Chapter 22

What is a micelle?

After chemical digestion by lipase is complete, the bile salts and digested lipids stay together in structures known as micelles (my-SELZ).

Which tongue papillae do not contain taste buds?

Filiform papillae play no role in taste, and are instead covered with stratified squamous keratinized epithelium.

What are haustra?

Their constant tension bunches the colon into pockets referred to as haustra (HAW-struh; singular, haustrum)

What do the sensory receptors in the digestive organs respond to?

These receptors respond to stimuli such as stretch, the concentration and pH of the fluids the organs contain, and the presence of certain products of digestion. When these receptors are stimulated, their axons contact neurons in the ANS or ENS, which then initiate the appropriate response such as triggering peristalsis or secretion from certain glands.

What is emulsification?

When bile is released into the duodenum, the bile salts coat the lipids and physically break them apart into smaller pieces, a process known as emulsification (ee-mul′-sih-fih-KAY-shun). Although emulsification is mechanical digestion, it is necessary for the chemical digestion and absorption of lipids,

What does the bolus turn into and where does this take place?

churn the incoming bolus into chyme

What passes through rapidly from stomach to duodenum?

chyme

What is the ENS?

enteric nervous system, or ENS the ENS is a self-contained branch of the autonomic nervous system that extends from the esophagus to the anus (the terminal portion of the large intestine)

Through what structure does bile leave the gall bladder?

gallbladder to release bile into the cystic duct (SIS-tik).

What are rugae?

the interior of the stomach contains folds called rugae (ROO-ghee). Rugae allow the stomach to expand considerably.

What is the mucosa in the large intestine rich with?

mucosa is rich with goblet cells that secrete protective and lubricating mucus.

What is the longest portion of the alimentary canal?

small intestine, or small bowel, is the longest portion of the alimentary canal

What makes up the portal triad?

we find three structures collectively referred to as a portal triad: (1) a branch of the hepatic artery called a hepatic arteriole, (2) a branch of the portal vein called a portal venule, and (3) a small bile duct that carries bile.

What are the two components of a tooth?

A tooth consists of two components: the crown, which is the visible portion above the gum line, and the root, which is embedded within the alveolus

What is the role of the gastroesophageal sphincter?

At the esophagus' inferior end is another sphincter, the gastroesophageal sphincter (gas′-troh-ee-sah-fah-JEE-al), also known as the lower esophageal sphincter, that regulates the passage of the bolus into the stomach. This sphincter also prevents the contents of the stomach from re-entering the esophagus.

What stimulates the release of bile?

Bile release is stimulated by the hormone CCK,

What are the functions of bile?

Bile serves two critical functions: (1) It is required for the digestion and absorption of lipids; and (2) it is the mechanism by which the liver excretes wastes and other substances that the kidneys cannot excrete.

What is the hardest substance in the body?

Enamel

What is the opening of the alimentary canal called?

Lumen

What factors contribute to it?

Multiple factors contribute to the development of GERD, including increased acid secretion, obesity, caffeine consumption, a hiatal hernia (abnormal enlargement of the esophageal hiatus in the diaphragm), and medications such as anti-inflammatory drugs. Another factor that contributes to a significant proportion of cases is infection with the bacterium Helicobacter pylori. This bacterium creates a state of chronic inflammation, which stimulates acid production.

What do CCK and secretin do?

One hormonal mediator of pancreatic secretion is cholecystokinin (kohl′-eh-sih-stoh-KY-nin), or CCK. CCK is produced by duodenal enteroendocrine cells in response to the presence of lipids and partially digested proteins in the duodenum. It acts on acinar cells to trigger the secretion of digestive enzymes and other proteins. Another stimulatory hormone is secretin, which is released by duodenal cells in response to acid and lipids in the duodenum. Secretin primarily triggers duct cells to secrete bicarbonate ions

Which branch of the ANS stimulates the digestive system?

Parasympathetic System

List the three salivary glands.

Parotid glands. The parotid glands (puh-ROT-id; para- = "beside," ot- = "ear") are large glands located over the masseter muscle just anterior to the ear. These glands secrete saliva through the parotid duct, which passes over the masseter muscle and pierces the buccinator muscle to open into the oral cavity near the second molar. The parotid glands secrete 25-30% of total saliva. Submandibular glands. The smaller submandibular glands are located just medial to the inferior portion of the body of the mandible. They secrete saliva through the submandibular duct, which empties into the floor of the oral cavity. In spite of their smaller size, the submandibular glands are very active, and secrete 65-70% of total saliva. Sublingual glands. As implied by their name, the sublingual glands (sub-LING-gwuhl) are situated inferior to the tongue. The sublingual glands secrete saliva through several small sublingual ducts that empty into the oral cavity just under the tongue. These are the smallest salivary glands and secrete only about 5% of total saliva.

