Chapter 15: The Digestion and Absorption of Food

The absorption of iron is typical of that of most trace metals in two major respects:

(1) Cellular storage proteins and plasma carrier proteins are involved; and (2) the control of absorption, rather than urinary excretion, is the major mechanism for the homeostatic control of the body's content of the trace metal.

what keeps the stomach from digesting itself?

(1) The surface of the mucosa is lined with cells that secrete slightly alkaline mucus that forms a thin layer over the luminal surface. Both the protein content of mucus and its alkalinity neutralize H+ in the immediate area of the epithelium. In this way, mucus forms a chemical barrier between the highly acidic contents of the lumen and the cell surface. (2) The tight junctions between the epithelial cells lining the stomach restrict the diffusion of H+ into the underlying tissues. (3) Damaged epithelial cells are replaced every few days by new cells arising by the division of cells within the gastric pits.

Gastrointestinal reflexes are initiated by a relatively small number of luminal stimuli:

(1) distension of the wall by the volume of the luminal contents; (2) chyme osmolarity (total solute concentra- tion); (3) chyme acidity; and (4) chyme concentrations of specific digestion products like monosaccharides, fatty acids, peptides, and amino acids. These stimuli act on mechanoreceptors, osmo- receptors, and chemoreceptors located in the wall of the tract and trigger reflexes that influence the effectors—the muscle layers in the wall of the tract and the exocrine glands that secrete sub- stances into its lumen.

emulsification of fat requires

(1) mechanical disruption of the large lipid droplets into smaller droplets and (2) an emulsifying agent, which acts to prevent the smaller droplets from reaggregating back into large droplets. The mechanical disruption is provided by the motility of the GI tract, occurring in the lower portion of the stomach and in the small intestine, which grinds and mixes the luminal contents. Phospholipids in food, along with phospholipids and bile salts secreted in the bile, pro- vide the emulsifying agents.

gastrointestinal (GI) tract

(or alimentary canal) consisting of the mouth, pharynx, esophagus, stomach, small intestine, and large intestine accessory organs are not part of the tract but secrete substances into it via connect- ing ducts. adult gastrointestinal tract is a tube approximately 9 m (30 feet) in length, running through the body from mouth to anus lumen of the tract is continuous with the external environment, which means that its contents are technically out- side the body. This fact is relevant to understanding some of the tract's properties. For example, the large intestine is colonized by billions of bacteria, most of which are harmless and even beneficial in this location. if the same bacteria enter the internal environment, as may happen, for example, if a portion of the large intestine is perforated, they may cause a severe infection also has a variety of immune functions, allowing it to produce antibodies and fight infectious microorganisms that are not destroyed by the acidity of the stomach.

small intestine secretion

1500 mL of fluid is secreted by the cells of the small intestine from the blood into the lumen each day One of the causes of water movement (secretion) into the lumen is that the intestinal epithelium at the base of the villi secretes a number of mineral ions—notably, Na+, Cl-, and HCO3- —into the lumen, and water follows by osmosis. These secretions, along with mucus, lubricate the surface of the intestinal tract and help protect the epithelial cells from excessive damage by the digestive enzymes in the lumen. Some damage to these cells still occurs, however, and the intestinal epithelium has one of the highest cell- renewal rates of any tissue in the body. water movement into the lumen also occurs when the chyme entering the small intestine from the stomach is hypertonic because of a high concentration of solutes in the meal and because digestion breaks down large molecules into many more small molecules. This hypertonicity causes the osmotic movement of water from the isotonic plasma into the intestinal lumen.

endoscope

A device used to diagnose gastric and duodenal ulcers is the endoscope uses either fiber-optic or video technology to directly visualize the gastric and duodenal mucosa in a procedure called endoscopy the endoscopist can apply local treatments and take samples of tissue (biopsy) during upper endoscopy Similar devices can be used to visualize the colon (flexible sigmoidoscopy or colonoscopy

protein intake

A healthy adult requires a minimum of about 40 to 50 g of protein per day to supply essential amino acids and replace the nitrogen contained in amino acids that are metabolized to urea. A typical American diet contains about 60 to 90 g of protein per day. This represents about one-sixth of the average daily caloric intake. a large amount of pro- tein, in the form of enzymes and mucus, is secreted into the GI tract or enters it via the death and disintegration of epithelial cells. Regardless of the source, most of the protein in the lumen is broken down into dipeptides, tripeptides, and amino acids, all of which are absorbed by the small intestine.

