Digestion

Endocrine

the body's "slow" chemical communication system; a set of glands that secrete hormones into the bloodstream

Adaptations of small intestine

1. Villi and microvilli increase surface area 2. Long length increases surface area 3. Rich blood supply helps carry absorbed molecules away to maintain a steep diffusion gradient (same for lacteal supply with lipids) 4. Thin wall means short diffusion distance for absorption (villi single layer epithelial cells) 5. Mitochondria present to supply ATP for active transport 6. High permeability as abundant channel proteins

Maltase

A digestive enzyme that breaks maltose into glucose.

Pancreatic lipase

A triglyceride molecule cannot be directly absorbed across the intestinal mucosa. Rather, it must first be digested into a 2-monoglyceride and two free fatty acids. The enzyme that performs this hydrolysis is pancreatic lipase, which is delivered into the lumen of the gut as a constituent of pancreatic juice.

Dipeptidases

A type of protease which hydrolyse the bond between the two amino acids of a dipeptide. Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.

Exopeptidases

A type of protease which hydrolyses the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. They progressively release dipeptides and single amino acids.

Pyloric sphincter

About every half minutes the stomach tries to empty its mostly digested content into the small intestine via the pyloric sphincter, a muscle that act as a security valve between the stomach and the small intestine. Mixing waves forces the sphincter open and let a small amount of chyme goes through (3mL). The rest of the chyme is pushed back into the stomach body to continue its digestion. The sphincter remains in an open or relaxed state two-thirds of the time, permitting small quantities of food to pass into the duodenum. When the duodenum begins to fill, pressure increases and causes the pyloric sphincter to contract and close. Muscular contractions (peristaltic waves) in the duodenum then push food deeper into the intestine.

Digestion in the oesophagus

After it is swallowed, the chewed food (now called a bolus) moves down the esophagus. The esophagus acts as a connection between the mouth and the stomach, but no digestion occurs here. The oesophagus passes from the pharynx, through the thoracic cavity and diaphragm, into the abdominal cavity where it joins to the stomach. The oesophagus is generally collapsed, but when swallowing occurs, it opens and receives the bolus. Peristalsis in the digestive tract begins in the oesophagus.

Digestion in the large intestine

Although the glands of the large intestine secrete mucus, they do not secrete digestive enzymes. Therefore, chemical digestion in the large intestine occurs exclusively because of bacteria in the lumen of the colon. Through the process of saccharolytic fermentation, bacteria break down some of the remaining carbohydrates. This results in the discharge of hydrogen, carbon dioxide, and methane gases that create flatus (gas) in the colon.

Amylase

Amylase (technically alpha-amylase) is the enzyme that hydrolyses starch to maltose (a glucose-glucose disaccharide)

Emulsification of lipids

As derivatives of cholesterol, bile acids have both hydrophilic and hydrophobic domains (i.e. they are amphipathic). On exposure to a large aggregate of triglyceride, the hydrophobic portions of bile acids intercalate into the lipid, with the hydrophilic domains remaining at the surface. Such coating with bile acids aids in breakdown of large aggregates or droplets into smaller and smaller droplets, providing a greater area of hydrolysis.

Salmonella transmission

Because salmonellosis is transmitted through contaminated food and undercooked meats, animals such as chickens, pigs, turkeys, and cows are the most common reservoirs. These can carry the bacteria without symptoms. Environmental sources harvest Salmonella bacteria as well. The bacteria can survive for several weeks in water and several years in soil if conditions of temperature, humidity, and pH are favourable, allowing for replication. Infected humans can also serve as reservoirs transmitting salmonellosis from human to human.

Bile

Bile contains bile acids, which are critical for digestion and absorption of fats and fat-soluble vitamins in the small intestine. Many waste products, including bilirubin, are eliminated from the body by secretion into bile and elimination in feces. Bile is produced continuously by the liver (liver bile), and stored and concentrated in the gallbladder.Bile acids are derivatives of cholesterol synthesized in the hepatocyte, therefore hepatic synthesis of bile acids accounts for the majority of cholesterol breakdown in the body.Venous blood from the ileum goes straight into the portal vein, and hence through the sinusoids of the liver. Hepatocytes extract bile acids very efficiently from sinusoidal blood, and little escapes the healthy liver into systemic circulation. Bile acids are then transported across the hepatocytes to be recycled. The net effect of this is that each bile salt molecule is reused about 20 times, often two or three times during a single digestive phase.