What are the functions of saliva?

Saliva performs several functions in the oral cavity, including moistening, lubricating, and cleansing the oral mucosa. In addition, its lysozyme and IgA deter the growth of pathogenic bacteria in the oral cavity. Saliva also functions in (1) mechanical digestion, by moistening and helping to mix ingested food into a bolus so it can be swallowed, and (2) chemical digestion, by the actions of salivary amylase. Finally, many food molecules dissolve in the water of saliva, and these molecules then stimulate taste receptors on the tongue.

Which ones produce the majority of saliva?

Submandibular glands.

Which organs does food actually pass through?

The alimentary canal is a continuous tube through which food passes directly. It consists of the oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine.

What two structures form the common bile duct?

The common bile duct joins the main pancreatic duct near the duodenum to form the hepatopancreatic ampulla (heh-PAH-toh-payn′-kree-at-ik am-POOL-uh). The ampulla is surrounded by a ring of smooth muscle called the hepatopancreatic sphincter, which controls the emptying of bile and pancreatic fluids into the duodenum. Recall that the contents of the hepatopancreatic ampulla then empty into the duodenum at the major duodenal papilla.

What is the function of the gall bladder?

The gallbladder stores bile, concentrates it (removing water), and releases it when stimulated.

What happens in GERD?

The gastroesophageal sphincter normally remains closed except during swallowing. When this mechanism fails, acidic chyme from the stomach regurgitates into the esophagus. If this occurs on a chronic basis, it is called gastroesophageal reflux disease, or GERD, and may lead to pain, difficulty swallowing, vocal cord damage, respiratory problems, and even esophageal cancer.

What takes nutrient rich blood to the liver for processing?

The hepatic portal vein then delivers the blood to the liver for processing. Blood drains from the liver by a set of hepatic veins, which in turn deliver blood to the inferior vena cava.

What is the main digestive function of the liver?

The liver's main digestive function is to produce bile

How does the muscularis externa change throughout the length of the esophagus?

The muscularis externa of the esophagus consists of two layers of muscle, but it differs from the remainder of the alimentary canal. Rather than being only smooth muscle, its superior third is composed of skeletal muscle, its middle third is a mixture of skeletal and smooth muscle (see Figure 22.8a), and its inferior third is composed of smooth muscle. At the junction of the pharynx and the esophagus, the muscularis externa is modified into a sphincter called the upper esophageal sphincter, which controls the passage of the bolus into the esophagus.

Where is the ileocecal valve located and what is its job?

Ileum ileocecal valve (ill′-ee-oh-SEE-kuhl) controls the movement of materials from the ileum into the cecum. This sphincter also prevents materials in the large intestine from flowing backward into the ileum. This function is key because, as we discuss shortly, the large intestine houses a great number of bacteria that could cause illness if they entered the ileum.

List and briefly describe the three types of teeth?

Incisors. The incisors are the central teeth that are broad and flat with a narrow crown. They are specialized for cutting off pieces of food. The middle two incisors are the central incisors, and those to either side are the lateral incisors. Canines. The canines (KAY-nynz), also known as cuspids, are on either side of the incisors. Their pointed crowns are specialized for ripping and tearing. Molars. The teeth posterior and lateral to the canines are the premolars and the molars.Both types of molars have broad crowns with rounded projections called cusps that are specialized for grinding.

Briefly describe lipid absorption. Use figure 22.33 as a guide.