upper esophageal sphincter

A ring of skeletal muscle surrounds the esophagus just below the pharynx

fat-soluble vitamins

A, D, E, and K follow the pathway for fat absorption They are solubilized in micelles; thus, any interference with the secretion of bile or the action of bile salts in the intestine decreases the absorp- tion of the fat-soluble vitamins, a pathological condition called malabsorption

migrating myoelectrical complex (MMC)

After most of a meal has been absorbed, the segmenting contractions cease and are replaced by a pattern of peristaltic activity known as the migrating myoelectrical complex (MMC). Beginning in the lower portion of the stomach, repeated waves of peristaltic activity travel about 2 feet along the small intestine and then die out. The next MMC starts slightly farther down the small intestine so that peristaltic activity slowly migrates down the small intestine, taking about 2 h to reach the large intestine. By the time the MMC reaches the end of the ileum, new waves are beginning in the stomach, and the process repeats. The MMC moves any undigested material still remaining in the small intestine into the large intestine and also prevents bacteria from remaining in the small intestine long enough to grow and multiply excessively. In diseases characterized by an aberrant MMC, bacterial overgrowth in the small intestine can become a major problem. Upon the arrival of a meal in the stomach, the MMC rapidly ceases in the intestine and is replaced by segmentation.

micelles in the digestive system

Although emulsification speeds up digestion, absorption of the water-insoluble products of the lipase reaction would still be very slow if it were not for a second action of the bile salts, the formation of micelles, similar in structure to emulsion droplets but much smaller—4 to 7 nm in diameter. Micelles consist of bile salts, fatty acids, monoglycerides, and phospholipids all clustered together with the polar ends of each molecule oriented toward the micelle's surface and the nonpolar portions forming the micelle's core

fatty acid --> triglyceride through epithelium

Although fatty acids and monoglycerides enter epithelial cells from the intestinal lumen, triglycerides are released on the other side of the cell into the interstitial fluid. In other words, during their passage through the epithelial cells, fatty acids and monoglycerides are resynthesized into triglycerides. This occurs in the smooth endoplasmic reticulum, where the enzymes for triglyceride synthesis are located. This process decreases the concentration of cytosolic free fatty acids and monoglycerides and thereby maintains a diffusion gradient for these molecules into the cell from the intestinal lumen. The resynthesized fat aggregates into small droplets coated with amphipathic proteins that perform an emulsifying function similar to that of bile salts.

How do micelles increase absorption?

Although fatty acids and monoglycerides have an extremely low solubility in water, a few molecules do exist in solution and are free to diffuse across the lipid portion of the apical plasma membranes of the epithelial cells lining the small intestine. Micelles, containing the products of fat digestion, are in equilibrium with the small concentration of fat-digestion products that are free in solution. Thus, micelles are continuously breaking down and reforming. As the luminal concentrations of free lipids decrease because of their diffusion into epithelial cells, more lipids are released into the free phase from micelles as they begin to break down

receptive relaxation

An empty stomach has a volume of only about 50 mL, and the diameter of its lumen is only slightly larger than that of the small intestine. When a meal is swallowed, however, the smooth muscles in the fundus and body relax before the arrival of food, allowing the stomach's volume to increase to as much as 1.5 L with little increase in pressure. called RECEPTIVE RELAXATION mediated by the parasympathetic nerves to the stomach's enteric nerve plexuses, with coordination provided by afferent input from the stomach via the vagus nerve and by the swallowing center in the brain. Nitric oxide and serotonin released by enteric neurons mediate this relaxation.

small intestine is divided into three segments:

An initial short segment, the duodenum is followed by the jejunum then by the longest segment, the ileum. Normally, most of the chyme entering from the stomach is fully digested and absorbed in the first quarter of the small intestine in the duodenum and part of the jejunum. Therefore, the small intestine has a very large reserve for the absorption of most nutrients; removal of portions of the small intestine as a treatment for disease does not necessarily result in nutritional deficiencies, depending on which part of the intestine is removed. Moreover, the remaining tissue can often increase its digestive and absorptive capacities to compensate in part for the removal of the diseased part.

