red blood cells, into the components heme and globin. Globin protein is further broken down and used as an energy source for the body. The iron-containing heme group cannot be recycled by the body and is converted into the pigment bilirubin and added to bile to be excreted from the body. Bilirubin gives bile its distinctive greenish color. Intestinal bacteria further convert bilirubin into the brown pigment stercobilin, which gives feces their brown color. Metabolism The hepatocytes of the liver are tasked with many of the important metabolic jobs that support the cells of the body. Because all of the blood leaving the digestive system passes through the hepatic portal vein, the liver is responsible for metabolizing carbohydrate, lipids, and proteins into biologically useful materials. Our digestive system breaks down carbohydrates into the monosaccharide glucose, which cells use as a primary energy source. Blood entering the liver through the hepatic portal vein is extremely rich in glucose from digested food. Hepatocytes absorb much of this glucose and store it as the macromolecule glycogen, a branched polysaccharide that allows the hepatocytes to pack away large amounts of glucose and quickly release glucose between meals. The absorption and release of glucose by the hepatocytes helps to maintain homeostasis and protects the rest of the body from dangerous spikes and drops in the blood glucose level. (See more about glucose in the body.) Fatty acids in the blood passing through the liver are absorbed by hepatocytes and metabolized to produce energy in the form of ATP. Glycerol, another lipid component, is converted into glucose by hepatocytes through the process of gluconeogenesis. Hepatocytes can also produce lipids like cholesterol, phospholipids, and lipoproteins that are used by other cells throughout the body. Much of the cholesterol produced by hepatocytes gets excreted from the body as a component of bile. Dietary proteins are broken down into their component amino acids by the digestive system before being passed on to the hepatic portal vein. Amino acids entering the liver require metabolic processing before they can be used as an energy source. Hepatocytes first remove the amine groups of the amino acids and convert them into ammonia and eventually urea. Urea is less toxic than ammonia and can be excreted in urine as a waste product of digestion. The remaining parts of the amino acids can be broken down into ATP or converted into new glucose molecules through the process of gluconeogenesis. Digestion The liver plays an active role in the process of digestion through the production of bile. Bile is a mixture of water, bile salts, cholesterol, and the pigment bilirubin. Hepatocytes in the liver produce bile, which then passes through the bile ducts to be stored in the gallbladder. When food containing fats reaches the duodenum, the cells of the duodenum release the hormone cholecystokinin to stimulate the gallbladder to release bile. Bile travels through the bile ducts and is released into the duodenum where it emulsifies large masses of fat. The emulsification of fats by bile turns the large clumps of fat into smaller pieces that have more surface area and are therefore easier for the body to digest. Bilirubin present in bile is a product of the liver's digestion of worn out red blood cells. Kupffer cells in the liver catch and destroy old, worn out red blood cells and pass their components on to hepatocytes. Hepatocytes metabolize hemoglobin, the red oxygen-carrying pigment of red blood cells, into the components heme and globin. Globin protein is further broken down and used as an energy source for the body. The iron-containing heme group cannot be recycled by the body and is converted into the pigment bilirubin and added to bile to be excreted from the body. Bilirubin gives bile its distinctive greenish color. Intestinal bacteria further convert bilirubin into the brown pigment stercobilin, which gives feces their brown color. Metabolism The hepatocytes of the liver are tasked with many of the important metabolic jobs that support the cells of the body. Because all of the blood leaving the digestive system passes through the hepatic portal vein, the liver is responsible for metabolizing carbohydrate, lipids, and proteins into biologically useful materials. Our digestive system breaks down carbohydrates into the monosaccharide glucose, which cells use as a primary energy source. Blood entering the liver through the hepatic portal vein is extremely rich in glucose from digested food. Hepatocytes absorb much of this glucose and store it as the macromolecule glycogen, a branched polysaccharide that allows the hepatocytes to pack away large amounts of glucose and quickly release glucose between meals. The absorption and release of glucose by the hepatocytes helps to maintain homeostasis and protects the rest of the body from dangerous spikes and drops in the blood glucose level. (See more about glucose in the body.) Fatty acids in the blood passing through the liver are absorbed by hepatocytes and metabolized to produce energy in the form of ATP. Glycerol, another lipid component, is converted into glucose by hepatocytes through the process of gluconeogenesis. Hepatocytes can also produce lipids like cholesterol, phospholipids, and lipoproteins that are used by other cells throughout the body. Much of the cholesterol produced by hepatocytes gets excreted from the body as a component of bile. Dietary proteins are broken down into their component amino acids by the digestive system before being passed on to the hepatic portal vein. Amino acids entering the liver require metabolic processing before they can be used as an energy source. Hepatocytes first remove the amine groups of the amino acids and convert them into ammonia and eventually urea. Urea is less toxic than ammonia and can be excreted in urine as a waste product of digestion. The remaining parts of the amino acids can be broken down into ATP or converted into new glucose molecules through the process of gluconeogenesis.

digestion

Digestion is the process of turning large pieces of food into its component chemicals. Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces. This mode of digestion begins with the chewing of food by the teeth and is continued through the muscular mixing of food by the stomach and intestines. Bile produced by the liver is also used to mechanically break fats into smaller globules. While food is being mechanically digested it is also being chemically digested as larger and more complex molecules are being broken down into smaller molecules that are easier to absorb. Chemical digestion begins in the mouth with salivary amylase in saliva splitting complex carbohydrates into simple carbohydrates. The enzymes and acid in the stomach continue chemical digestion, but the bulk of chemical digestion takes place in the small intestine thanks to the action of the pancreas. The pancreas secretes an incredibly strong digestive cocktail known as pancreatic juice, which is capable of digesting lipids, carbohydrates, proteins and nucleic acids. By the time food has left the duodenum, it has been reduced to its chemical building blocks—fatty acids, amino acids, monosaccharides, and nucleotides.

appendicitis

Doctors typically remove an appendix if it becomes inflamed, and even a healthy appendix may be removed during abdominal surgeries such as a hysterectomy. A doctor's justification for this removal is that the appendix is susceptible to bacterial infections that lead to appendicitis, a fairly common and dangerous inflammation of the appendix. Often one of the first signs of appendicitis is pain and tenderness near the navel, often growing sharper and spreading downward into the lower right abdomen. The pain can grow quite severe over the course of a few hours, so much so that it may be impossible to get comfortable or to move without pain. Applying pressure to the area will commonly cause pain that can sharpen after releasing the pressure (a phenomenon called "rebound tenderness"), though this is not always the case. Additional common symptoms include nausea, vomiting, fever and others. Tenderness and growing pain in the right abdomen that is noticeable enough to cause considerable discomfort during movement or at rest warrants medical attention in order to reach a diagnosis and receive any necessary treatment. Untreated appendicitis can lead to the rupture of the appendix, a serious medical emergency wherein fecal matter leaks out of the cecum. Left untreated, the bacteria-laden fecal matter spreads throughout the abdominal cavity, where the bacteria begin to digest the peritoneum that lines the cavity. The infection and inflammation of the peritoneum, known as peritonitis, is a severely painful and potentially fatal consequence of appendicitis.

appendix

Extending from the inferior end of the large intestine's cecum, the human appendix is a narrow pouch of tissue whose resemblance to a worm inspired its alternate name, vermiform (worm-like) appendix. It is located in the right iliac region of the abdomen (in the lower right-hand abdominal area), measuring about four inches long and roughly a quarter of an inch in diameter. Like the rest of the digestive tract, the appendix is made of an inner layer of mucosa with submucosa, muscularis, and serosa layers surrounding it. Unlike the rest of the large intestine, however, the submucosa of the appendix contains many masses of lymphoid tissue. The presence of lymphoid tissue suggests that the appendix may play a role in the immune system in addition to the digestive system.

Gingiva (Gums)

Gingiva, commonly called the gum, is the soft tissue that covers and protects the root of the tooth. The gum is not attached to the tooth. Continuously, instead, between the tooth and the area around the tooth is a shallow v-shaped groove called the sulcus

anatomy of the liver

Gross Anatomy The liver is a roughly triangular organ that extends across the entire abdominal cavity just inferior to the diaphragm. Most of the liver's mass is located on the right side of the body where it descends inferiorly toward the right kidney. The liver is made of very soft, pinkish-brown tissues encapsulated by a connective tissue capsule. This capsule is further covered and reinforced by the peritoneum of the abdominal cavity, which protects the liver and holds it in place within the abdomen. The peritoneum connects the liver in 4 locations: the coronary ligament, the left and right triangular ligaments, and the falciform ligament. These connections are not true ligaments in the anatomical sense; rather, they are condensed regions of peritoneal membrane that support the liver. The wide coronary ligament connects the central superior portion of the liver to the diaphragm. Located on the lateral borders of the left and right lobes, respectively, the left and right triangular ligaments connect the superior ends of the liver to the diaphragm. The falciform ligament runs inferiorly from the diaphragm across the anterior edge of the liver to its inferior border. At the inferior end of the liver, the falciform ligament forms the round ligament (ligamentum teres) of the liver and connects the liver to the umbilicus. The round ligament is a remnant of the umbilical vein that carries blood into the body during fetal development. The liver consists of 4 distinct lobes — the left, right, caudate, and quadrate lobes. The left and right lobes are the largest lobes and are separated by the falciform ligament. The right lobe is about 5 to 6 times larger than the tapered left lobe. The small caudate lobe extends from the posterior side of the right lobe and wraps around the inferior vena cava. The small quadrate lobe is inferior to the caudate lobe and extends from the posterior side of the right lobe and wraps around the gallbladder. Bile Ducts The tubes that carry bile through the liver and gallbladder are known as bile ducts and form a branched structure known as the biliary tree. Bile produced by liver cells drains into microscopic canals known as bile canaliculi. The countless bile canaliculi join together into many larger bile ducts found throughout the liver. These bile ducts next join to form the larger left and right hepatic ducts, which carry bile from the left and right lobes of the liver. Those two hepatic ducts join to form the common hepatic duct that drains all bile away from the liver. The common hepatic duct finally joins with the cystic duct from the gallbladder to form the common bile duct, carrying bile to the duodenum of the small intestine. Most of the bile produced by the liver is pushed back up the cystic duct by peristalsis to arrive in the gallbladder for storage, until it is needed for digestion. Blood Vessels The blood supply of the liver is unique among all organs of the body due to the hepatic portal vein system. Blood traveling to the spleen, stomach, pancreas, gallbladder, and intestines passes through capillaries in these organs and is collected into the hepatic portal vein. The hepatic portal vein then delivers this blood to the tissues of the liver where the contents of the blood are divided up into smaller vessels and processed before being passed on to the rest of the body. Blood leaving the tissues of the liver collects into the hepatic veins that lead to the vena cava and return to the heart. The liver also has its own system of arteries and arterioles that provide oxygenated blood to its tissues just like any other organ. Lobules The internal structure of the liver is made of around 100,000 small hexagonal functional units known as lobules. Each lobule consists of a central vein surrounded by 6 hepatic portal veins and 6 hepatic arteries. These blood vessels are connected by many capillary-like tubes called sinusoids, which extend from the portal veins and arteries to meet the central vein like spokes on a wheel. Each sinusoid passes through liver tissue containing 2 main cell types: Kupffer cells and hepatocytes. Kupffer cells are a type of macrophage that capture and break down old, worn out red blood cells passing through the sinusoids. Hepatocytes are cuboidal epithelial cells that line the sinusoids and make up the majority of cells in the liver. Hepatocytes perform most of the liver's functions — metabolism, storage, digestion, and bile production. Tiny bile collection vessels known as bile canaliculi run parallel to the sinusoids on the other side of the hepatocytes and drain into the bile ducts of the liver.