Just as lipid digestion is more complicated than that of carbohydrates and proteins, lipid absorption is also more complex. Again this is due to lipids' nonpolar nature. Lipids face several water-based barriers that deter them from passing into the cytosol of the enterocytes, including the mucus lining of the small intestine and the polar phosphate heads of the enterocytes' plasma membranes. For this reason, lipids require assistance to approach the enterocyte membranes and eventually cross them. Let's walk through the steps of lipid absorption (Figure 22.33): Micelles escort lipids to the enterocyte plasma membrane. Digested triglycerides and other lipids such as cholesterol and fat-soluble vitamins (such as vitamin D) remain associated with bile salts in micelles. Notice that these bile salts have their polar portions on the outside, facing the water-based environment, and their nonpolar portions on the inside, facing the digested lipids. The polar outer portion of the micelle allows it to approach the polar mucus layer and phosphate heads of the plasma membrane. Lipids diffuse through the phospholipid bilayer and enter the cytosol. Once the lipids in the micelles are at the plasma membrane, the attraction of the fatty acid tails of the phospholipid bilayer draws the lipids into the cytosol. This generally occurs by simple diffusion, although some lipids use facilitated diffusion by carrier-mediated transport. Note that the leftover bile salts in the micelles remain in the lumen of the small intestine and are reabsorbed by active transport mechanisms in the ileum. Lipids are reassembled into triglycerides and packaged into chylomicrons. Within the enterocyte, enzymes catalyze reactions that turn the free fatty acids and monoglycerides back into triglycerides. The triglycerides are packaged with cholesterol, other dietary lipids, phospholipids, and lipid-binding protein molecules known as apoproteins into structures called chylomicrons (ky-loh-MY-krahnz). Chylomicrons are similar to micelles in that their nonpolar lipids face the inside and their polar portions face the outside. This allows chylomicrons to travel with the polar water molecules in blood. Chylomicrons are released into the interstitial fluid by exocytosis and then enter a lacteal. The newly formed chylomicrons are packaged into vesicles by the Golgi apparatus and released by exocytosis from the basal enterocyte membrane into the interstitial fluid in the core of the villus. Chylomicrons vary in size but are generally large—in fact, too large to enter the capillaries in the villus. However, lacteals, which are also found in the core of the villus, have valves in their walls that allow large substances to enter and exit. Chylomicrons therefore enter a lacteal, where they join the lymph.

What is the key factor that determines the rate of gastric emptying?

key factor that determines the rate of gastric emptying is the amount and composition of chyme in the duodenum.

What is the role of the mucus?

mucus coats the epithelium and protects it and the underlying tissues from ingested food and chemicals secreted by digestive organs.

What are the four main layers of the canal?

regions of the alimentary canal have four named tissue layers (Figure 22.2): mucosa, submucosa, muscularis externa, and either the serosa or adventitia.

What four main processes occur in the small intestine?

Four main processes occur in the small intestine: Secretion Digestion Absorption Propulsion

What is the basic unit of the liver?

The basic unit of the liver is the liver lobule

What happens to nutrients as a result of digestion?

All nutrients go through the same mechanisms of mechanical digestion—mastication in the mouth, churning in the stomach, and segmentation in the small intestine. However, chemical digestion varies for each nutrient because digestive enzymes are specific for a single type of bond in a single type of nutrient. Most digestive enzymes catalyze hydrolysis reactions—those that use a water molecule to break a bond between two molecules, as illustrated here (this example shows a peptide bond between two amino acids): This is one reason why so much water is secreted with fluids like gastric and pancreatic juices—the water molecules are key components of the reactions that chemically break down food molecules. These hydrolysis reactions would occur without digestive enzymes, but they would occur far too slowly and we would be unable to extract nutrients from the food we eat. Our digestive enzymes speed up the reactions, a process known as enzymatic hydrolysis. Once nutrients are digested to their component molecules, they must enter the body. Remember that the mucosa of the alimentary canal is actually an external membrane, so even though it might seem that the food is "inside" the body, it technically is not until it is absorbed. Nutrients and other molecules must be absorbed across the epithelial lining of the alimentary canal and enter the bloodstream before they can be delivered to body cells.

What are the regions of the stomach?

As you saw in Figure 22.1, the stomach sits primarily in the left upper quadrant just inferior to the diaphragm. The stomach's convex left side is known as its greater curvature, and its concave right side is its lesser curvature. Figure 22.10 shows that the stomach has five anatomical regions: Cardia. The region where the esophagus empties into the stomach is the cardia (KAR-dee-ah). The cardia receives the bolus when the gastroesophageal sphincter relaxes. Fundus. The dome-shaped top of the stomach is its fundus. Body. The largest portion of the stomach is its body. Pyloric antrum. The inferior portion of the stomach is the pyloric antrum (py-LOR-ik AN-trum). Pylorus. The terminal portion of the stomach is the pylorus (pylor- = "gatekeeper"), which abuts the first portion of the small intestine, the duodenum. Like the cardia, the pylorus contains a sphincter that controls the flow of ingested food. In the pylorus, a thick ring of smooth muscle called the pyloric sphincter regulates the flow of materials between the stomach and the small intestine.

What digestive processes occur in the oral cavity?

Four digestive processes take place here: ingestion, secretion, chemical and mechanical digestion, and propulsion. Although the oral cavity is technically part of the alimentary canal, it houses two accessory organs: the teeth and the tongue. In addition, three pairs of accessory organs, the salivary glands, are located in and around the oral cavity. Together, these organs turn ingested food into a moist, chewed mass called a bolus (BOW-lus; "ball"). The posterior oral cavity, pharynx, and esophagus then deliver the bolus to the stomach through the process of swallowing. In this module, we look at the structure and function of the oral cavity and its associated accessory organs, the pharynx, and the esophagus

What is the purpose of mastication?