rhythm of segmentation

As with the slow waves in the stomach, this intestinal basic electrical rhythm produces oscillations in the smooth muscle membrane potential. If threshold is reached, action potentials are triggered that increase muscle contraction. The frequency of segmentation is set by the frequency of the intestinal basic electrical rhythm; unlike the stomach, however, which normally has a single rhythm (three per minute), the intestinal rhythm varies along the length of the intestine, each successive region having a slightly lower frequency than the one above. For example, segmentation in the duodenum occurs at a frequency of about 12 contractions/min, whereas in the last portion of the ileum the rate is only 9 contractions/min. Segmentation produces, there- fore, a slow migration of the intestinal contents toward the large intestine because more chyme is forced toward the large intestine, on average, than in the opposite direction.

pyloric sphincter.

At the junction between the antrum and the small intestine is this ring of contractile smooth muscle

ulcers

At times, these protective mechanisms against the acidity of the stomach can prove inadequate, and erosion (ulcers) of the gastric surface can develop. Ulcers can occur not only in the stomach but also in the lower part of the esophagus and in the duodenum. , duodenal ulcers are about 10 times more frequent than gastric ulcers, affecting about 10% of the U.S. population. Damage to blood vessels in the tissues underlying the ulcer may cause bleeding into the gastrointestinal lumen On occasion, the ulcer may penetrate the entire wall, resulting in leakage of the luminal contents into the abdominal cavity.

intrinsic factor

B12 (cya- nocobalamin), is a very large, charged molecule. To be absorbed, vitamin B12 must first bind to a protein known as intrinsic factor secreted by the acid-secreting cells in the stomach. Intrinsic factor with bound vitamin B12 then binds to specific sites on the epithelial cells in the lower portion of the ileum, where vitamin B12 is absorbed by endocytosis.

pernicious anemia

B12 is required for erythrocyte formation, and deficiencies result in pernicious anemia. This form of anemia may occur when the stomach either has been removed (for example, to treat ulcers or gastric cancer) or fails to secrete intrinsic factor (often due to autoimmune destruction of acid-producing cells). Because the absorption of vitamin B12 occurs in the lower part of the ileum, removal or dysfunction of this segment due to disease can also result in pernicious anemia. Although healthy individuals can absorb oral vita- min B12, it is not very effective in patients with pernicious anemia because of the absence of intrinsic factor. Therefore, the treatment of pernicious anemia usually requires injections of vitamin B12.

large intestine

Because most substances are absorbed in the small intestine, only small quan- tities of water, ions, and undigested material pass on to the large intestine. The large intestine temporarily stores the undigested material (some of which is metabolized by bacteria) and concentrates it by absorbing ions and water.

what happens when a lactose intolerant person consumes dairy?

Because the absorption of water requires prior absorption of solute to provide an osmotic gradient, the unabsorbed lactose in persons with lactose intolerance prevents some of the water from being absorbed. This lactose-containing fluid is passed on to the large intestine, where bacteria digest the lactose. They then metabolize the released monosaccharides, producing large quantities of gas (which distends the colon, producing pain) and short-chain fatty acids, which cause fluid movement into the lumen of the large intestine, producing diarrhea. The response to ingestion of milk or dairy products by adults whose lactase levels have diminished varies from mild discomfort to severely dehydrating diarrhea, according to the volume of milk and dairy products ingested and the amount of lactase present in the intestine. The person can avoid these symptoms by either drinking milk in which the lactose has been predigested with added lactase enzyme or taking pills containing lactase along with the milk.

gall stone lodged in common bile duct

Because the duct from the pancreas joins the common bile duct just before it enters the duodenum, a gallstone that becomes lodged at this point prevents or limits both bile and pancreatic secretions from entering the intestine. This results in failure to both neutralize acid and adequately digest most organic nutrients, not just fat. The end results are severe nutritional deficiencies. The buildup of very high pressure in a blocked common bile duct is transmitted back to the liver and interferes with the further secretion of bile. As a result, bilirubin, which is normally secreted into the bile by uptake from the blood in the liver, accumulates in the blood and diffuses into tissues, producing a yellowish coloration of the skin and eyes known as jaundice.