Digestion in the mouth

Both mechanical and chemical digestion occur in the mouth. Teeth grind and break up food so as to increase surface area for enzyme hydrolysis, while an enzyme in saliva called amylase begins to break down carbohydrates into smaller carbohydrates. Breakdown in the mouth is minimal, and insufficient for absorption due to the short time food remains in the mouth, mainly for taste.

Digestion

Breakdown of large, insoluble organic compounds into small soluble molecules so that they can be absorbed.

Enzymic secretions in the duodenum

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. Chyme is mixed with bile and pancreatic juice 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 for hydrolysis by lipase. Pancreatic juice contains many enzymes to break carbohydrates, lipids, proteins and nucleic acids into their monomer subunits. These secretions are thoroughly mixed with the chyme by contractions of the duodenum muscularis layer until all of the digestible material is chemically digested.

Lower oesophageal sphincter

Bundle of muscles at the low end of the oesophagus, where it meets the stomach. When the LES is closed, it prevents acid and stomach contents from traveling backwards from the stomach, preventing heartburn. When the sphincter relaxes, the bolus can pass into the stomach. The LES muscles are not under voluntary control.

Upper oesophageal sphincter

Bundle of skeletal muscles at the top of the oesophagus. The muscles of the UES are under conscious control, used when breathing, eating, belching, and vomiting. They keep food and secretions from going down the windpipe.

Effects of cyanide

Cyanide is considered to be toxic because it binds to cytochrome c oxidase ie. the fourth complex in the electron transport chain. It attaches to the iron within this protein complex and inhibits the normal activity of the complex system. It binds tightly so that it cannot transport any electrons to oxygen. This blocks the further passage of electrons throughout the chain, halting ATP production. Cyanide stops/inhibits respiration Resp creates atp -> No atp for active transport of Na+ at Na+/K+ pump on the basolateral side High concentration Na+ in epithelial cell Co-transport of glucose-sodium from lumen does not take place so no absorption of glucose from food

E.coli

E. coli is a facultative (aerobic and anaerobic growth) gram-negative, rod shaped bacteria. When E. coli locates in human large intestines, it can help digestion processes, food breakdown and absorption, and vitamin K production. Different strains of E. coli can be found in different type of animals, so we can determine the source (from human or from other animals) of the stools by examining which strain of E. coli is present in the stools. Although E. coli in human large intestine can assist with waste processing and food absorption, some strains of E. coli can cause severe infections in many animals, such as humans, sheep, horses, dogs, etc. Urinary tract infection, for example, can be caused by ascending infections of urethra. E. coli's ability to induce injury in humans is a result of its ability to produce numerous virulence factors, most notably Shiga toxin (Stx), which is one of the most potent toxins known to man, Stx acts by inhibiting protein synthesis in endothelial and other cells that line the interior surface of blood vessels. In addition to Shiga toxin, E. coli O157:H7 produces numerous other putative virulence factors, including proteins which aid in the attachment and colonization of the bacteria in the intestinal wall and which can lyse red blood cells and liberate iron to help support E. coli metabolism.

Structure of villus

Each villus has a central core composed of one artery and one vein, a strand of muscle, a centrally located lymphatic capillary (lacteal), and connective tissue that adds support to the structures. The blood vessels are thought to transport proteins and carbohydrates absorbed by the cells of the villi, while the lymphatic capillary removes droplets of emulsified fat. The muscle strand allows the villi to contract and expand; it is believed that these contractions empty the contents of the lacteal into larger lymphatic vessels. Covering the core of a villus is the surface mucous-membrane layer. This is mainly composed of two cell types: tall, narrow, columnar cells that absorb the substances passed into the blood and lymphatic vessels; and goblet cells, rounded at the end, that secrete mucus into the intestinal cavity. Crypts (of Lieberkuhn) are moat-like invaginations of the epithelium around the villi, and are lined largely with younger epithelial cells which are involved primarily in secretion. Importantly, toward the base of the crypts are stem cells, which continually divide and provide the source of all the epithelial cells in the crypts and on the villi.