Mastication aids digestion by increasing the overall surface area of the food, giving digestive enzymes more places to catalyze the reactions of chemical digestion.

What is the role of the serosa?

Serosa or adventitia. The outer connective tissue layer is the serosa in the organs within the peritoneal cavity and the adventitia in organs outside the cavity. The serosa, also called the visceral peritoneum, is composed of simple squamous epithelial tissue and loose connective tissue, whereas the adventitia is composed of dense irregular connective tissue. Both structures support digestive organs and anchor them to surrounding structures.

List the organs included in each and give a brief description of the digestive process occurring in each one. (See figures 22.19 and 22.27 for help)

The alimentary canal is a continuous tube through which food passes directly. It consists of the oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine. The accessory organs are not part of the alimentary canal but assist in digestion in some way. They are located around the alimentary canal and include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

What is in the pancreatic juice? Why is it rich in bicarbonate ions?

The collective secretions of the pancreatic acinar and duct cells are known as pancreatic juice. It consists of water and multiple digestive enzymes and other proteins. In addition, the duct cells secrete bicarbonate ions, a base, which make pancreatic juice alkaline. This helps to neutralize the acidic chyme that enters the duodenum from the stomach and protects the duodenum from damage by the acid. The digestive enzymes, secreted by acinar cells, are crucial in chemical digestion and catalyze the reactions that digest carbohydrates, proteins, lipids, and nucleic acids.

What are the two main types (categories) of organs in the digestive system?

The digestive system consists of two types of organs: the organs of the alimentary canal, also known as the gastrointestinal (GI) tract or digestive tract; and the accessory organs.

What are the seven basic digestive functions/processes?

The digestive system must perform the following six basic processes to carry out these functions: Ingestion. Food and water are brought into the digestive system by ingestion, which occurs via the mouth under normal conditions. Secretion. Digestive organs contain both endocrine and exocrine glands that secrete a variety of substances—such as mucus, enzymes, acid, and hormones—to aid other digestive processes. Propulsion. Ingested food and liquids pass from one digestive organ to the next by the process of propulsion. Propulsion is accomplished largely by rhythmic contractions of the smooth muscle of the alimentary canal called peristalsis (pehr-uh-STAL-sis) and is aided by mucus secreted by multiple organs. Digestion. Food breakdown occurs by the process of digestion. There are two kinds of digestion. In mechanical digestion, digestive organs physically break food down into smaller pieces via processes such as chewing and mixing food by movements by the muscles of the alimentary canal. In chemical digestion, enzymes secreted by digestive organs catalyze reactions that break the chemical bonds within food particles until only small molecules remain. Absorption. Once food particles are mechanically and chemically digested, nutrients move through the wall of the alimentary canal into blood or lymphatic vessels by a process called absorption. Water, electrolytes, and vitamins are also absorbed into the blood in the same manner. Defecation. Certain ingested materials are not digestible or usable by the body. Such materials continue their transit through the alimentary canal until they exit the body as feces through defecation.Defecation also provides the body with a way to eliminate certain metabolic wastes. Note that defecation is simply a specialized form of propulsion.

What is the digestive role of the esophagus?

The esophagus (eh-SOF-ah-gus) is a muscular tube about 25 cm (10 in.) long found posterior to the trachea. It transmits the bolus from the pharynx to the stomach. Like the pharynx, the esophageal mucosa is lined with stratified squamous nonkeratinized epithelium (Figure 22.8). The mucosa and submucosa contain esophageal glands (eh-sah′-fah-JEE-uhl) that secrete mucus to lubricate the bolus as it passes through the esophagus.

What three layers make up the mucosa?

The innermost mucosa consists of three components: First is a layer of epithelium facing the lumen, followed by a thin layer of loose connective tissue called the lamina propria, and finally two thin layers of smooth muscle together known as the muscularis mucosae.The epithelium from the stomach to the end of the large intestine is simple columnar epithelium with copious goblet cells that secrete an alkaline mucus. This mucus coats the epithelium and protects it and the underlying tissues from ingested food and chemicals secreted by digestive organs. The mucosa also houses regenerative epithelial cells that have a high rate of mitosis. These cells allow the mucosa to replace epithelial cells as they are damaged or sloughed off in the alimentary canal. The lamina propria houses blood and lymphatic vessels, glands, and mucosa-associated lymphatic tissue (MALT; see Chapter 20). The two layers of the muscularis mucosae are arranged in different directions—the inner layer is circular and the outer layer is longitudinal. As discussed later, this arrangement allows motility as the two layers contract alternately.

List the subdivisions of the large intestine.