gallbladder

Between meals, secreted bile is stored here a small sac underneath the liver that branches from the common hepatic duct. The gallbladder concentrates the organic molecules in bile by absorbing some ions and water. Dur- ing a meal, the smooth muscles in the gallbladder wall are stimulated to contract, causing a concentrated bile solution to be injected into the duodenum via the common bile duct

common hepatic duct

Bile is secreted by the liver into small ducts that join to form the common hepatic duct. the gallbladder is a small sac underneath the liver that branches from the common hepatic duct. During a meal, the smooth muscles in the gallbladder wall are stimulated to contract, causing a concentrated bile solution to be injected into the duodenum via the common bile duct, an extension fo the common hepatic duct

defecation

Contractions of the rectum, the final segment of the large intestine, and relaxation of associated sphincter muscles expel the feces in a process called defecation.

Valsalva maneuver

Defecation is normally assisted by a deep breath, followed by closure of the glottis and contraction of the abdominal and thoracic muscles, producing an increase in abdominal pressure that is transmitted to the contents of the large intestine and rectum. This maneuver (termed the Valsalva maneuver) also causes an increase in intrathoracic pressure, which leads to a transient increase in blood pressure followed by a decrease in pressure as the venous return to the heart is decreased. The cardiovascular changes resulting from excessive strain during defecation may in rare instances precipitate a stroke or heart attack, especially in constipated elderly people with cardiovascular disease.

rate of acid secretion during meal

During a meal, the rate of acid secretion increases markedly as stimuli arising from the cephalic, gastric, and intestinal phases alter the release of the four chemical messengers During the cephalic phase, increased activity of efferent parasympathetic neural input to the stomach's enteric nervous system results in the release of ACh from the plexus neurons, gastrin from the gastrin-releasing G cells, and histamine from ECL cells Once food has reached the stomach, the gastric phase stimuli—distension from the volume of ingested material and the presence of peptides and amino acids released by the digestion of luminal proteins—produce a further increase in acid secre- tion. These stimuli use some of the same neural pathways used during the cephalic phase. Neurons in the mucosa of the stomach respond to these luminal stimuli and send action potentials to the cells of the enteric nervous system, which in turn can relay signals to the gastrin-releasing cells, histamine-releasing cells, and parietal cells. In addition, peptides and amino acids can act directly on the gastrin-releasing enteroendocrine cells to promote gastrin secretion.

results of vomiting

Excessive vomiting can lead to large losses of the water and ions that normally would be absorbed in the small intestine. This can result in severe dehydration, upset the body's ion balance, and produce circulatory problems due to a decrease in plasma volume. The loss of acid from vomiting results in metabolic alkalosis A variety of centrally acting anti- emetic drugs can suppress vomiting.

neurons in GI?

Finally, a critical component in the control of gastrointestinal functions is the role of the central nervous system. The CNS receives information from the GI tract (afferent input) and has a vital influence on GI function (efferent output).

GLUT

Fructose enters the epithelial cells by facilitated diffusion via a glucose transporter (GLUT) in the small intestine These monosac- charides then leave the epithelial cells and enter the interstitial fluid by way of facilitated diffusion via various GLUT proteins in the basolateral membranes of the epithelial cells. From there, the monosaccharides diffuse into the blood through capillary pores. Most ingested carbohydrates are digested and absorbed within the first 20% of the small intestine.

trypsin and chymotrypsin

Further breakdown of protein is completed in the small intestine by these enzymes the major proteases secreted by the pancreas. These peptide fragments can be absorbed if they are small enough or are further digested to free amino acids by At least 20 different peptidases that are located on the apical membrane of the epithelial cells, with various specificities for the peptide bonds they attack. two specific are carboxy- and aminopeptidases

stimuli in the early portion of the small intestine influence acid secretion by the stomach