Proteases

Enzymes that continue the breakdown of polypeptides in the small intestine. Digestion of proteins is initiated by pepsin in the stomach, but the bulk of protein digestion is due to the pancreatic proteases. Several proteases are synthesized in the pancreas and secreted into the lumen of the small intestine. The two major pancreatic proteases are trypsin and chymotrypsin, which are synthesized and packaged into secretory vesicles as the inactive proenzymes trypsinogen and chymotrypsinogen. Trypsin and chymotrypsin digest proteins into peptides and peptides into smaller peptides, but they cannot digest proteins and peptides to single amino acids.

Digestion of fats

Fats are digested by lipases that hydrolyze the glycerol fatty acid bonds. Of particular importance in fat digestion and absorption are the bile salts, which emulsify the fats to allow for their solution as micelles in the chyme, and increase the surface area for the pancreatic lipases to operate. Large aggregates of dietary triglyceride, which are virtually insoluble in an aqueous environment, must be broken down physically and held in suspension - emulsification. Triglyceride molecules must be enzymatically digested to yield monoglyceride and fatty acids, both of which can efficiently diffuse or be transported into the enterocyte The key players in these two transformations are bile acids and pancreatic lipase, both of which are mixed with chyme and act in the lumen of the small intestine.

E.coli transmission

Foods associated with E. coli have included raw or undercooked meats (e.g., ground beef), deli meats, unpasteurised fruit juices and dairy products, and produce. Species such as cows and pigs which do not carry the receptors specific to the toxin can carry the bacteria without symptoms. You can be infected with the E. coli bacteria if you don't use a high temperature to cook your beef, or if you don't cook it long enough. When you eat undercooked beef, the germs go into your stomach and intestines. Waterborne transmission occurs by swimming in contaminated lakes or drinking untreated water.

Digestion in the stomach

From the bolus, alcohol is absorbed here but no food substances, normally the stomach empties in 2-6 hours when it leaves as chyme. Food is broken down chemically, by gastric juice, and mechanically, by contraction of the three layers of smooth muscle in the muscular externa layer. The mucosa is full of gastric glands and pits, and there is a prominent layer of smooth muscle - the muscularis mucosa. The contraction of this muscle helps to expel the contents of the gastric glands. The muscularis externa layer has three layers of muscle, unlike other parts of the digestive tract, which have 2 layers of muscles, going in every direction: a longitudinal, circular and oblique muscle. That way in addition to moving food down the tract it can also churn food and break it down more easily.

Stomach mucosa

Gastric pits are indentations in the stomach which denote entrances to tubular shaped gastric glands which release gastric juice into the stomach lumen. Goblet cells - secrete mucus to form a protective layer around the stomach lining from the other acidic secretion and prevent the stomach from digesting itself. The parietal cells secrete the hydrochloric acid (called HCI, it has a pH 2) which aids the digestion by breaking down anything from nutrients to bacteria or medication and activates the protein-digesting enzyme called pepsin. The chief cells, at the bottom of the funnel, secrete pepsinogen, the inactive version of pepsin which will be activated thanks to the HCI. Pepsin is capital in the breakdown of protein into smaller molecule.

Hormone control of digestive juices

In the duodenum, digestive secretions from the liver, pancreas, and gallbladder play an important role in digesting chyme during the intestinal phase. In order to neutralize the acidic chyme, a hormone called secretin stimulates the pancreas to produce alkaline bicarbonate solution and deliver it to the duodenum. Secretin acts in tandem with another hormone called cholecystokinin (CCK). Not only does CCK stimulate the pancreas to produce the requisite pancreatic juices, it also stimulates the gallbladder to release bile into the duodenum.

Peristalsis

Involuntary waves of muscle contraction that keep food moving along in one direction through the digestive system. During peristalsis, the longitudinal muscles within the small intestine wall contract, and then the circular muscles contract, pushing the food down the tract. This coordinated contraction of smooth muscle keeps food moving on its one-way path through your digestive system.