The large intestine is made up of three segments: the cecum, the colon, and the rectum. As you read in the last module, the first portion of the large intestine is the cecum, a blind pouch (cecum- = "blind") that is intraperitoneal and located in the right lower quadrant of the abdomen. The cecum features a smaller blind-ended pouch extended from its posteroinferior end, called the vermiform appendix (VER-muh-form uh-PEN-diks; "wormlike appendage"), which is generally shortened to simply appendix (see Figure 22.18a), The appendix was long considered a vestigial organ, or one left over from evolution that served no function. However, it is now understood that it houses multiple lymphatic nodules and plays a role in the immune system. Despite its small size, an inflamed appendix can lead to big problems, The next and longest portion of the large intestine is the colon (KOH-lun), which is divided into four portions: Ascending colon. The retroperitoneal ascending colon travels superiorly along the right side of the abdomen from the right lower quadrant to the right upper quadrant. When it reaches the liver, it makes a sharp left-hand turn at a junction called the hepatic flexure, also known as the right colic flexure. Transverse colon. At the hepatic flexure, the ascending colon becomes the intraperitoneal transverse colon, so named because it passes transversely across the superior abdominal cavity. At the spleen, it takes a sharp turn inferiorly at a junction called the splenic flexure, also known as the left colic flexure. Descending colon. The splenic flexure gives rise to the retroperitoneal descending colon, which passes along the left side of the abdominal cavity. Sigmoid colon. In the left lower quadrant, the descending colon becomes the S-shaped sigmoid colon (sigmoid = "S-shaped"), which is intraperitoneal and passes toward the sacrum. After the sigmoid colon, the large intestine continues as the rectum (REK-tum). This portion of the large intestine runs anterior to the sacrum and is retroperitoneal. The walls of the rectum feature horizontal folds called rectal valves, which allow the passage of flatus (gas) without risking the simultaneous passage of feces. The rectum ends at the anal canal, the last portion of the large intestine (Figure 22.17b). Recall from the muscular system chapter that the anal canal passes through the levator ani muscle in the floor of the pelvic cavity (see Chapter 9). The walls of the anal canal feature longitudinal grooves called anal columns.Between the anal columns are anal sinuses, which secrete mucus when feces pass through the anal canal during defecation. The terminal portion of the anal canal has two sphincters. The first is the involuntary internal anal sphincter, which is simply the thickened circular layer of the muscularis externa. The internal anal sphincter is supplied by parasympathetic motor neurons. The second is the voluntary external anal sphincter, which is composed of skeletal muscle. As this sphincter is voluntary, it is innervated by somatic motor neurons controlled by the cerebral cortex.

What is the role of the large intestine?

The large intestine receives material from the small intestine that was not digested or absorbed. You might think this means the small intestine does the "important" work and the large intestine is simply a passageway for the leftovers, known as feces , or fecal matter. However, the large intestine is very active in absorbing water and electrolytes. Absorption in the large intestine is critical for maintaining fluid, electrolyte, and acid-base homeostasis. When this absorption is disrupted, as with bacterial or viral gut infections, dehydration as well as electrolyte and acid-base disturbances may result.

What structure surrounds the small intestine?

The large intestine, or large bowel, runs along the border of the abdominal cavity, surrounding the small intestine and other abdominal organs like a frame

What are the three types of folds in the small intestine? What is their purpose?

The largest type of folds, which are visible with the naked eye as ridges in the wall, are called circular folds (or plicae circulares; Figure 22.15a). Circular folds involve both the mucosa and submucosa of the small intestine. These folds not only increase surface area but also slow down the transit of chyme through the small intestine, which gives the nutrients more time to be digested, and the small intestine cells, called enterocytes (EN-tehr-oh-sytz), more time to absorb nutrients. The smaller two types of folds are not visible to the naked eye. The mucosa folds into projections called villi (VILL-ahy; singular, villus; Figure 22.15b). Notice that each villus consists of a layer of enterocytes and occasional mucus-secreting goblet cells surrounding a central core of blood capillaries and a lymphatic vessel called a lacteal. Between villi, the mucosa indents to form intestinal crypts, which house glands that also contain enteroendocrine cells (which secrete a variety of hormones). The smallest folds, the microvilli, are found in the plasma membrane of the enterocytes (Figure 22.15c). Each enterocyte has as many as 3000 microvilli, which gives the cell the appearance of a bristle brush, or brush border, on microscopic examination. Associated with the brush border are numerous digestive enzymes produced and secreted by enterocytes, such as sucrase, maltase, and lactase, which catalyze the reactions that break down disaccharides, and peptidases, which catalyze reactions that break down peptides.

How is the absorption of water-soluble and fat-soluble vitamins different?