High acidity in the duodenum triggers reflexes that inhibit gastric acid secretion. This inhibition is beneficial because the digestive activity of enzymes and bile salts in the small intestine is strongly inhibited by acidic solutions. This reflex limits gastric acid production when the H+ concentration in the duodenum increases due to the entry of chyme from the stomach. Acid, distension, hypertonic solutions, solutions contain- ing amino acids, and fatty acids in the small intestine reflexively inhibit gastric acid secretion. The extent to which acid secretion is inhibited during the intestinal phase varies, depending upon the amounts of these substances in the intestine; the net result is the same, however—balancing the secretory activity of the stomach with the digestive and absorptive capacities of the small intestine. The inhibition of gastric acid secretion during the intestinal phase is mediated by short and long neural reflexes and by hormones that inhibit acid secretion by influencing the four signals that directly control acid secretion: ACh, gastrin, histamine, and somatostatin.

not all neural reflexes start within the tract

Hunger, the sight or smell of food, and the emotional state of an individual can have significant effects on the gastrointestinal tract, effects that are mediated by the CNS via autonomic neurons.

result of gall stone

If a gallstone is small, it may pass through the common bile duct into the intestine with no complications. A larger stone may become lodged in the opening of the gallbladder, causing painful contractile spasms of the smooth muscle. A more serious complication arises when a gallstone lodges in the common bile duct, thereby preventing bile from entering the intestine. A large decrease in bile can decrease fat digestion and absorption. Furthermore, impaired absorption of the fat-soluble vitamins A, D, K, and E can occur, leading to, for example, clotting problems (vitamin K deficiency) and Ca2+ malabsorption (due to vitamin D deficiency). The fat that is not absorbed enters the large intestine and eventually appears in the feces (a condition known as steator- rhea). bacteria in the large intestine convert some of this fat into fatty acid derivatives that alter ion and water move- ments, leading to a net flow of fluid into the large intestine. The results are diarrhea and fluid and nutrient loss.

liver and cholesterol

In addition to synthesizing bile salts from cholesterol, the liver also secretes cholesterol extracted from the blood into the bile. Bile secretion, followed by excretion of cholesterol in the feces, is one of the mechanisms for maintaining cholesterol homeostasis in the blood and is also the process by which some cholesterol-lowering drugs work. Dietary fiber also sequesters bile and thereby lowers plasma cholesterol. occurs because the sequestered bile salts escape the enterohepatic circulation. Therefore, the liver must either synthesize new cholesterol, or remove it from the blood, or both to produce more bile salts. Cholesterol is insoluble in water, and its solubility in bile is achieved by its incorporation into micelles (whereas in blood, cholesterol is incorporated into lipoproteins).

small intestine mobility

In contrast to the peristaltic waves that sweep over the stomach, the most common motion in the small intestine during digestion of a meal is a stationary contraction and relaxation of intestinal segments, with little apparent net movement toward the large intestine Each contracting segment is only a few centimeters long, and the contraction lasts a few seconds. The chyme in the lumen of a contracting segment is forced both up and down the intestine. This rhythmic contraction and relaxation= segmentation

how do various substances get through the small intestine

In the small intestine, monosaccharides and amino acids are absorbed by specific transporter-mediated processes in the plasma membranes of the intestinal epithelial cells, whereas fatty acids enter these cells primarily by diffusion. Most mineral ions are actively absorbed by transporters, and water diffuses passively down osmotic gradients.

H+/K+- ATPase protein movt to membrane

Increased acid secretion results from the transfer of H+/K+- ATPase proteins from the membranes of intracellular vesicles to the plasma membrane by fusion of these vesicles with the apical membrane, thereby increasing the number of pump proteins in the apical plasma membrane. Three chemical messengers stimulate the insertion of H+/ K+-ATPases into the plasma membrane and therefore acid secretion: gastrin (a gastric hormone), acetylcholine (ACh, a neurotransmitter), and histamine (a paracrine substance). By contrast, somatostatin—another paracrine substance—inhibits acid secretion. Parietal cell membranes contain receptors for all four of these molecules These chemical messengers not only act directly on the parietal cells but also influence each other's secretion. For example, histamine markedly potentiates the response to the other two stimuli, gastrin and ACh, and gastrin and ACh both stimulate histamine secretion.