Layers of the duodenum

Mucosa 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. Mucous glands secrete mucus into the lumen to lubricate the intestinal wall and protect it from friction and acidic chyme. 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. Contractions of the muscularis mix chyme and propel it through the duodenum toward the rest of the small 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.

Three salivary glands in the mouth

Parotid 25% produce a serous, watery secretion Sublingual 5% secrete a saliva that is predominantly mucous in character Submandibular 70% produce a mixed serous and mucous secretion In serous secretions, the main type of protein secreted is alpha-amylase, an enzyme that breaks down starch into maltose and glucose, whereas in mucous secretions the main protein secreted is mucin, which acts as a lubricant. The mucus in saliva is extremely effective in binding masticated food into a slippery bolus that (usually) slides easily through the esophagus without inflicting damage to the mucosa. Saliva also coats the oral cavity and esophagus, and food basically never directly touches the epithelial cells of those tissues.

Salmonella

Salmonella are a genus of bacteria under which there are multiple species. All members of the genus Salmonella are also classified as being enterobacteria, this means that they live in the intestines of animals. Salmonella is not very infective, therefore large numbers of the gram-negative bacteria are required to cause infection. The most common symptoms are diarrhea, fever, abdominal cramps, and vomiting. The bacteria can attach to the endothelial cells lining the intestines where they produce toxins and attack the intestinal cells.

Salmonella prevention

Salmonella is mainly found in faecal material so failure to wash hands after defamation can lead to transmission through contaminated food. Washing hands with warm, soapy water for twenty seconds before and after handling food as well as washing surfaces often can further prevent spread. Cooking utensils, cutting boards, and counter tops should be washed after each item being cooked. Dish rags and sponges harvest bacteria. Salmonella bacterial is killed at 60.C but the toxin is not destroyed so contaminated meat must not be eaten (not denatured). Keeping meats, poultry, and seafood separate when grocery shopping is also recommended, so fluid from uncooked meat cannot drip onto cooked or other food. Flies cannot also cross contaminate.

Gastrin

Secreted by G-cells in the stomach Stimulated by chewing food or when anticipation of eating stimulates nerves within the brain medulla which signal to the stomach and stimulate the release of gastrin. Stimulates the gastric glands to produce gastric juices

Secretin

Secreted by the duodenum Stimulated by the movement of food into the duodenum Stimulates the pancreas to produce alkaline fluid (sodium hydrogen carbonate) to neutralise acidic secretions

Cholecystokinin

Secreted by the duodenum Stimulated by the movement of food into the duodenum Stimulates the pancreas to secrete pancreatic juice containing enzymes Stimulates gallbladder to release bile

Hydrolysis of lipids

Shortly after a meal, lipase is present within the small intestine in rather huge quantities after release from the pancreas. But lipase can act only on the surface of triglyeride droplets, for a given volume of lipid, the smaller the droplet size, the greater the surface area, which means more lipase molecules can get to work. Hydrolysis of triglyceride into monoglyceride and free fatty acids is accomplished predominantly by pancreatic lipase. The activity of this enzyme is to clip the fatty acids at positions 1 and 3 of the triglyceride, leaving two free fatty acids and a 2-monoglyceride.

Movement through small intestine

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. Segmentation, which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents. By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates absorption.

E.coli symptoms

Symptoms may vary, but usually include diarrhea (usually bloody), vomiting, and severe stomach cramps.

Digestion in the duodenum

The chyme is slowly transported into the small intestine, where most chemical digestion and absorption takes place. It's function is mainly regulation of the stomach emptying, inhibition of the stomach's acid liquid and final breakdown of food via enzymes. Additionally, the gallbladder and the pancreas are connected to the duodenum and send their substance (bile and enzymes respectively) to further aid the digestion of nutrients present in the chyme.

Diaphragm

The diaphragm increases abdominal pressure to help the body get rid of vomit, urine, and feces. It also places pressure on the esophagus to prevent acid reflux.

Cephalic phase

The earliest phase of digestion, in which the brain thinks about and prepares the digestive organs for the consumption of food. About 30% of total acid secretion occurs before food enters the stomach. These sensory and mental inputs converge on the hypothalamus, which relays signals to the medulla oblongata. Vagus nerve fibers from the medulla stimulate the parasympathetic nervous system of the stomach which, in turn, stimulates gastric secretion (via parietal and G cells). Chewing and swallowing also stimulates G-cells to secrete gastrin, which in term stimulates the stomach gastric glands to release gastric acid allowing the breakdown of food.