There are two types of vitamins: water-soluble vitamins, which are polar molecules; and fat-soluble vitamins, which are lipid-based and mostly nonpolar. Most of the water-soluble vitamins are absorbed in the small intestine by diffusing through the enterocytes' plasma membranes. One important exception is vitamin B12, which must bind to intrinsic factor—a molecule produced by the parietal cells of the stomach—to be absorbed in the ileum. A shortage of intrinsic factor can impair vitamin B12 absorption, as discussed in A&P in the Real World. Fat-soluble vitamins are packaged into micelles with fats and other lipids and are absorbed with them. The fat-soluble vitamins include A, D, E, and K.

What are other functions of the liver?

The liver performs a host of other functions, including the following (note that some of these functions are only peripherally related to the digestive system, so they are not discussed in detail here): Nutrient metabolism. The liver processes nutrients obtained from the diet. Carbohydrates and proteins absorbed from the alimentary canal are delivered to the liver by the hepatic portal vein, and lipids by the hepatic arteries. Within the liver, some of these nutrients are stored for later use, modified into another form, or used to synthesize other molecules. For example, some of the glucose taken in by the liver is stored in the form of glycogen, and many of the dietary amino acids are used to synthesize plasma proteins such as albumin and clotting proteins. We discuss the liver's role in multiple metabolic processes in the metabolism chapter (see Chapter 23). Detoxification. The liver detoxifies substances produced by the body, such as the previously mentioned bilirubin. In addition, the liver processes substances that we eat or drink, some of which are toxins harmful to the body (such as alcohol). These substances are generally delivered to the liver first via the hepatic portal vein, where the liver converts them into less harmful materials that can be excreted in bile or in urine. The liver also metabolizes many drugs, such as antibiotics. Individuals with impaired liver function metabolize drugs more slowly than those with normal liver function, and the drugs remain in their systems much longer. Does the liver ever need help with detoxification? For more information, see A&P in the Real World: Do We Really Need to "Detox"? Excretion. Recall that the liver directly excretes bilirubin in bile. Several other substances the liver processes are excreted in bile, particularly drugs such as certain antibiotics. The liver also modifies substances so that they can be excreted by the kidneys.

What are the functions of the digestive system?

The most fundamental function of the digestive system is to take food into the body and break it down into its component nutrients so that they can be used by body cells. But the digestive system also plays a critical role in fluid, electrolyte, and acid-base homeostasis by taking in water and electrolytes via the diet and delivering them to the blood. Other functions of the digestive system include ingesting vitamins and minerals, producing hormones, and excreting metabolic wastes.

Where is the lacteal located?

The mucosa folds into projections called villi (VILL-ahy; singular, villus; Figure 22.15b). Notice that each villus consists of a layer of enterocytes and occasional mucus-secreting goblet cells surrounding a central core of blood capillaries and a lymphatic vessel called a lacteal.

How is the mucosa in the large intestine different from that in the small intestine?

The mucosa of the large intestine lacks villi and its cells lack microvilli. These structural adaptations reflect the fact that nutrient absorption is not the large intestine's primary function. Like most of the alimentary canal, its mucosa is rich with goblet cells that secrete protective and lubricating mucus.

What makes up the muscularis externa?

The muscularis externa is a thick muscular layer composed of smooth muscle in most of the alimentary canal. We generally find two layers of smooth muscle that are arranged in the same manner as in the muscularis mucosae, with inner circular and outer longitudinal layers. The motility of the muscularis externa is regulated by groups of nerves of the enteric nervous system called the myenteric plexus (my- = "muscle," enter- = "intestine"), or Auerbach's plexus.

What is the primary function of the pharynx?

The primary function of the pharynx is propulsion in the form of swallowing, during which the bolus passes through the pharynx and into the esophagus. Recall that the pharynx is surrounded by three pairs of skeletal muscles: the upper, middle, and lower pharyngeal constrictor muscles (see Chapter 9). These muscles contract sequentially during swallowing and propel the bolus inferiorly.

Where does the chemical digestion of lipids begin? Where does the majority take place?

The process of lipid digestion proceeds as follows (Figure 22.31): Lipids are broken apart by stomach churning and broken down in reactions catalyzed by gastric lipase. Lipid digestion begins in the stomach with the help of the enzyme gastric lipase. Gastric lipase catalyzes the reactions that remove one fatty acid from triglycerides, leaving some free fatty acids and diglycerides. About 15% of total dietary fats are digested this way. Lipids enter the small intestine and are emulsified by bile salts. The undigested and partially digested triglycerides enter the small intestine, where they interact with bile salts. Bile salts coat the lipids and physically break them apart into smaller pieces. This is purely a physical process—no bonds are broken during emulsification. Pancreatic lipase catalyzes reactions that digest the lipids into free fatty acids and monoglycerides. The pancreas releases an enzyme, pancreatic lipase, that catalyzes lipid breakdown. In this process, triglycerides are digested into monoglycerides and free fatty acids. Bile salts remain associated with the digested lipids to form micelles. After chemical digestion by lipase is complete, the bile salts and digested lipids stay together in structures known as micelles (my-SELZ). If the digested lipids did not remain associated with bile salts, they would re-form into large globules and absorption would not be possible.