gross anatomy of the gallbladder

Hollow, muscular and pear-shaped, the gallbladder is a small organ — only about 3 inches in length and 1.5 inches in width at its widest point. The larger end of the gallbladder extends inferiorly and to the right while the tapered end points superiorly and medially. The tapered end of the gallbladder narrows into a small bile duct known as the cystic duct. The cystic duct connects to the common hepatic duct that carries bile from the liver. These ducts merge to form the common bile duct that extends to the wall of the duodenum.

pancreatic health problems

If the ducts leading from the pancreas are blocked in some way — such as when a gallstone blocks the ampulla of Vater - pancreatic fluids can build up in the pancreas and may then become activated so that they digest the pancreas itself. This condition is known as acute pancreatitis. If the onset is gradual and longer-term, we call it chronic pancreatitis. Pancreatic cancer has one of the direst prognoses of any of the types of cancer, in part because it tends to be very metastatic (it spreads rapidly) and because it is often undiagnosed at an early stage. Pancreatic surgery can be quite problematic for several reasons: The pancreas' soft, spongy, tissue is very blood-rich, but its texture makes it extremely difficult to suture. Tumors are often advanced by the time they are detected. Due to the complexities, candidates for surgery are often strongly advised to seek their treatment in a facility that conducts a higher volume of such procedures. Hereditary hemochromatosis and diabetes mellitus involve the pancreas as well. You can test for your inherited genetic risk of hemochromatosis using modern DNA health testing.

digestive disorders

Many diseases and health conditions - such as ulcers, GERD, IBD and celiac disease, just to name a few - lead to dysfunction in our digestive system. Learn about them by visiting our section on digestive diseases and conditions. (Also, now you can test for your genetic risk of acquiring celiac disease - learn more about DNA health testing.)

microscopic anatomy of the stomach

Microscopic analysis of the stomach's structure reveals that it is made of several distinct layers of tissue: the mucosa, submucosa, muscularis, and serosa layers. Mucosa The innermost layer of the stomach is known as the mucosa, and is made of mucous membrane. The mucous membrane of the stomach contains simple columnar epithelium tissue with many exocrine cells. Small pores called gastric pits contain many exocrine cells that secrete digestive enzymes and hydrochloric acid into the lumen, or hollow region, of the stomach. Mucous cells found throughout the stomach lining and gastric pits secrete mucus to protect the stomach from its own digestive secretions. The mucosa of the stomach is much thicker than the mucosa of the other organs of the gastrointestinal tract due to the depth of the gastric pits. Deep inside the mucosa is a thin layer of smooth muscle known as the muscularis mucosae. The muscularis mucosae layer allows the mucosa to form folds and increase its contact with the stomach's contents. Submucosa Surrounding the mucosa is the submucosa layer of the stomach. The submucosa is made up of various connective tissues, blood vessels, and nerves. Connective tissues support the tissues of the mucosa and connect it to the muscularis layer. The blood supply of the submucosa provides nutrients to the wall of the stomach. Nervous tissue in the submucosa monitors the contents of the stomach and controls smooth muscle contraction and secretion of digestive substances. Muscularis The muscularis layer of the stomach surrounds the submucosa and makes up a large amount of the stomach's mass. The muscularis is made of 3 layers of smooth muscle tissue arranged with its fibers running in 3 different directions. These layers of smooth muscle allow the stomach to contract to mix and propel food through the digestive tract. Serosa The outermost layer of the stomach surrounding the muscularis layer is the serosa — a thin serous membrane made of simple squamous epithelial tissue and areolar connective tissue. The serosa has a smooth, slippery surface and secretes a thin, watery secretion known as serous fluid. The smooth, wet surface of the serosa helps to protect the stomach from friction as it expands with food and moves to mix and propel the food.

absorption

Once food has been reduced to its building blocks, it is ready for the body to absorb. Absorption begins in the stomach with simple molecules like water and alcohol being absorbed directly into the bloodstream. Most absorption takes place in the walls of the small intestine, which are densely folded to maximize the surface area in contact with digested food. Small blood and lymphatic vessels in the intestinal wall pick up the molecules and carry them to the rest of the body. The large intestine is also involved in the absorption of water and vitamins B and K before feces leave the body.

columnar epithelial cells

Secrete all sorts of stuff as well as absorb and process various nutrients mucus

physiology of the stomach

Storage In the mouth, we chew and moisten solid food until it becomes a small mass known as a bolus. When we swallow each bolus, it then passes through the esophagus to the stomach where it is stored along with other boluses and liquids from the same meal. The size of the stomach varies from person to person, but on average it can comfortably contain 1-2 liters of food and liquid during a meal. When stretched to its maximum capacity by a large meal or overeating, the stomach may hold up to 3-4 liters. Distention of the stomach to its maximum size makes digestion difficult, as the stomach cannot easily contract to mix food properly and leads to feelings of discomfort. After the stomach has been filled with food from a meal, it stores the food for about 1-2 hours. During this time, the stomach continues the digestive process that began in the mouth and allows the intestines, pancreas, gallbladder, and liver to prepare to complete the digestive process. At the inferior end of the stomach, the pyloric sphincter controls the movement of food into the intestines. The pyloric sphincter is normally closed to keep food and stomach secretions within the stomach. Once chyme is ready to leave the stomach, the pyloric sphincter opens to allow a small amount of chyme to pass into the duodenum. This process, known as gastric emptying, slowly repeats over the 1-2 hours that food is stored in the stomach. The slow rate of gastric emptying helps to spread out the volume of chyme being released from the stomach and maximizes the digestion and absorption of nutrients in the intestines. Secretion The stomach produces and secretes several important substances to control the digestion of food. Each of these substances is produced by exocrine or endocrine cells found in the mucosa. The main exocrine product of the stomach is gastric juice — a mixture of mucus, hydrochloric acid, and digestive enzymes. Gastric juice is mixed with food in the stomach to promote digestion. Specialized exocrine cells of the mucosa known as mucous cells secrete mucus into the lumen of the stomach and into the gastric pits. This mucus spreads across the surface of the mucosa to coat the lining of the stomach with a thick, acid- and enzyme-resistant barrier. Stomach mucus is also rich in bicarbonate ions, which neutralize the pH of stomach acid. Parietal cells found in the gastric pits of the stomach produce 2 important secretions: intrinsic factor and hydrochloric acid. Intrinsic factor is a glycoprotein that binds to the vitamin B12 in the stomach and allows the vitamin to be absorbed in the small intestine. Vitamin B12 is an essential nutrient for the formation of red blood cells. Hydrochloric acid protects the body by killing pathogenic bacteria naturally found in food. Hydrochloric acid also helps to digest proteins by denaturing them into an unfolded shape that is easier for enzymes to digest. The protein digesting enzyme pepsin is activated by exposure to hydrochloric acid inside the stomach. Chief cells, also found within the gastric pits of the stomach, produce two digestive enzymes: pepsinogen and gastric lipase. Pepsinogen is the precursor molecule of the very potent protein-digesting enzyme pepsin. Because pepsin would destroy the chief cells that produce it, it is secreted in its inactive pepsinogen form. When pepsinogen reaches the acidic pH found in the stomach thanks to hydrochloric acid, it changes shape and becomes the active enzyme pepsin. Pepsin then breaks dietary proteins into their amino acid building blocks. Gastric lipase is an enzyme that digests fats by removing a fatty acid from a triglyceride molecule. G cells are endocrine cells found at the bottom of the gastric pits. G cells release the hormone gastrin into the bloodstream in response to many stimuli, such as signals from the vagus nerve; the presence of amino acids in the stomach from digested proteins; and the stretching of the stomach wall during a meal. Gastrin travels through the blood to various receptor cells throughout the stomach where it stimulates the glands and muscles of the stomach. Glandular stimulation by gastrin leads to increased secretion of gastric juice to increase digestion. Stimulation of smooth muscles by gastrin leads to stronger contractions of the stomach and the opening of the pyloric sphincter to move food into the duodenum. Gastrin also binds to receptor cells in the pancreas and gallbladder where it increases the secretion of pancreatic juice and bile. Digestion Digestion in the stomach can be divided into 2 classes: mechanical digestion and chemical digestion. Mechanical digestion is the physical division of a mass of food into smaller masses while chemical digestion is the chemical conversion of larger molecules into smaller molecules. The mixing action of the stomach walls allows mechanical digestion to occur in the stomach. The smooth muscles of the stomach produce contractions known as mixing waves that mix the boluses of food with gastric juice. This mixing leads to the production of the thick liquid known as chyme. While food is being physically mixed with gastric juice to produce chyme, the enzymes present in the gastric juice chemically digest large molecules into their smaller subunits. Gastric lipase splits triglyceride fats into fatty acids and diglycerides. Pepsin breaks proteins into smaller amino acids. The chemical digestion begun in the stomach will not be completed until chyme reaches the intestines, but the stomach prepares hard-to-digest proteins and fats for further digestion. Hormonal Control The activity of the stomach is under the control of several hormones that regulate the production of stomach acid and the release of food into the duodenum. Gastrin, produced by the G cells of the stomach itself, increases the activity of the stomach by stimulating increased gastric juice production, muscle contraction, and gastric emptying through the pyloric sphincter. Cholecystokinin (CCK), produced by the mucosa of the duodenum, is a hormone that acts to slow gastric emptying by contracting the pyloric sphincter. CCK is released in response to food rich in proteins and fats, which are difficult for the body to digest. By inhibiting gastric emptying, CCK allows food to be stored in the stomach longer to promote improved digestion by the stomach and to give the pancreas and gallbladder time to release enzymes and bile to increase digestion in the duodenum. Secretin, another hormone produced by the duodenum's mucosa, responds to the acidity of chyme entering the duodenum from the stomach. Secretin travels through the bloodstream to the stomach where it slows the production of gastric juice by the exocrine glands of the mucosa. Secretin also promotes the production of pancreatic juice and bile that contain acid-neutralizing bicarbonate ions. The net effect of secretin is to protect the intestines from the damaging effects of acidic chyme.