goblet cells

Interspersed between these absorptive epithelial cells with microvilli secrete mucus that lubricates and protects the inner surface of the wall of the small intestine.

absorption of iron and food content

Iron absorption also depends on the types of food ingested because it binds to many negatively charged ions in food, which can retard its absorption. For example, iron in ingested liver is much more absorbable than iron in egg yolk because the latter contains phosphates that bind the iron to form an insoluble and unabsorbable complex.

parietal cells

Lining the walls of the glands in the stomach secrete acid and intrinsic factor (helps absorb vitamin B12 bc it's large and in charge)

chief cells

Lining the walls of the glands in the stomach secrete pepsinogen

pancreatic amylase

Most (∼95% or more) starch digestion is completed in the small intestine by pancreatic amylase The products of both salivary and pancreatic amylase are the disaccharide maltose and a mixture of short, branched chains of glucose molecules. These products, along with ingested sucrose and lactose, are broken down into monosaccharides—glucose, galactose, and fructose—by enzymes located on the apical mem- branes of the small-intestine epithelial cells (brush border). These monosaccharides are then transported across the intestinal epithe- lium into the blood.

digestion

Most food enters the gastrointestinal tract as large particles containing macromolecules, such as proteins and polysaccha- rides, which are unable to cross the intestinal epithelium. Before ingested food can be absorbed, therefore, it must be dissolved and broken down into small molecules. (Small nutrients such as vitamins and minerals do not need to be broken down and can cross the epithelium intact.) accomplished by the action of hydrochloric acid in the stomach, bile from the liver, and a variety of digestive enzymes released by the system's exocrine glands.

how are most proteins absorbed?

Most of the products of protein digestion are absorbed as short chains of two or three amino acids by secondary active transport coupled to the H+ gradient The absorption of small peptides contrasts with carbohydrate absorption, in which molecules larger than monosaccharides are not absorbed. Free amino acids, by contrast, enter the epithelial cells by secondary active transport coupled to Na+ There are many different amino acid transporters that are specific for the different amino acids Within the cytosol of the epithelial cell, the dipeptides and tripeptides are hydrolyzed to amino acids; these, along with free amino acids that entered the cells, then leave the cell and enter the interstitial fluid through facilitated-diffusion transporters in the basolateral membranes. As with carbohydrates, protein digestion and absorption are largely completed in the upper portion of the small intestine. Very small amounts of intact proteins are able to cross the intestinal epithelium and gain access to the interstitial fluid. They do so by a combination of endocytosis and exocytosis. The absorptive capacity for intact proteins is much greater in infants antibodies (proteins involved in the immunologic defense system of the body) secreted into the mother's milk can be absorbed intact by the infant, providing some immunity until the infant's immune system matures.

small intestine absorption

Normally, virtually all of the fluid secreted by the small intestine is absorbed back into the blood. In addition, a much larger volume of fluid, which includes salivary, gastric, hepatic, and pancreatic secretions, as well as ingested water, is simultaneously absorbed from the intestinal lumen into the blood. overall there is a large net absorption of water from the small intestine. Absorption is achieved by the transport of ions, primarily via Na+ and nutrient cotransport from the intestinal lumen into the blood, with water following by osmosis.

hemochromatosis

Once iron has entered the blood, the body has very little means of excreting it, so it accumulates in tissues. Although the control mechanisms for iron absorption tend to maintain the iron content of the body within a narrow homeostatic range, a very large ingestion of iron can overwhelm them, leading to an increased deposition of iron in tissues and producing toxic effects such as changes in skin pigmentation, diabetes mellitus, liver and heart failure, and decreased testicular function. Some people have genetically defec- tive control mechanisms and therefore develop hemochromatosis even when iron ingestion is normal.

hepatic portal vein

Other absorbed nutrients (than fats taht go into the lacteals) enter the blood capillaries. The venous drainage from the small intestine—as well as from the large intestine, pancreas, and portions of the stomach—does not empty directly into the vena cava but passes first, via the hepatic portal vein, to the liver. There it flows through a second capillary network before leaving the liver to return to the heart. Because of this portal circulation, material that is absorbed into the intestinal capillaries, in contrast to the lacteals, can be processed by the liver before enter- ing the general circulation. This is important because the liver con- tains enzymes that can metabolize (detoxify) harmful compounds that may have been ingested, thereby preventing them from enter- ing the circulation.