How are the proximal and distal parts of the large intestine different in function?

The proximal large intestine is the primary site of water and electrolyte absorption and bacterial activity, and exhibits two main types of motility. The distal large intestine performs a small amount of absorption, primarily of water, but its main role is to store fecal material until it is ready to be expelled during defecation. For this reason, the distal large intestine is much less motile than the proximal large intestine. When mass movements force fecal material into the normally empty rectum, it initiates the parasympathetic-mediated defecation reflex

What are the two types of movement in the large intestine?

The proximal large intestine is the primary site of water and electrolyte absorption and bacterial activity, and exhibits two main types of motility. The first is a type of segmentation, or churning, similar to what we saw in the small intestine, in which the circular muscle of each haustrum contracts repeatedly. This swirls the material around in the haustrum, which aids in water and electrolyte absorption. These contractions are controlled primarily by local neurons of the ENS and are triggered by stretch. The other type of motility in the proximal large intestine is a propulsive motion known as a mass movement, or mass peristalsis.During a mass movement, multiple haustra undergo peristalsis, which propels their contents toward the distal large intestine. Mass movements occur three to four times per day, and appear to be triggered by food consumption, which initiates reflexes controlled by the ENS.

What are the three division of the small intestine?

The small intestine consists of three divisions: Duodenum Jejunum Ileum

What are the three digestive functions of the large intestine?

The tasks of the large intestine also include secretion (primarily in the form of mucus), propulsion, and defecation. In addition, it houses numerous bacteria that perform important functions such as synthesizing vitamins. The upcoming sections examine the gross and microscopic anatomy of the large intestine, the bacteria that inhabit it, and its basic functions.

What is a chylomicron?

The triglycerides are packaged with cholesterol, other dietary lipids, phospholipids, and lipid-binding protein molecules known as apoproteins into structures called chylomicrons (ky-loh-MY-krahnz). Chylomicrons are similar to micelles in that their nonpolar lipids face the inside and their polar portions face the outside. This allows chylomicrons to travel with the polar water molecules in blood.

What purpose do the gut flora serve?

These bacteria, the normal flora or gut flora, consist of about 500 different bacterial species that coexist with humans in a symbiotic (mutually beneficial) relationship. Humans provide the bacteria with the environment they need to survive, and the bacteria perform a number of useful functions for humans, including the following: Produce vitamins. Bacteria produce vitamins such as vitamin K, which is necessary for blood clotting. Metabolize undigested materials. Bacteria metabolize carbohydrates such as soluble fibers that the small intestine is unable to digest, converting them into fatty acids and other molecules the body can absorb and use. This also aids in the absorption of certain vitamins and electrolytes. A somewhat unfortunate byproduct of this metabolism is the production of gas within the intestine that is released as flatus. Deter the growth of harmful bacteria. The normal flora prevent the growth of pathogenic, or disease-causing, microorganisms by competing for nutrients and producing chemicals that kill certain harmful bacterial species. Stimulate the immune system. During infancy, the normal flora induce immune tolerance to their own antigens. At the same time, they stimulate the development of mucosa-associated lymphatic tissue (MALT) and the production of antibodies to pathogens. This creates a favorable environment for the normal flora while also protecting the host from pathogenic bacteria.

List the four cell types found in the gastric glands and their functions.

We find four main types of cells in or near gastric glands, each of which secretes a different product. From superficial to deep, these cell types are as shown in Figure 22.11b and c. Mucous neck cells. The cells located near the top, or "neck," of the gland are called mucous neck cells. As their name implies, these cells secrete mucus much like goblet cells. However, goblet cells secrete alkaline mucus, whereas mucous neck cells secrete acidic mucus. This prevents their mucus from neutralizing the acid produced by other cells known as parietal cells. Parietal cells. The next cells are the parietal cells, which secrete the hydrochloric acid (HCl) that is responsible for the acidic pH of gastric juice. Acid is an important component of gastric juice because it is required to activate a precursor enzyme called pepsinogen (pep-SIN-oh-jen) and also because it destroys most disease-causing organisms we ingest. In addition, parietal cells produce the chemical intrinsic factor, which is required for intestinal absorption of vitamin B12, found in various foods. Chief cells. Next down are the chief cells, which secrete the inactive precursor enzyme pepsinogen. When pepsinogen encounters an acidic pH, it becomes the active enzyme pepsin, which begins protein digestion in the stomach. Enteroendocrine cells. Located at the very bottom of the gland are the enteroendocrine cells, which secrete several hormones that influence digestion. For example, enteroendocrine cells called G cells secrete the hormone gastrin, which stimulates acid secretions from the parietal cells. As you can see in Figure 22.11c, enteroendocrine cells are closest to the blood vessels in the underlying submucosa, so their hormones have ready access to the blood.