physiology of the gallbladder

Storage The gallbladder acts as a storage vessel for bile produced by the liver. Bile is produced by hepatocytes cells in the liver and passes through the bile ducts to the cystic duct. From the cystic duct, bile is pushed into the gallbladder by peristalsis (muscle contractions that occur in orderly waves). Bile is then slowly concentrated by absorption of water through the walls of the gallbladder. The gallbladder stores this concentrated bile until it is needed to digest the next meal. Stimulation Foods rich in proteins or fats are more difficult for the body to digest when compared to carbohydrate-rich foods (see Macronutrients). The walls of the duodenum contain sensory receptors that monitor the chemical makeup of chyme (partially digested food) that passes through the pyloric sphincter into the duodenum. When these cells detect proteins or fats, they respond by producing the hormone cholecystokinin (CCK). CCK enters the bloodstream and travels to the gallbladder where it stimulates the smooth muscle tissue in the walls of the gallbladder. Secretion When CCK reaches the gallbladder, it triggers the smooth muscle tissue in the muscularis layer of the gallbladder to contract. The contraction of smooth muscle forces bile out of the gallbladder and into the cystic duct. From the cystic duct, bile enters the common bile duct and flows into the ampulla of Vater, where the bile ducts merge with the pancreatic duct. Bile then flows from the ampulla of Vater into the duodenum where it breaks the fats into smaller masses for easier digestion by the enzyme pancreatic lipase. Gallstones Gallstones are hard masses of bile salts, pigments, and cholesterol that develop within the gallbladder. These solid masses form when the components of bile crystallize. Growing slowly over many years as more crystallization occurs, gallstones may reach up to an inch in diameter. Most gallstones remain in the gallbladder and are harmless, but they can be pushed out of the gallbladder along with bile and potentially block the neck of the gallbladder or one of the bile ducts. Blockage of the gallbladder or cystic duct may result in cholecystitis, a painful inflammation of the gallbladder. Even worse, blockage of the common bile duct may result in jaundice and liver damage, while blockage of the ampulla of Vater can lead to pancreatitis. Both liver damage and pancreatitis are potentially life-threatening conditions. Gallstones are most often treated by a cholecystectomy, the surgical removal of the gallbladder.

salivary glands

Surrounding the mouth are 3 sets of salivary glands. The salivary glands are accessory organs that produce a watery secretion known as saliva. Saliva helps to moisten food and begins the digestion of carbohydrates. The body also uses saliva to lubricate food as it passes through the mouth, pharynx, and esophagus.

teeth

Teeth. The teeth are 32 small, hard organs found along the anterior and lateral edges of the mouth. Each tooth is made of a bone-like substance called dentin and covered in a layer of enamel—the hardest substance in the body. Teeth are living organs and contain blood vessels and nerves under the dentin in a soft region known as the pulp. The teeth are designed for cutting and grinding food into smaller pieces.

anus

The anus, or anal canal, is the final segment of the gastrointestinal tract. It acts as the orifice that feces pass through during defecation. Anatomy The anus is a short tube at the end of the rectum that ends at the body's exterior. It is around 1 inch (2-3 cm) long and varies widely in its diameter depending on how distended it is. The superior end of the anus is continuous with the tissue of the rectum and is lined with simple columnar epithelial tissue. This tissue forms folds known as anal columns, with valleys known as anal valves between the folds At the inferior end of the anal columns is the pectinate line, where the tissue lining the anus changes from simple columnar epithelium to stratified squamous epithelium. The stratified squamous epithelium is non-keratinized inside the anus, but becomes keratinized epithelium as it approaches the body's exterior. The change from non-keratinized to keratinized occurs at a visible landmark known as Hilton's line. Two sphincter muscles, the internal and external anal sphincters, surround the anus and control its opening and closing. The internal anal sphincter is made of visceral muscle and is continuous with the muscularis layer of the rectum. Closer to the body's exterior, the external anal sphincter is made of skeletal muscle and is continuous with the surrounding skeletal muscles of the perianal region. A layer of keratinized stratified squamous epithelium covers the external anal sphincter and is continuous with the skin of the perianal region. Physiology The anus plays an important role in controlling the elimination of solid wastes through defecation. When the rectum is empty or slightly filled, both the internal and external anal sphincters remain closed to hold waste material inside the rectum and prevent defecation. Once enough fecal matter fills the rectum to exert pressure on the rectal walls, pressure receptors in the rectum send signals to the brain, which in turn send signals to relax the internal anal sphincter. The external anal sphincter continues to hold feces in the rectum until voluntary signals from the cerebral cortex cause it to relax during the defecation reflex.

What does the appendix do?

The appendix is not a vital organ and medical researchers still debate its exact function in our bodies. One hypothesis suggests that it is a vestigial remnant of a once larger cecum. This larger cecum would have been used by vegetarian ancestors to digest cellulose from plants. Supporters of this hypothesis therefore conclude that the appendix no longer serves any purpose for us. Another hypothesis suggests that the appendix acts as a storage area for beneficial bacteria during times of illness. Beneficial bacteria living in the appendix could survive being flushed out of the large intestine by diarrhea. The appendix would therefore help a person to recover more rapidly from illness by enabling the bacteria to re-colonize the intestines after the illness has passed.

ascending colon

The ascending colon is one of the four major regions of the colon, which is itself one of the parts of our large intestine. The ascending colon carries feces from the cecum superiorly along the right side of our abdominal cavity to the transverse colon. In the ascending colon, bacteria digest the transitory fecal matter in order to release vitamins. The intestinal wall absorbs water, nutrients, and vitamins from the feces and deposits these materials into our bloodstream. Gross Anatomy The ascending colon is a hollow tube about 2.5 inches in diameter and about 8 inches long, with many small pouches along its length to increase its surface area. The inferior end of the ascending colon connects to the cecum of the large intestine in the right iliac region of the abdominal cavity. From the cecum, the ascending colon rises superiorly toward the right inferior border of the liver. Just before reaching the liver, the ascending colon turns about 90 degrees to the left at the hepatic flexure. From the hepatic flexure the colon continues onward as the transverse colon. Physiology About 90% of the nutrients present in digested food have been absorbed by the time it reaches the large intestine. This food is mixed with bacteria in the cecum to form feces. Waves of peristalsis move the feces slowly up the length of the ascending colon. As the feces pass through the ascending colon, bacteria digest the waste material that the human body cannot digest and liberate vitamins K, B1, B2, and B12. The walls of the colon absorb these vitamins along with most of the water present in the feces. Under normal conditions fecal matter enters the colon as chunky liquid waste and exits the colon as a condensed solid waste. The absorption of water by the colon helps to maintain water homeostasis in the body and prevent dehydration. Histology The mucosa forms the innermost layer of the ascending colon. Mucosa is made of epithelial tissue and secretes mucus from goblet cells to lubricate feces passing through the lumen. The cells of the mucosa absorb the vitamins and water from feces and transfer these substances to blood in nearby capillaries. Deep to the mucosa is the submucosa layer that contains the blood vessels, nerves, and connective tissues that support the mucosa. Next, the muscularis layer surrounds the submucosa and provides several layers of smooth muscle tissue to move food through the colon via peristalsis. The pouches of the colon are formed by the contraction of smooth muscle in the muscularis. The outermost layer of the ascending colon is covered by the peritoneum on the anterior side and by areolar connective tissue on the posterior side. These tissues hold the colon in place, provide blood vessels to the colon, and protect the colon from friction caused by the body's movement.

cecum

The cecum is the cul-de-sac at the beginning of the large intestine that descends from the union of the ileum and the large intestine. It provides a space for the mixing of bacteria with partially digested food from the small intestine to form feces. Anatomy The cecum is a short, pouch-like region of the large intestine between the ascending colon and vermiform appendix. It is located in the lower right quadrant of the abdominal cavity inferior and lateral to the ileum. Histology The cecum consists of four distinct tissue layers that work together to achieve the organ's function. The innermost layer, the mucosa, is made of smooth mucous membrane with many goblet cells. Goblet cells secrete mucus to lubricate and protect the surface of the cecum. Absorption of nutrients is performed by the epithelial cells forming the surface of the mucous membrane. Surrounding the mucosa is the submucosa layer that contains the blood vessels and nerves that support the surrounding tissues. The next layer, the muscularis, contains bands of smooth muscle tissue arranged in longitudinal and transverse bands to contract the walls of the cecum. Contraction of the muscularis results in the mixing of chyme with bacteria and the propulsion of chyme into the ascending colon. The outermost layer of the cecum is the serosa, a layer of simple squamous epithelial tissues. The serosa produces a slick serous fluid that lubricates the exterior of the cecum and protects it from friction with the surrounding tissues of the abdomen. Physiology The cecum plays an important role in the digestive system by assisting in the formation of feces. Partially digested food, known as chyme, passes through the small intestine where it is digested and most of its nutrients are absorbed. The ileocecal sphincter at the end of the small intestine opens and closes to allow small amounts of chyme to enter the cecum at the beginning of the large intestine. Chyme is next mixed with bacteria by contractions in the walls of the cecum, before being pushed upward into the ascending colon.

descending colon

The descending colon is a segment of the large intestine and is the third and penultimate segment of the colon. It transports feces from the transverse colon inferiorly along the left side of the abdominal cavity to the sigmoid colon. Feces passing through the descending colon are stored until they are ready to be eliminated from the body. The walls of the descending colon absorb water as well as remaining nutrients and vitamins from the feces, depositing these valuable substances into our bloodstream. The descending colon is a hollow tube that is part of the gastrointestinal (GI) tract. Its diameter is roughly 2.5 inches (7 cm), while its length is approximately 9 to 10 inches (25 cm). It contains many small pouches, known as haustra, along its length that increase its surface area and help to move feces through the colon. At its superior end, the descending colon connects to the transverse colon at the splenic flexure just inferior to the spleen. From the splenic flexure, the descending colon extends inferiorly toward the left hip before turning about 90 degrees to the right and forming the sigmoid colon. Like all parts of the gastrointestinal tract, the descending colon consists of four major tissue layers surrounding a hollow lumen. The mucosa forms the innermost layer that surrounds the lumen and is in contact with fecal matter stored in the colon. Mucosa is a mucous membrane made of simple columnar epithelial tissue and a thin underlying layer of areolar connective tissue. The mucus secreted from goblet cells in the mucosa layer provides lubrication to feces moving through the lumen, protecting the delicate tissues of the colon. Epithelial cells in the mucosa absorb any remaining nutrients, vitamins, and water present in the feces. Surrounding the mucosa are the supportive tissues of the submucosa layer, where we find blood vessels, nerves and connective tissues. The muscularis layer is found deep to the submucosa and enables movement of the descending colon. Several layers of smooth muscle make up the muscularis and allow the descending colon to form the pocket-like haustra. During defecation, these muscle cells contract in waves of peristalsis to push feces down the colon toward the sigmoid colon. Finally, the outermost anterior layer of the descending colon is visceral peritoneum; on the posterior of the descending colon, the outermost layer is areolar connective tissue (known as adventitia). These tissues anchor the colon along the posterior body wall; provide blood flow to the colon; and offer protection from friction as our bodies move. By the time feces reach the descending colon, the vast majority of nutrients, vitamins and water have been extracted by the ascending and transverse colon, leaving mostly waste products. Still, some absorption of water and vitamins produced by bacterial fermentation of feces — including vitamins K, B1, B2 and B12 — does occur in the descending colon. Its primary function, however, is the storage and accumulation of feces prior to defecation. During defecation, the descending colon helps to propel feces toward the sigmoid colon and rectum and eventually out of the body by contraction of its smooth muscle tissue.