Brush border

The enzymes responsible for this terminal stage of digestion are not free in the intestinal lumen, but rather, tethered as integral membrane proteins in the plasma membrane of the enterocyte. The apical plasma membrane housing these enzymes is composed of numerous microvilli which extend from the cell and constitute the "brush border". Hence, the enzymes embedded in those microvilli are referred to as brush border enzymes.Villi increase the internal surface area of the intestinal walls. This increased surface area allows for more intestinal wall area to be available for absorption. An increased absorptive area is useful because digested nutrients (including sugars and amino acids) pass into the villi, which is semi-permeable, through diffusion, which is effective only at short distances.

Large intestine

The large intestine is the terminal part of the alimentary canal. The primary function of this organ is to finish absorption of nutrients and water, synthesize certain vitamins, form feces, and eliminate feces from the body. The large intestine runs from the appendix to the anus. The large intestine is subdivided into four main regions: the cecum, the colon, the rectum, and the anus. The ileocecal valve, located at the opening between the ileum and the large intestine, controls the flow of chyme from the small intestine to the large intestine. Few enzyme-secreting cells are found in the wall of the large intestine, and there are no circular folds or villi. Other than in the anal canal, the mucosa of the colon is simple columnar epithelium made mostly of enterocytes (absorptive cells) and goblet cells. In addition, the wall of the large intestine has far more intestinal glands, which contain a vast population of enterocytes and goblet cells. These goblet cells secrete mucus that eases the movement of feces and protects the intestine from the effects of the acids and gases produced by enteric bacteria. The enterocytes absorb water and salts as well as vitamins produced by your intestinal bacteria.

Pancreatic juice

The pancreas is located behind the stomach in the upper left abdomen. The pancreas contains exocrine glands that produce enzymes important to digestion. These enzymes include trypsin and chymotrypsin to digest proteins; amylase for the digestion of carbohydrates; and lipase to break down fats. When food enters the stomach, these pancreatic juices are released into a system of ducts that culminate in the main pancreatic duct. The pancreatic duct joins the common bile duct to form the ampulla of Vater which is located at the first portion of the small intestine, called the duodenum. The common bile duct originates in the liver and the gallbladder and produces another important digestive juice called bile. The pancreatic juices and bile that are released into the duodenum, help the body to digest fats, carbohydrates, and proteins. Bicarbonate makes the pancreatic secretions alkaline in nature. This flow of alkaline fluid into the small intestine helps to neutralize the acidic chyme that comes from the stomach.

Movement in the large intestine

The residue of chyme that enters the large intestine contains few nutrients except water, which is reabsorbed as the residue lingers in the large intestine, typically for 12 to 24 hours. In the large intestine, mechanical digestion begins when chyme moves from the ileum into the cecum, an activity regulated by the ileocecal sphincter. Right after you eat, peristalsis in the ileum forces chyme into the cecum. When the cecum is distended with chyme, contractions of the ileocecal sphincter strengthen. Once chyme enters the cecum, colon movements begin. The presence of food residues in the colon stimulates a slow-moving haustral contraction. This type of movement involves sluggish segmentation, primarily in the transverse and descending colons. These movements also mix the food residue, which helps the large intestine absorb water. The second type of movement is peristalsis, which, in the large intestine, is slower than in the more proximal portions of the alimentary canal. The third type is a mass movement. These strong waves start midway through the transverse colon and quickly force the contents toward the rectum.

Absorption in the large intestine

The small intestine absorbs about 90 percent of the water you ingest (either as liquid or within solid food). The large intestine absorbs most of the remaining water, a process that converts the liquid chyme residue into semisolid feces ("stool"). Feces is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old epithelial cells from the GI mucosa, inorganic salts, and enough water to let it pass smoothly out of the body. The entry of feces into the rectum activates the defecation reflex.

Exocrine

gland that secretes its products through excretory ducts to the surface of an organ or tissue or into a vessel