What is intrinsic factor required for?

intrinsic factor, which is required for intestinal absorption of vitamin B12, found in various foods.

Which division is most active in chemical digestion and absorption?

jejunum

How are the mucosa and muscularis externa in the stomach modified?

muscularis externa of the alimentary canal usually has two layers of smooth muscle: an inner circular layer and an outer longitudinal layer. Figure 22.10 shows you how this differs in the stomach—there is an additional inner layer of smooth muscle in the stomach's body with its fibers oriented obliquely. This oblique layer of smooth muscle allows the stomach to perform churning, a motion that pummels the food into a liquid called chyme (KYME). the mucosa of the stomach is heavily indented to form deep structures called gastric pits.Between the gastric pits we find columnar cells and numerous goblet cells, both of which secrete a thick mucus that lines and protects the cells of the stomach from its own secretions. Conditions that decrease the amount of mucus secreted by these cells can lead to a gastric ulcer (also called a peptic ulcer), in which acid and enzymes eat away at the mucosa and expose the underlying tissues. Ulcers are often associated with the presence of certain bacteria and/or excessive production of acid.

How is the muscularis externa unique in the large intestine?

muscularis externa of the large intestine is unique in that its longitudinal layer is not continuous throughout most of its length. Instead, this layer is gathered into three bands or ribbons of muscle called taeniae coli (TEE-nee-ee KOHL-ahy; taeniae = "ribbons"; see Figure 22.17a).

What are the functions of peristalsis and segmentation in the small intestine?

peristalsis in the small intestine is accomplished by alternating contractions of the longitudinal and circular layers of smooth muscle in the muscularis externa (see Figure 22.16a). Its primary function is to propel chyme toward the ileum and ultimately through the ileocecal valve to the cecum. segmentation, also known as intestinal churning, involves contractions of only the circular layer of smooth muscle, which produces a squeezing motion (see Figure 22.16b).The primary functions of segmentation are mechanical digestion and mixing the chyme with intestinal and pancreatic enzymes as well as bile. The vagus nerve appears to regulate both peristalsis and segmentation.

Where does the majority of absorption of water and electrolytes take place?

the alimentary canal absorbs large quantities of water, electrolytes (such as sodium, potassium, and calcium ions), and vitamins. Much of this absorption occurs in the small intestine, but a significant quantity occurs in the large intestine as well. The next subsections examine the absorption of each of these substances.

What are: enamel, dentin, pulp cavity, root canal?

the enamel—covering a soft, inner gelatinous substance called the pulp.Enamel is composed almost fully of secreted calcium hydroxyapatite crystals with only a small amount of organic material. This makes enamel the hardest substance in the body, allowing it to endure the forces that accompany chewing. The cells that secrete enamel deteriorate after the tooth erupts, so the body cannot repair damaged enamel. The inner layer of mineralized tissue in both the root and crown is dentin, which is composed of about 70% calcium hydroxyapatite crystals. This makes dentin the second hardest material in the body. However, dentin also has some degree of elasticity, which is critical for preventing the overlying enamel from fracturing when chewing hard substances. Unlike enamel, dentin is formed throughout life by cells called odontoblasts that line its inner surface. he final component of a tooth is the inner pulp, which is composed of loose connective tissue and housed within the central pulp cavity. Pulp contains blood vessels and nerves that supply the other tissues of the tooth with nutrients and innervation. The pulp cavity extends into the root via the thin root canal

Where are the microvilli located?

the microvilli, are found in the plasma membrane of the enterocytes

What is the role of the pyloric sphincter?

the pylorus contains a sphincter that controls the flow of ingested food. In the pylorus, a thick ring of smooth muscle called the pyloric sphincter regulates the flow of materials between the stomach and the small intestine.

What are the three digestive functions of the stomach?

the stomach performs three primary functions: secretion, propulsion, and digestion. Note that absorption of food molecules is not a main role of the stomach, but small amounts of chemicals such as alcohol may be absorbed here.

What is food turned into?

these organs turn ingested food into a moist, chewed mass called a bolus (BOW-lus; "ball").