Digestive System of the Lower Torso

The digestive organs of the lower torso include lower gastrointestinal (GI) tract, which consists of the small intestine, large intestine, and anus. Several accessory organs, such as the liver and pancreas, assist the lower GI tract with the digestion of food to release many essential nutrients. By the time this food exits the lower GI tract through the anus, the food has been completely digested and almost all of the nutrients have been absorbed into the bloodstream. Bacteria assist with the digestion of food and its conversion into feces while water is absorbed to leave only solid waste material to be excreted. Acidic, partially digested food known as chyme reaches the lower GI tract through the pyloric sphincter of the stomach. Chyme passing through the pyloric sphincter first enters the duodenum, a one-foot long, C-shaped segment of the small intestine located inferior to and to the right of the stomach. In addition to chyme, the duodenum also receives bile from the gallbladder and liver, as well as pancreatic juice from the pancreas. Bile and pancreatic juice mix with chyme in the duodenum to neutralize the chyme's acidity, emulsify lipids, and chemically digest the chyme into its most basic building blocks. A small amount of absorption of nutrients also takes place in the walls of the duodenum, but most of the absorption takes place in the jejunum. The jejunum is the eight-foot long, convoluted middle section of the small intestine that is located between the duodenum and the ileum. Chyme entering the jejunum has been thoroughly digested and is ready to have its nutrients absorbed through the intestinal mucosa. The mucosa of the jejunum contains many finger-like projections, or villi, along with many circular folds along its length. These structures greatly increase the surface area of the jejunum, which increases the jejunum's ability to absorb nutrients. By the time chyme moves from the jejunum to the ileum, about 90% of its nutrient content has been absorbed into the bloodstream. Chyme continues along the length of the small intestine and enters the ileum, a highly folded, 10-foot long tube that leads to the cecum of the large intestine. Chyme entering the ileum is very low in nutrients, so the ileum is specialized for the absorption of the very last remaining traces of nutrients before moving the chyme on to the large intestine. The ileum contains villi and folds similar to the jejunum to increase its surface area and to increase the absorption of nutrients. Small masses of lymphatic tissue known as Peyer's patches line the ileum and monitor the contents of the intestines for pathogens to prevent illness. Upon reaching the end of the ileum, chyme passes through the ileocecal sphincter and enters the large intestine. The cecum is a dead-end pouch on the right inferior end of the large intestine. Chyme entering the cecum is mixed with bacterial flora to produce feces. The feces are slowly moved by peristalsis out of the cecum and into the ascending colon while bacteria begin to break down indigestible waste material. Attached to the inferior end of the cecum is the vermiform appendix, a thin tube that stores beneficial bacteria. Feces next pass through the colon, the longest section of the large intestine that extends from the cecum to the rectum. There are four major regions of the colon: the ascending colon, transverse colon, descending colon, and sigmoid colon. The ascending colon connects to the cecum and runs superiorly toward the right inferior corner of the liver. Just below the liver, the ascending colon turns to the left and becomes the transverse colon. The transverse colon crosses to the left side of the abdomen, turns to the downward, and becomes the descending colon. The descending colon runs down the left side of the abdomen to the sigmoid colon. Finally, the sigmoid colon forms an S-shaped end to the colon that terminates in the rectum. Feces passing through the large intestine are mixed with and fermented by bacteria. The bacteria release B vitamins and vitamin K from the feces as they move along the length of the colon. The smooth walls of the colon absorb the released vitamins along with water and any other remaining nutrients. By the time that feces reach the end of the colon, they have been dried, condensed, and stripped of all vitamins and nutrients. The rectum is a short, straight tube near the end of the large intestine that stores feces until the body is ready to expel them during defecation. It is lined with many sensory receptors that monitor pressure and stretching in the walls of the rectum. When the rectum is filled with feces, these receptors send signals to the brain to let the conscious mind know that feces are ready for defecation. The anal canal is last segment of the large intestine that ends at the anus and controls the defecation of feces. Feces entering the rectum and anal canal exert pressure on the internal anal sphincter, causing its smooth muscle tissue to relax and dilate. The skeletal muscles of the external anal sphincter hold feces in the anal canal until voluntary signals from the brain trigger its dilation. Once both anal sphincters are opened, smooth muscle contractions in the rectum and sigmoid colon push feces through the anal canal and out of the body.

Digestive system

The digestive system is a group of organs working together to convert food into energy and basic nutrients to feed the entire body. Food passes through a long tube inside the body known as the alimentary canal or the gastrointestinal tract (GI tract). The alimentary canal is made up of the oral cavity, pharynx, esophagus, stomach, small intestines, and large intestines. In addition to the alimentary canal, there are several important accessory organs that help your body to digest food but do not have food pass through them. Accessory organs of the digestive system include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. Food begins its journey through the digestive system in the mouth, also known as the oral cavity. Inside the mouth are many accessory organs that aid in the digestion of food—the tongue, teeth, and salivary glands. Teeth chop food into small pieces, which are moistened by saliva before the tongue and other muscles push the food into the pharynx.

Digestive System Physiology

The digestive system is responsible for taking whole foods and turning them into energy and nutrients to allow the body to function, grow, and repair itself. The six primary processes of the digestive system include: Ingestion of food Secretion of fluids and digestive enzymes Mixing and movement of food and wastes through the body Digestion of food into smaller pieces Absorption of nutrients Excretion of wastes

Digestive System of the Head and Neck

The digestive system of the head and neck contains the structures responsible for the ingestion, chewing, swallowing, and initial digestion of food. These structures include the parts of the mouth, the salivary glands that produce saliva, and the pharynx. The process of digestion begins with the ingestion of food through the oral cavity, commonly known as the mouth. The mouth contains the teeth that masticate, or chew, large masses of food into smaller pieces that have more surface area and are able to be swallowed This process of physically separating food into smaller pieces is known as mechanical digestion. The tongue, lips, and cheeks assist in mechanical digestions by holding food in the mouth and moving it around so that it can be effectively chewed by the teeth. Three groups of salivary glands surround the mouth and secrete saliva into the oral cavity. The parotid glands are the largest salivary glands and are found on either side of the jaw just anterior to the ears. The parotid glands secrete saliva into the posterior of the mouth. Just anterior and slightly inferior to the parotid glands are the two submandibular glands that rest below the jaw and secrete into the middle of the mouth. Finally, just below the tongue are the sublingual glands that secrete saliva into the anterior portion of the mouth. The saliva secreted by all of these glands is mostly made of water, but also contains the enzymes salivary amylase and lingual lipase. Saliva both moistens and softens dry food in the oral cavity to protect the delicate mucosa of the digestive system and aid in the swallowing of food. Salivary amylase begins the process of chemically digestion in carbohydrates by breaking starches into simple sugars that can be used as a fast energy source. Lingual lipase similarly digests fats into fatty acids, but is not activated until food reaches the acidic environment of the stomach. Masticated food that has been mixed with saliva forms a paste-like substance that the mouth and tongue roll into a mass known as a bolus. This mass is placed on the tongue and swallowed into the pharynx, or throat, through a complex interaction of the muscles of the mouth, tongue, palate, and throat. Swallowed food passes into the funnel-shaped pharynx where it passes through the oropharynx region in the back of the mouth and into the laryngopharynx that connects to the esophagus and larynx, or voice box. The epiglottis, a flap of flexible fibrocartilage at the top of the larynx, moves to cover the opening of the larynx during the swallowing process to direct food into the esophagus. This movement by the epiglottis prevents the swallowed food from blocking the airway and causing life-threatening asphyxiation. The bolus of food passes into the esophagus, where it will be moved to the stomach by waves of smooth muscle contraction known as peristalsis.

Digestive System of the Upper Torso

The digestive system of the upper abdomen and the chest is also known as the upper gastrointestinal (GI) tract. It includes several of the digestive system's most important organs, including the esophagus, stomach, duodenum, liver, and gallbladder. These organs are responsible for significant breakdown of food and for transporting the partially digested food further along the alimentary canal. Food entering the long, tube-like esophagus from the throat is moved by peristalsis toward the stomach. Upon reaching the stomach, food is stored and digested by being mixed with acid and digestive enzymes to form a liquid called chyme. Next, this chyme passes slowly into the duodenum, the first segment of the small intestine. Bile from the liver and gallbladder along with enzymes from the pancreas are added to chyme in the duodenum to further digest it into small nutrients that can be absorbed by the intestines.

mixing and movement

The digestive system uses 3 main processes to move and mix food: Swallowing. Swallowing is the process of using smooth and skeletal muscles in the mouth, tongue, and pharynx to push food out of the mouth, through the pharynx, and into the esophagus. Peristalsis. Peristalsis is a muscular wave that travels the length of the GI tract, moving partially digested food a short distance down the tract. It takes many waves of peristalsis for food to travel from the esophagus, through the stomach and intestines, and reach the end of the GI tract. Segmentation. Segmentation occurs only in the small intestine as short segments of intestine contract like hands squeezing a toothpaste tube. Segmentation helps to increase the absorption of nutrients by mixing food and increasing its contact with the walls of the intestine.

duodenum

The duodenum is the first and shortest segment of the small intestine. It receives partially digested food (known as chyme) from the stomach and plays a vital role in the chemical digestion of chyme in preparation for absorption in the small intestine. Many chemical secretions from the pancreas, liver and gallbladder mix with the chyme in the duodenum to facilitate chemical digestion. Located inferior to the stomach, the duodenum is a 10-12 inch (25-30 cm) long C-shaped, hollow tube. The duodenum is a part of the gastrointestinal (GI) tract, attached to the pyloric sphincter of the stomach on its superior end and to the jejunum of the small intestine on its inferior end. The pancreas, liver and gallbladder all deliver their digestive secretions into the duodenum through an orifice known as the ampulla of Vater, which is located roughly in the middle of the duodenum on the left side. The walls of the duodenum are made of four layers of tissue that are consistent with the structure of the rest of the gastrointestinal tract: The innermost layer, the mucosa, lines the inner surface of the duodenum and is in contact with chyme passing through the intestinal lumen. It is made of simple columnar epithelial tissue with microvilli on its surface to increase its surface area and improve the absorption of nutrients. Plentiful mucous glands secrete mucus into the lumen to lubricate the intestinal wall and protect it from friction and acidic chyme. Surrounding the mucosa layer is the submucosa, a layer of connective tissue that supports the other tissue layers. Many blood vessels and nerves pass through the submucosa, while protein fibers give strength and elasticity to the duodenum. Surrounding the submucosa is the muscularis layer that contains the smooth muscle tissue of the duodenum. Contractions of the muscularis mix chyme and propel it through the duodenum toward the rest of the small intestine. Lastly, the serosa is the outermost layer of the duodenum that acts as the outer skin of the intestine. Serous membrane made of simple squamous epithelium provides a smooth, slick surface to prevent friction between the duodenum and the surrounding organs. The serosa also secretes serous fluid to further reduce friction and keep the duodenum's surface moist. After being stored and mixed with hydrochloric acid in the stomach for about 30 to 60 minutes, chyme slowly enters the duodenum through the pyloric sphincter. Next, Brunner's glands in the mucosa of the duodenum secrete alkaline mucus containing a high concentration of bicarbonate ions to neutralize the hydrochloric acid present in the chyme. This alkaline mucus both protects the walls of the duodenum and helps the chyme to reach a pH conducive to chemical digestion in the small intestine. Upon reaching the ampulla of Vater in the middle of the duodenum, chyme is mixed with bile from the liver and gallbladder, as well as pancreatic juice produced by the pancreas. These secretions complete the process of chemical digestion that began in the mouth and stomach by breaking complex macromolecules into their basic units. Bile produced in the liver and stored in the gallbladder acts as an emulsifier, breaking lipids into smaller globules to increase their surface area. Pancreatic juice contains many enzymes to break carbohydrates, lipids, proteins and nucleic acids into their monomer subunits. For example, pancreatic lipase breaks triglycerides, or fats, into glycerol and fatty acids that can be absorbed into the bloodstream by the intestinal wall. These secretions are thoroughly mixed with the chyme by contractions of the duodenum until all of the digestible material is chemically digested. Slow waves of smooth muscle contraction known as peristalsis flow down the length of the gastrointestinal tract to push chyme through the duodenum. Each wave begins at the stomach and pushes chyme a short distance toward the jejunum. It takes many peristaltic contractions over the course of an hour for chyme to travel through the entire length of the duodenum. Small regional contractions of the intestinal wall, known as segmentations, help to mix chyme with the digestive secretions in the duodenum and increase the rate of digestion. Segmentations also increase the contact of chyme with the mucosal cells to increase the absorption of nutrients through the intestinal wall.

esophagus expanded

The esophagus is a long, thin, and muscular tube that connects the pharynx (throat) to the stomach. It forms an important piece of the gastrointestinal tract and functions as the conduit for food and liquids that have been swallowed into the pharynx to reach the stomach. The esophagus is about 9-10 inches (25 centimeters) long and less than an inch (2 centimeters) in diameter when relaxed. It is located just posterior to the trachea in the neck and thoracic regions of the body and passes through the esophageal hiatus of the diaphragm on its way to the stomach. At the superior end of the esophagus is the upper esophageal sphincter that keeps the esophagus closed where it meets the pharynx. The upper esophageal sphincter opens only during the process of swallowing to permit food to pass into the esophagus. At the inferior end of the esophagus, the lower esophageal sphincter opens for the purpose of permitting food to pass from the esophagus into the stomach. Stomach acid and chyme (partially digested food) is normally prevented from entering the esophagus, thanks to the lower esophageal sphincter. If this sphincter weakens, however, acidic chyme may return to the esophagus in a condition known as acid reflux. Acid reflux can cause damage to the esophageal lining and result in a burning sensation known as heartburn. If these symptoms occur with enough frequency, they are known as GERD (gastroesophageal reflux disease). Like the rest of the gastrointestinal tract, the esophagus is made of four distinct tissue layers. The mucosa layer forms the inner lining of the esophagus and is the only tissue layer that has direct contact with substances passing through the esophagus. Non-keratinized stratified squamous epithelial tissue makes up the majority of the mucosa layer and provides protection to the esophagus from rough food particles and acid from the nearby stomach. Mucous glands in the mucosa produce mucus to lubricate the esophagus and help shield the mucosa from stomach acid. Deep to the mucosa is the submucosa layer that contains connective tissue and provides blood and nerve supply to the mucosa and other tissues of the esophagus. Surrounding the submucosa is the muscularis layer that allows the esophagus to contract and expand to move substances. Skeletal muscle is mostly found in the superior region of the esophagus to aid in the swallowing reflex while smooth muscle in the inferior esophagus pushes substances toward the stomach via peristalsis. Finally, the adventitia layer forms an outer covering of loose connective tissue around the esophagus and attaches it loosely to the surrounding organs. The esophagus is involved in the processes of swallowing and peristalsis to move substances from the mouth to the stomach. The swallowing food begins in the mouth and continues with the contraction of skeletal muscles in the pharynx and esophagus. The upper esophageal sphincter dilates to permit the swallowed substance to enter the esophagus. From this point, waves of muscle contraction called peristalsis move food toward the stomach. In peristalsis, regions of the esophagus closer to the stomach open to permit food to pass through while the region just above the food contracts to push the food onward. Peristalsis works so well that food can be swallowed even while the body is lying down, upside down, or even in zero-gravity. A final function of the esophagus is its participation in the vomiting reflex to void the contents of the stomach. Peristalsis is reversed in the esophagus during vomiting to forcefully remove toxic or pathogen-laden food from the body.

esophagus

The esophagus is a muscular tube connecting the pharynx to the stomach that is part of the upper gastrointestinal tract. It carries swallowed masses of chewed food along its length. At the inferior end of the esophagus is a muscular ring called the lower esophageal sphincter or cardiac sphincter. The function of this sphincter is to close of the end of the esophagus and trap food in the stomach

excretion

The final function of the digestive system is the excretion of waste in a process known as defecation. Defecation removes indigestible substances from the body so that they do not accumulate inside the gut. The timing of defecation is controlled voluntarily by the conscious part of the brain, but must be accomplished on a regular basis to prevent a backup of indigestible materials.

ingestion

The first function of the digestive system is ingestion, or the intake of food. The mouth is responsible for this function, as it is the orifice through which all food enters the body. The mouth and stomach are also responsible for the storage of food as it is waiting to be digested. This storage capacity allows the body to eat only a few times each day and to ingest more food than it can process at one time.

Gallbladder

The gallbladder is a small storage organ located inferior and posterior to the liver. Though small in size, the gallbladder plays an important role in our digestion of food. The gallbladder holds bile produced in the liver until it is needed for digesting fatty foods in the duodenum of the small intestine. Bile in the gallbladder may crystallize and form gallstones, which can become painful and potentially life threatening.

illeum

The ileum is the last part of the three part tube that makes up the small intestine. It is where the remaining nutrients are absorbed before moving into the large intestine.

inferior nasal concha

The inferior nasal concha is a curved bony plate on the lateral wall within the nasal fossa. It is the largest of the three bony plates and separates the inferior and middle meatuses of the nose

jejunum

The jejunum is the middle segment of the small intestine found between the duodenum and the ileum. Most of the nutrients present in food are absorbed by the jejunum before being passed on to the ileum for further absorption. Anatomy The jejunum is a continuation of the small intestine following the duodenum. It begins at the duodenojejunal flexure, where the small intestine turns sharply toward the anterior direction From the duodenojejunal flexure, the jejunum follows a convoluted path through the abdomen before continuing as the ileum. While the jejunum does not have an anatomical landmark to separate it from the ileum, it slowly changes its anatomical structure along its length as it transitions into the ileum. Histology The jejunum is made of four distinct tissue layers that work together to give the organ its function. The innermost layer, the mucosa, surrounds the hollow lumen and provides contact between the jejunum and chyme. It is made of folds of epithelial tissue specialized for absorption of nutrients. Many goblet cells in the mucosa produce mucus to protect the intestinal walls and to lubricate chyme passing through the jejunum. Deep to the mucosa is the submucosa layer that supports the other tissue layers. Many blood vessels and nerves pass through the submucosa to provide oxygen, nutrients, and nerve signals to tissues of the jejunum. The muscularis is the next layer of the jejunum that surrounds the submucosa and contains smooth muscle tissue. Contractions of the smooth muscle in the muscularis allow food to be mixed and propelled through the jejunum. Finally, the serosa forms the outermost layer of the jejunum and functions as the skin of the intestine. Serosa is made of simple squamous epithelial tissue and secretes a thin slippery liquid known as serous fluid. Serous fluid lubricates the exterior of the jejunum and protects it from friction between organs of the abdominal cavity. Physiology Partially digested food, known as chyme, enters the jejunum from the duodenum. As chyme enters the jejunum, it is mixed by segmentations, or localized smooth muscle contractions in the walls of the jejunum. These segmentations help to circulate chyme and increase its contact with the walls of the jejunum. The walls of the jejunum are folded many times over to increase its surface area and allow it to absorb nutrients. Each epithelial cell on the surface of the jejunum contains microscopic folds of cell membrane called microvilli that create tiny pockets and increase the contact between the cells and chyme. The entire wall of the jejunum is also folded into microscopic finger-like ridges known as villi that form larger pockets and further increase the surface area of the jejunum. At the macroscopic level, the inner surface of the jejunum contains many wrinkles of tissue known as circular folds, which create even more pockets for chyme and further increase the surface area available for absorption. Thus, the entire structure of the jejunum is optimized for the absorption of nutrients from chyme. By the time chyme has passed through the jejunum and enters the ileum, around 90% of all available nutrients have been absorbed into the body.

large intestine

The large intestine is a long, thick tube about 2.5 inches in diameter and about 5 feet long. It is located just inferior to the stomach and wraps around the superior and lateral border of the small intestine. The large intestine absorbs water and contains many symbiotic bacteria that aid in the breaking down of wastes to extract some small amounts of nutrients. Feces in the large intestine exit the body through the anal canal.

large intestine expanded

The large intestine is the final section of the gastrointestinal tract that performs the vital task of absorbing water and vitamins while converting digested food into feces. Although shorter than the small intestine in length, the large intestine is considerably thicker in diameter, thus giving it its name. The large intestine is about 5 feet (1.5 m) in length and 2.5 inches (6-7 cm) in diameter in the living body, but becomes much larger postmortem as the smooth muscle tissue of the intestinal wall relaxes. The large intestine wraps around the border of the abdominal body cavity from the right side of the body, across the top of the abdomen, and finally down the left side. Beginning on the right side of the abdomen, the large intestine is connected to the ilium of the small intestine via the ileocecal sphincter. From the ileocecal sphincter, the large intestine forms a sideways "T," extending both superiorly and inferiorly. The inferior region of the large intestine forms a short dead-end segment known as the cecum that terminates in the vermiform appendix. The superior region forms a hollow tube known as the ascending colon that climbs along the right side of the abdomen. Just inferior to the diaphragm, the ascending colon turns about 90 degrees toward the middle of the body at the hepatic flexure and continues across the abdomen as the transverse colon. At the left side of the abdomen, the transverse colon turns about 90 degrees at the splenic flexure and runs down the left side of the abdomen as the descending colon. At the end of the descending colon, the large intestine bends slightly medially at the sigmoid flexure to form the S-shaped sigmoid colon before straightening into the rectum. The rectum is the enlarged final segment of the large intestine that terminates at the anus. Like the rest of the gastrointestinal canal, the large intestine is made of four tissue layers: The innermost layer, known as the mucosa, is made of simple columnar epithelial tissue. The mucosa of the large intestine is smooth, lacking the villi found in the small intestine. Many mucous glands secrete mucus into the hollow lumen of the large intestine to lubricate its surface and protect it from rough food particles. Surrounding the mucosa is a layer of blood vessels, nerves and connective tissue known as the submucosa, which supports the other layers of the large intestine. The muscularis layer surrounds the submucosa and contains many layers of visceral muscle cells that contract and move the large intestine. Continuous contraction of smooth muscle bands in the muscularis produces lumpy, pouch-like structures known as haustra in the large intestine. Finally, the serosa forms the outermost layer. The serosa is a thin layer of simple squamous epithelial tissue that secretes watery serous fluid to lubricate the surface of the large intestine, protecting it from friction between abdominal organs and the surrounding muscles and bones of the lower torso. The large intestine performs the vital functions of converting food into feces, absorbing essential vitamins produced by gut bacteria, and reclaiming water from feces. A slurry of digested food, known as chyme, enters the large intestine from the small intestine via the ileocecal sphincter. Chyme passes through the cecum where it is mixed with beneficial bacteria that have colonized the large intestine throughout a person's lifetime. The chyme is then slowly moved from one haustra to the next through the four regions of the colon. Most of the movement of chyme is achieved by slow waves of peristalsis over a period of several hours, but the colon can also be emptied quickly by stronger waves of mass peristalsis following a large meal. While chyme moves through the large intestine, bacteria digest substances in the chyme that are not digestible by the human digestive system. Bacterial fermentation converts the chyme into feces and releases vitamins including vitamins K, B1, B2, B6, B12, and biotin. Vitamin K is almost exclusively produced by the gut bacteria and is essential in the proper clotting of blood. Gases such as carbon dioxide and methane are also produced as a byproduct of bacterial fermentation and lead to flatulence, or gas passed through the anus. The absorption of water by the large intestine not only helps to condense and solidify feces, but also allows the body to retain water to be used in other metabolic processes. Ions and nutrients released by gut bacteria and dissolved in water are also absorbed in the large intestine and used by the body for metabolism. The dried, condensed fecal matter is finally stored in the rectum and sigmoid colon until it can be eliminated from the body through the process of defecation.

lips

The lips are highly mobile structures that surround the mouth opening. They contain skeletal muscles and a variety of sensory nerves that are useful in judging the temperature and texture of foods. Their normal reddish color is due to an abundance of blood vessels near their surfaces. The external borders of the lips mark the boundaries between the skin of the face and the mucous membrane that lines the alimentary cana

liver and gallbladder

The liver is a roughly triangular accessory organ of the digestive system located to the right of the stomach, just inferior to the diaphragm and superior to the small intestine. The liver weighs about 3 pounds and is the second largest organ in the body. The liver has many different functions in the body, but the main function of the liver in digestion is the production of bile and its secretion into the small intestine. The gallbladder is a small, pear-shaped organ located just posterior to the liver. The gallbladder is used to store and recycle excess bile from the small intestine so that it can be reused for the digestion of subsequent meals.

middle nasal concha

The middle nasal concha is one of the bony plates on the lateral wall within the nasal fossa that separates the superior and middle meatuses of the nose. It is the lower of the two thin bony processes of the ethmoid bone.

Microscopic Anatomy of the gallbladder

The mucosa, which forms the innermost layer of the gallbladder, lines the gallbladder with simple columnar epithelial tissue. The columnar epithelial tissue contains microvilli on its surface, increasing the surface area and allowing the lining to absorb water and concentrate the dilute bile. Beneath the columnar tissue is a thin lamina propria layer made of connective tissue and capillaries that support and anchor the epithelial layer. Deep to the lamina propria is the muscularis layer that contains smooth muscle tissue. Contraction of the muscularis pushes bile out of the gallbladder and into the cystic duct. Surrounding the muscularis is a thin layer of fibrous connective tissue that helps to reinforce and strengthen the wall of the gallbladder. Finally, the serosa forms the outermost layer of the gallbladder. The serosa is an epithelial layer that forms part of the peritoneum, or lining of the abdominal cavity. The serosa gives the gallbladder a smooth, slick surface to prevent friction between moving organs.

omentum

The omentum is an apron-like double fold of fatty membrane that hangs down in front of the intestines. It contains blood vessels, nerves, lymph vessels and lymph nodes. It acts as a storage for fat and also may limit the spread of infection in the abdominal cavity.

palatine glands

The palatine glands are in the back of the mouth, on both sides of the tongue and closely associated with the palate. These masses of lymphatic tissue are also called palatine tonsils, or sometimes just tonsils. These structures lie beneath the lining of the mouth and, like other lymphatic tissues, they help to protect the body against infections. These are common sites of infection; when they become inflamed, the condition is called tonsillitis.

pancreas expanded

The pancreas is a glandular organ in the upper abdomen, but really it serves as two glands in one: a digestive exocrine gland and a hormone-producing endocrine gland. Functioning as an exocrine gland, the pancreas excretes enzymes to break down the proteins, lipids, carbohydrates, and nucleic acids in food. Functioning as an endocrine gland, the pancreas secretes the hormones insulin and glucagon to control blood sugar levels throughout the day. Both of these diverse functions are vital to the body's survival.

pancreas

The pancreas is a large gland located just inferior and posterior to the stomach. It is about 6 inches long and shaped like short, lumpy snake with its "head" connected to the duodenum and its "tail" pointing to the left wall of the abdominal cavity. The pancreas secretes digestive enzymes into the small intestine to complete the chemical digestion of foods.

gross anatomy of pancreas

The pancreas is a narrow, 6-inch long gland that lies posterior and inferior to the stomach on the left side of the abdominal cavity. The pancreas extends laterally and superiorly across the abdomen from the curve of the duodenum to the spleen. The head of the pancreas, which connects to the duodenum, is the widest and most medial region of the organ. Extending laterally toward the left, the pancreas narrows slightly to form the body of the pancreas. The tail of the pancreas extends from the body as a narrow, tapered region on the left side of the abdominal cavity near the spleen. Glandular tissue that makes up the pancreas gives it a loose, lumpy structure. The glandular tissue surrounds many small ducts that drain into the central pancreatic duct. The pancreatic duct carries the digestive enzymes produced by endocrine cells to the duodenum.

Microscopic Anatomy of the pancreas

The pancreas is classified as a heterocrine gland because it contains both endocrine and exocrine glandular tissue. The exocrine tissue makes up about 99% of the pancreas by weight while endocrine tissue makes up the other 1%. The exocrine tissue is arranged into many small masses known as acini. Acini are small raspberry-like clusters of exocrine cells that surround tiny ducts. The exocrine cells in the acini produce digestive enzymes that are secreted from the cells and enter the ducts. The ducts of many acini connect to form larger and larger ducts until the products of many acini run into the large pancreatic duct. The endocrine portion of the pancreas is made of small bundles of cells called islets of Langerhans. Many capillaries run through each islet to carry hormones to the rest of the body. There are 2 main types of endocrine cells that make up the islets: alpha cells and beta cells. Alpha cells produce the hormone glucagon, which raises blood glucose levels. Beta cells produce the hormone insulin, which lowers blood glucose levels.

parotid duct

The parotid duct is a salivary gland, also referred to as Stensen's duct, or Steno's duct. It is located beneath and in front of the ear.

parotid gland

The parotid gland is the largest of the three major pairs of salivary glands. It is located anteriorly and inferiorly to the ear between the skin and the muscle of chewing, the masseter. The parotid duct carries its contents and drains into the mouth. It is the parotid gland that becomes swollen and infected with the mumps or parotitis.

pharynx

The pharynx, or throat, is a funnel-shaped tube connected to the posterior end of the mouth. The pharynx is responsible for the passing of masses of chewed food from the mouth to the esophagus. The pharynx also plays an important role in the respiratory system, as air from the nasal cavity passes through the pharynx on its way to the larynx and eventually the lungs. Because the pharynx serves two different functions, it contains a flap of tissue known as the epiglottis that acts as a switch to route food to the esophagus and air to the larynx.

rectum

The rectum is the final segment of the large intestine that connects the colon to the anus. It stores fecal matter produced in the colon until the body is ready to eliminate the waste through the process of defecation. Anatomy The rectum is a hollow muscular tube about 8 inches (20 cm) in length and 2.5 inches in diameter at its widest point. It extends from the inferior end of the sigmoid colon along the anterior surface of the sacrum and coccyx in the posterior of the pelvic cavity. At its inferior end, the rectum tapers slightly before ending at the anus. Histology The mucosa forms the innermost layer of the rectum that is in contact with fecal matter. The mucosa is made of epithelial tissue that secretes mucus from specialized cells known as goblet cells. Mucus helps to protect the walls of the rectum and lubricate the feces as they pass through the rectum. Deep to the mucosa is the submucosa layer that supports the other layers of the rectum. Many blood vessels and nerves pass through the submucosa to provide nutrients, oxygen, and nerve signals to the mucosa and muscle tissue. Next is the muscularis layer, which contains layers of visceral (smooth) muscle. Contractions of the muscularis allow the rectum to expel feces during defecation. Finally, the serosa forms the outermost layer of the rectum and protects it from external damage. The serosa is made of a thin layer of simple squamous epithelium that secretes serous fluid to lubricate the exterior of the rectum and prevent damage caused by friction between the moving organs of the pelvic cavity. Physiology Feces enter the rectum from the sigmoid colon, where they are stored until they can be eliminated through defecation. While feces are stored in the rectum, the walls of the rectum absorb some water and return it to the blood supply. Bacteria continue the fermentation of organic fecal matter that began in the colon and liberate some remaining nutrients that are absorbed by the rectal walls. As feces accumulate and fill the rectum, they exert increasing pressure on the rectal walls. The distention of the rectum stimulates stretch receptors in the rectal walls to send nerve impulses to the brain. These impulses are integrated in the brain and result in feelings of discomfort and mounting pressure to empty the rectum through defecation. They also cause relaxation of the smooth muscle of the internal anal sphincter to allow defecation to proceed.

sigmoid colon

The sigmoid colon is a curved, S-shaped region of the large intestine and is the final segment of the colon. It transports fecal matter from the descending colon to the rectum and anus. Feces are stored in the sigmoid colon until they are ready to be eliminated from the body through the anal canal. The intestinal wall of the sigmoid colon also plays a small role in the absorption of water, nutrients and vitamins from feces. The sigmoid colon is a hollow tube approximately 2.5 inches (7 cm) in diameter and 16 inches (40 cm) long. A band of smooth muscle tissue running the length of the colon contracts to form many small pouches called haustra in the walls of the sigmoid colon. Haustra help to increase the surface area of the colon, while their smooth muscle tissue helps to move feces toward the end of the colon. At its superior end, the sigmoid colon connects to the descending colon near the iliac crest in the left iliac region of the abdominal cavity. From this junction, the sigmoid colon curves medially and anteriorly toward the body's midline, before abruptly turning about 90 degrees to face inferiorly to form the rectum. Four major tissue layers surround the hollow lumen of the sigmoid colon and provide its structure and function. The innermost layer, the mucosa, is made of a mucous membrane consisting of simple stratified columnar epithelium and an underlying areolar connective tissue. The mucosa is the only tissue layer in contact with the contents of the sigmoid colon, acting as the interface between the colon and feces. Nutrients, vitamins and water present in feces are absorbed through the epithelium of the mucosa, while goblet cells produce thin, slick mucus to help feces move through the colon. The next layer outside the mucosa is the submucosa, a layer of connective tissues and nerves that support the mucosa. Blood vessels in the submucosa transport nutrients to and from the sigmoid colon, while nervous tissue monitors the contents of the sigmoid colon to manage the defecation reflex. The muscularis layer surrounds the submucosa and contains several layers of smooth muscle tissue. Contraction of smooth muscle tissue both forms the haustra of the sigmoid colon and propels feces out of the sigmoid colon and into the rectum during defecation. The outermost layer of the sigmoid colon is covered by the serosa layer, which extends as a thin membrane known as the mesentery. Although it is made of thin simple squamous epithelial tissue, the serosa protects the sigmoid colon from external friction by giving it a smooth surface and secreting lubricating serous fluid into the abdominal body cavity. The mesentery secures the colon along the posterior body wall and provides blood vessels and nerves to the colon, while the serosa protects the colon from friction caused by the body's movement. By the time digested food (known as chyme) reaches the large intestine, almost all of the available nutrients present in the chyme have been absorbed. Chyme is mixed with bacteria in the large intestine, which ferments the chyme to form feces. The fermentation process releases ions and vitamins from the chyme that are absorbed by the mucosa of the colon. Waves of peristalsis in the muscularis slowly push the feces from haustrum to haustrum through the ascending, transverse, and descending colon to the sigmoid colon. Feces entering the sigmoid colon have completed most of their bacterial fermentation and have had most of the water, ions and vitamins absorbed from them in the prior sections of the colon. Thus, the primary roles of the sigmoid colon are the storage of feces and the propulsion of feces into the rectum. The muscularis in the sigmoid colon plays an important role in controlling the expulsion of feces during defecation. Some absorption of water and vitamins K, B1, B2, and B12 also occurs as the feces are being stored in the sigmoid colon. The sigmoid colon is clinically important. As it is the region of the colon closest to the body's exterior, the sigmoid colon is commonly used to screen for and prevent colon cancer. A sigmoidoscopy is a procedure wherein an endoscope is inserted through the anus and rectum and used to observe the mucosa of the sigmoid colon. Any growths (known as polyps) that are found during a sigmoidoscopy are removed and examined by a pathologist to check for abnormal cells. This procedure helps to detect and prevent colon cancer before it becomes incurable.

Sinusoids and Bile Canaliculi

The sinusoids and bile canaliculi neighbor each other in the liver. The sinusoids are capillary-like vessels that the blood is conveyed to the inferior vena cava by the hepatic veins. Bile is produced in the liver by the hepatocytes and secreted into thin channels called bile canaliculi located within each hepatic plate. The canaliculi are drained peripherally by bile ducts that in turn drain into hepatic ducts that carry bile away from the liver. As a result, blood travels in the sinusoids and bile travels in the opposite direction so blood and bile never mix in the lobules of the liver under normal conditions.

small intestine expanded

The small intestine is a long, highly convoluted tube in the digestive system that absorbs about 90% of the nutrients from the food we eat. It is given the name "small intestine" because it is only 1 inch in diameter, making it less than half the diameter of the large intestine. The small intestine is, however, about twice the length of the large intestine and usually measures about 10 feet in length. The small intestine winds throughout the abdominal cavity inferior to the stomach. Its many folds help it to pack all 10 feet of its length into such a small body cavity. A thin membrane known as the mesentery extends from the posterior body wall of the abdominal cavity to surround the small intestine and anchor it in place. Blood vessels, nerves, and lymphatic vessels pass through the mesentery to support the tissues of the small intestine and transport nutrients from food in the intestines to the rest of the body. The small intestine can be divided into 3 major regions: The duodenum is the first section of intestine that connects to the pyloric sphincter of the stomach. It is the shortest region of the small intestine, measuring only about 10 inches in length. Partially digested food, or chyme, from the stomach is mixed with bile from the liver and pancreatic juice from the pancreas to complete its digestion in the duodenum. The jejunum is the middle section of the small intestine that serves as the primary site of nutrient absorption. It measures around 3 feet in length. The ileum is the final section of the small intestine that empties into the large intestine via the ileocecal sphincter. The ileum is about 6 feet long and completes the absorption of nutrients that were missed in the jejunum. Like the rest of the gastrointestinal tract, the small intestine is made up of four layers of tissue. The mucosa forms the inner layer of epithelial tissue and is specialized for the absorption of nutrients from chyme. Deep to the mucosa is the submucosa layer that provides blood vessels, lymphatic vessels, and nerves to support the mucosa on the surface. Several layers of smooth muscle tissue form the muscularis layer that contracts and moves the small intestines. Finally, the serosa forms the outermost layer of epithelial tissue that is continuous with the mesentery and surrounds the intestines. The interior walls of the small intestine are tightly wrinkled into projections called circular folds that greatly increase their surface area. Microscopic examination of the mucosa reveals that the mucosal cells are organized into finger-like projections known as villi, which further increase the surface area. Each square inch of mucosa contains around 20,000 villi. The cells on the surface of the mucosa also contain finger-like projections of their cell membranes known as microvilli, which further increase the surface area of the small intestine. It is estimated that there are around 130 billion microvilli per square inch in the mucosa of the small intestine. All of these wrinkles and projections help to greatly increase the amount of contact between the cells of the mucosa and chyme to maximize the absorption of vital nutrients. The small intestine processes around 2 gallons of food, liquids, and digestive secretions every day. To ensure that the body receives enough nutrients from its food, the small intestine mixes the chyme using smooth muscle contractions called segmentations. Segmentation involves the mixing of chyme about 7 to 12 times per minute within a short segment of the small intestine so that chyme in the middle of the intestine is moved outward to the intestinal wall and contacts the mucosa. In the duodenum, segmentations help to mix chyme with bile and pancreatic juice to complete the chemical digestion of the chyme into its component nutrients. Villi and microvilli throughout the intestines sway back and forth during the segmentations to increase their contact with chyme and efficiently absorb nutrients. Once nutrients have been absorbed by the mucosa, they are passed on into tiny blood vessels and lymphatic vessels in the middle of the villi to exit through the mesentery. Fatty acids enter small lymphatic vessels called lacteals that carry them back to the blood supply. All other nutrients are carried through veins to the liver, where many nutrients are stored and converted into useful energy sources. Chyme is slowly passed through the small intestine by waves of smooth muscle contraction known as peristalsis. Peristalsis waves begin at the stomach and pass through the duodenum, jejunum, and finally the ileum. Each wave moves the chyme a short distance, so it takes many waves of peristalsis over several hours to move chyme to the end of the ileum. This important organ is susceptible to a variety of diseases and disorders, including IBS and IBD. Did you know that some of us are genetically at higher risk of developing celiac disease, a gluten intolerance that harms our small intestine? Learn more about DNA health testing to find out how taking a test can tell you whether it's true for you.

small intestine

The small intestine is a long, thin tube about 1 inch in diameter and about 10 feet long that is part of the lower gastrointestinal tract. It is located just inferior to the stomach and takes up most of the space in the abdominal cavity. The entire small intestine is coiled like a hose and the inside surface is full of many ridges and folds. These folds are used to maximize the digestion of food and absorption of nutrients. By the time food leaves the small intestine, around 90% of all nutrients have been extracted from the food that entered it.

spleen

The spleen is a brown, flat, oval-shaped lymphatic organ that filters and stores blood to protect the body from infections and blood loss. Protected by our ribs, the spleen is located between the stomach and the diaphragm in the left hypochondriac region of the abdominal body cavity. The splenic artery branches off from the aorta and the celiac trunk to deliver oxygenated blood to the spleen, while the splenic vein carries deoxygenated blood away from the spleen to the hepatic portal vein. A tough connective tissue capsule surrounds the soft inner tissue of the spleen. Spongy inner tissue within the spleen contains many tiny blood vessels and hollow sinuses that store blood. The spleen can release its stored blood into circulation to replace blood lost during a traumatic injury. Many platelets are also stored with the blood in the spleen to help form blood clots to prevent further blood loss. Around the vessels and sinuses of the spleen are regions of red pulp and white pulp with a marginal zone in between. The red pulp regions contain many net-like reticular fibers that filter worn-out red blood cells from the blood flowing through the spleen. Captured red blood cells are digested to recycle the iron and protein components of hemoglobin. The marginal zone between the red and white pulp acts as a filter to capture pathogens in the blood and pass these pathogens on to the white pulp. White pulp regions of the spleen are made of lymphatic tissue containing macrophages, T lymphocytes, and B lymphocytes that destroy pathogens in the blood and produce antibodies. The spleen may enlarge during certain infections due to an increase in the number of white blood cells, captured pathogens and antibodies inside the spleen. The spleen is not a vital organ — its functions are useful but not essential for life. Red bone marrow, the liver, and lymph nodes can complete the filtration and blood recycling functions of the spleen in its absence. Because it is not a vital organ and is so soft, spongy, and vascular, damage to the spleen is almost always treated by its complete removal. Untreated damage to the spleen can quickly lead to massive internal hemorrhaging and eventual death.

stomach

The stomach is a muscular sac that is located on the left side of the abdominal cavity, just inferior to the diaphragm. In an average person, the stomach is about the size of their two fists placed next to each other. This major organ acts as a storage tank for food so that the body has time to digest large meals properly. The stomach also contains hydrochloric acid and digestive enzymes that continue the digestion of food that began in the mouth.

gross anatomy of the stomach

The stomach is a rounded, hollow organ located just inferior to the diaphragm in the left part of the abdominal cavity. Located between the esophagus and the duodenum, the stomach is a roughly crescent-shaped enlargement of the gastrointestinal tract. The inner layer of the stomach is full of wrinkles known as rugae (or gastric folds). Rugae both allow the stomach to stretch in order to accommodate large meals and help to grip and move food during digestion. The stomach can be divided into four regions based on shape and function: The esophagus connects to the stomach at a small region called the cardia. The cardia is a narrow, tube-like region that opens up into the wider regions of the stomach. Within the cardia is the lower esophageal sphincter, a band of muscle tissue that contracts to hold food and acid inside of the stomach. The cardia empties into the body of the stomach, which forms the central and largest region of the stomach. Superior to the body is a dome shaped region known as the fundus. Inferior to the body is a funnel shaped region known as the pylorus. The pylorus connects the stomach to the duodenum and contains the pyloric sphincter. The pyloric sphincter controls the flow of partially digested food (known as chyme) out of the stomach and into the duodenum.

stomach

The stomach is the main food storage tank of the body. If it were not for the stomach's storage capacity, we would have to eat constantly instead of just a few times each day. The stomach also secretes a mixture of acid, mucus, and digestive enzymes that helps to digest and sanitize our food while it is being stored

sublingual duct

The sublingual duct, also known as the major sublingual duct, is located in the mouth. It is the largest of numerous excretory ducts which provide drainage for the sublingual and submandibular glands.

sublingual gland

The sublingual gland, as its name implies, lies under the floor of the mouth and on the side of the tongue. It is one of the three major pairs of salivary glands. Each sublingual gland possesses several small sublingual ducts that empty into the floor of the mouth in an area posterior to the submandibular duct

submandibulur duct

The submandibular duct is located in the mouth; it provides drainage for the submandibular gland. It is also known as Wharton's duct, or the submaxillary duct.

submandibular gland

The submandibular gland is one of the three major sets of salivary glands. It is located inferiorly to the mandible or jawbone midway along the inner side of the jaw. It has a muscular covering and empties its contents by way of the submandibular duct into the floor of the mouth on both sides. The sublingual gland, as its name implies, lies under the floor of the mouth and on the side of the tongue.

superior nasal concha

The superior nasal concha is one of two delicate scroll-shaped plates called superior and middle nasal conchae, which project inward from the sides of the ethmoid bone toward the perpendicular plate. These bones, which are called the turbinate bones, support mucous membranes that line the nasal cavity.

teeth expanded

The teeth are a group of hard organs found in the oral cavity. We use teeth to masticate (or chew) food into tiny pieces. They also provide shape to the mouth and face and are important components in producing speech. A tooth can be divided into two main parts: the crown and root. Found above the gum line, the crown is the enlarged region of the tooth involved in chewing. Like an actual crown, the crown of a tooth has many ridges on its top surface to aid in the chewing of food. Below the gum line is the region of the tooth called the root, which anchors the tooth into a bony socket known as an alveolus. Roots are tapered structures resembling the roots of plants, and each tooth may have between one to three roots. The exterior surface of the root is covered in a bone-like mixture of calcium and collagen fibers known as cementum. Cementum provides grip for the periodontal ligaments that anchor the root to the surrounding alveolus. Each tooth is an organ consisting of three layers: the pulp, dentin, and enamel. The pulp of the tooth is a vascular region of soft connective tissues in the middle of the tooth. Tiny blood vessels and nerve fibers enter the pulp through small holes in the tip of the roots to support the hard outer structures. Stem cells known as odontoblasts form the dentin of the tooth at the edge of the pulp. Surrounding the pulp is the dentin, a tough, mineralized layer of tissue. Dentin is much harder than the pulp due to the presence of collagen fibers and hydroxylapatite, a calcium phosphate mineral that is one of the strongest materials found in nature. The structure of the dentin layer is very porous, allowing nutrients and materials produced in the pulp to spread through the tooth. The enamel — the white, outer layer of the crown — forms an extremely hard, nonporous cap over the dentin. Enamel is the hardest substance in the body and is made almost exclusively of hydroxylapatite. Teeth are classified into four major groups: incisors, canines, premolars, and molars. Incisors are chisel-shaped teeth found in the front of the mouth and have a flat apical surface for cutting food into smaller bits. Canine teeth, also known as cuspids, are sharply pointed, cone-shaped teeth that are used for ripping tough material like meat. They flank the incisors on both sides. Premolars (bicuspids) and molars are large, flat-surfaced teeth found in the back of the mouth. Peaks and valleys on the flat apical surface of premolars and molars are used for chewing and grinding food into tiny pieces. Babies are born without teeth, but grow a temporary set of twenty deciduous teeth (eight incisors, four canines, and eight molars) between the ages of six months and three years. Baby teeth fill the child's tiny jaws and allow the child to chew food while larger, stronger adult teeth develop inside the mandible and maxilla bones. At about six years of age the deciduous teeth are slowly shed one at a time and replaced by permanent adult teeth. Adult teeth develop while hidden within the maxilla and mandible after the deciduous teeth have erupted. When an adult tooth erupts, it triggers the roots of the deciduous tooth above it to atrophy. This causes the baby tooth to become loose and eventually fall out. The new permanent tooth slowly pushes up through the gums to replace the baby tooth. Eventually, a total of thirty-two permanent adult teeth form and erupt. The adult teeth are arranged in both the upper and lower jaws from the midline of the mouth as follows: central incisor, lateral incisor, canine (cuspid), first premolar (bicuspid), second premolar, first molar, second molar, and third molar. The first twenty-eight adult teeth are fully erupted by the age of eleven to thirteen with the third molars, known as wisdom teeth, erupting in the back of the jaw several years later in early adulthood. Sometimes the wisdom teeth become impacted when they grow and become wedged at an abnormal position in the jaws and fail to erupt. In some cases there is not enough room in the jaw to accommodate a third set of molars. In both cases the wisdom teeth are surgically removed, as they are not needed to properly chew food. Mastication, or chewing, is the main function of the teeth. The teeth are aligned in the jaws so that the peaks of one tooth align with the valleys of its counterpart on the other jaw. Every bite forces food into the interface of the teeth to be chopped, while lateral motion of the jaw is used to grind food in the premolars and molars. Tooth decay and cavities are important health concerns related to the teeth. The enamel that covers the crown in each tooth can be broken down by acids produced by bacteria that live in the mouth and assist in digestion of small bits of food. This process of enamel erosion by acids is called decay. To prevent decay, good oral hygiene, consisting of daily brushing and flossing, is necessary. Decay can eventually lead to cavities, also known as dental caries, where holes appear in the enamel and expose the dentin. Cavities require medical intervention to prevent their growth, usually resulting in the removal of the affected tissue and the filling of the cavity with a hard material to restore the strength and function of the tooth.

tongue expanded

The tongue is anchored to the floor of the mouth and slung at the rear from muscles attached to a spiky outgrowth at the base of the skull. It is a strong muscle that is covered by the lingual membrane and has special areas that detect the flavor of food. The tongue is made up of muscles covered by mucous membranes. These muscles are attached to the lower jaw and to the hyoid bone (a small, U-shaped bone, which lies deep in the muscles at the back of the tongue) above the larynx. There are very small nodules, called papillae, from the top surface of the tongue, which give it its rough texture. Between the papillae at the sides and base of the tongue are small, bulblike structures that are sensory organs, called taste buds, which enable us to enjoy the sensations of flavor and warn us when food is unfit to eat. The muscle fibers are heavily supplied with nerves, so it can manipulate food in the mouth and place it between the teeth for chewing-without being bitten in the process. The tongue also aids in the formation of sounds of speech and coordinates its movements to aid in swallowing.

tongue

The tongue is located on the inferior portion of the mouth just posterior and medial to the teeth. It is a small organ made up of several pairs of muscles covered in a thin, bumpy, skin-like layer. The outside of the tongue contains many rough papillae for gripping food as it is moved by the tongue's muscles. The taste buds on the surface of the tongue detect taste molecules in food and connect to nerves in the tongue to send taste information to the brain. The tongue also helps to push food toward the posterior part of the mouth for swallowing.

transverse colon

The transverse colon is the longest region of the colon and is located between the ascending colon and descending colon. It is named for the fact that it crosses the abdominal cavity transversely from the right side to left side just below the stomach. Much of the absorption and feces formation of the colon takes place in the transverse colon, making it a very important region of the digestive system. Anatomy The colon is the main portion of the large intestine and is divided into four regions: the ascending, transverse, descending, and sigmoid colon. The transverse colon begins at the hepatic flexure, a sharp bend at the superior end of the ascending colon just inferior to the liver on the right side of the abdominal cavity. At the hepatic flexure, the colon turns to the left and crosses to the left side of the abdominal cavity as the transverse colon. Upon reaching the left side, the colon again turns sharply toward the inferior direction at the splenic flexure before continuing on as the descending colon. The transverse colon is a hollow tube about 2.5 inches (6 cm) in diameter and around 18 inches (46 cm) in length. It curves along its length so that the middle of the transverse colon is inferior and posterior to the hepatic and splenic flexures at its ends. Many pouches, or haustra, are formed in the walls of the transverse colon by contraction of the smooth muscle of the intestinal walls. Histology The walls of the transverse colon are made of four tissue layers: the mucosa, submucosa, muscularis, and serosa. The mucosa is the innermost layer of epithelial tissue that borders the hollow lumen and is in contact with feces. Its name is derived from the mucus produced by goblet cells that line the surface of the mucosa to lubricate the transverse colon. Mucus protects the intestinal walls from friction as rough fecal matter passes along its length. The mucosa is also responsible for the absorption of nutrients and water from feces and the delivery of these substances to the bloodstream. Deep to the mucosa is the submucosa layer that supports the other three layers of the intestinal wall. Many blood vessels and nerves travel through the submucosa to provide nutrients, oxygen, and nerve impulses to the tissues of the mucosa, muscularis, and serosa. Nutrients and water extracted from feces are also returned to the body through the veins of the submucosa. The next layer, the muscularis, is made of several bands of smooth muscle that mix and move feces through the transverse colon. Lastly, the serosa is the outermost layer of the transverse colon that acts as the skin of the colon. Just like the skin of our arms protects what is beneath it, the serosa protects the transverse colon from damage caused by its surroundings. The serosa secretes serous fluid, a thin, slick secretion that prevents friction between the transverse colon and the surrounding tissues. Physiology The transverse colon mixes feces by contracting small regions of the intestinal wall in a process known as segmentation. As the feces are mixed, bacteria ferment the waste material to release vitamins and a few trace nutrients remaining in the waste. Water, nutrients, and vitamins are absorbed through the walls of the colon to be used by the tissues of the body. The colon then uses slow longitudinal waves of muscle contraction known as peristalsis to push the feces along its length.

muscularis externa of stomach

Three layers of smooth muscle Inner oblique layer allows stomach to churn, mix, move, and physically break down food

liver

Weighing in at around 3 pounds, the liver is the body's second largest organ; only the skin is larger and heavier. The liver performs many essential functions related to digestion, metabolism, immunity, and the storage of nutrients within the body. These functions make the liver a vital organ without which the tissues of the body would quickly die from lack of energy and nutrients. Fortunately, the liver has an incredible capacity for regeneration of dead or damaged tissues; it is capable of growing as quickly as a cancerous tumor to restore its normal size and function

monomers

building blocks of polymers sugars, fatty acids amino acids, nucleotide

alimentary canal

digestive tube that extends from the mouth to the anus gastrointestinal tract it's a barrier that allows for the selective movement of materials between them

both you and food

have energy stored in the bonds between those atoms

mucosal layer

inner most layer of the stomach epithelium lamina propria muscularis mucosae

poop

is actually mostly water (about 75%) but also indigestible fiber, protein, fats, salts, various cellular waste material, mucous, and the living and dead gut bacteria that also help us digest food, and stercoblin from the breakdown of hemoglobin

polymers

large compound formed from combinations of many monomers

submucosal layer of small intestine

loose areolar connnective tissue elasticity and blood supply

two main ways food is broken down

mechanically and chemically

stratified squamous epithelial tissue

mouth esophagus anus

hollow organs

mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus

secretion

n the course of a day, the digestive system secretes around 7 liters of fluids. These fluids include saliva, mucus, hydrochloric acid, enzymes, and bile. Saliva moistens dry food and contains salivary amylase, a digestive enzyme that begins the digestion of carbohydrates. Mucus serves as a protective barrier and lubricant inside of the GI tract. Hydrochloric acid helps to digest food chemically and protects the body by killing bacteria present in our food. Enzymes are like tiny biochemical machines that disassemble large macromolecules like proteins, carbohydrates, and lipids into their smaller components. Finally, bile is used to emulsify large masses of lipids into tiny globules for easy digestion.

enzymes

proteins that act as biological catalysts break down food

propulsion

swallowing voluntary peristalsis involuntary

accessory digestive organs

teeth, tongue, salivary glands, liver, gallbladder, pancreas enzymes secreted usually

caloric value

the amount of energy that nutrients or foods supply to the body

biological molecules

the chemical compounds that provide physical structure and that bring about movement, energy use, and other cellular functions macromolecules lipids, carbohydrates, proteins, and nucleic acids

Defication

the discharge of feces from the body.

digestive organs function

to break down and absorb the right nutrient at the right time

Why do we mechanically break down food?

to increase the surface area of that bite of food