flatus

Other bacterial products include gas (flatus), which is a mixture of nitrogen and carbon dioxide, with small amounts of the gases hydrogen, methane, and hydrogen sulfide. Bacterial fermentation of undigested polysaccharides produces these gases in the colon (except for nitrogen, which is derived from swallowed air) at the rate of about 400 to 700 mL/day. Certain foods (beans, for example) contain large amounts of carbohydrates that cannot be digested by intestinal enzymes but are readily metabolized by bacteria in the large intestine, producing large amounts of gas.

pancreatic function during a meal

Pancreatic secretion increases during a meal, mainly as a result of stimulation by the hormones secretin and CCK Secretin is the primary stimulant for HCO3- secretion, whereas CCK mainly stimulates enzyme secretion. Because the function of pancreatic HCO3- is to neutralize acid entering the duodenum from the stomach, it is appropriate that the major stimulus for secretin release is increased acidity in the duodenum In analogous fashion, CCK stimulates the secretion of digestive enzymes, including those for fat and protein digestion, so it is appropriate that the stimuli for its release are fatty acids and amino acids in the duodenum Luminal acid and fatty acids also act on afferent neuron endings in the intestinal wall, initiating reflexes that act on the pancreas to increase both enzyme and HCO32 secretion. In these ways, the organic nutrients in the small intestine initiate neural and endocrine reflexes that control the secretions involved in their own digestion. cephalic and gastric stimuli also contribute by way of the parasympathetic nerves to the pancreas. Thus, the taste of food or the distension of the stomach by food will lead to increased pancreatic secretion.

pepsin

Proteins are first partially bro- ken down to peptide fragments in the stomach by the enzyme pepsin Pepsin is not essential for protein digestion because in its absence, as occurs in some pathological conditions, protein can be completely digested by enzymes in the small intestine. How- ever, pepsin accelerates protein digestion and normally accounts for about 20% of total protein digestion. It is also important in the digestion of collagen contained in the connective-tissue matrix of meat. This is useful because it helps shred meat into smaller, more easily processed pieces with greater surface area for digestion.

phlebotomy

Some people have genetically defec- tive control mechanisms and therefore develop hemochromatosis even when iron ingestion is normal. They can be treated with frequent blood withdrawal (phlebotomy), which removes iron contained in red blood cells (hemoglobin) from the body.

sphincter of Oddi

Surrounding the common bile duct at the point where it enters the duodenum is this ring of smooth muscle Although bile secretion is greatest during and just after a meal, the liver is always secreting some bile When this sphincter is closed, the dilute bile secreted by the liver is shunted into the gallbladder. Here, the organic components of bile become concentrated as some NaCl and water are absorbed into the blood. Shortly after the beginning of a fatty meal, the sphincter of Oddi relaxes and the gallbladder contracts, discharging concentrated bile into the duodenum. The signal for gallbladder contraction and sphincter relaxation is the intestinal hormone CCK—appropriately so, because, as we have seen, the presence of fat in the duodenum is a major stimulus for this hormone's release. It is from this ability to cause contraction of the gallbladder that cholecystokinin received its name: chole, "bile"; cysto, "bladder"; and kinin, "to move."

enteric nervous system

The GI tract has its own local neural control, a division of the autonomic nervous system The cells in this system form two networks or plexuses of neurons, the myenteric plexus and the submucosal plexus contains adrenergic and cholinergic neurons as well as neurons that release other neurotransmitters, such as nitric oxide, several neuropeptides, and ATP. neurons either synapse with other neurons within a given plexus or end near smooth muscles, glands, and epithelial cells. Many axons leave the myenteric plexus and synapse with neurons in the submucosal plexus, and vice versa, so that neural activity in one plexus influences the activity in the other. Moreover, stimulation at one point in the plexus can lead to impulses that are conducted longitudinally up and down the tract. For example, stimuli in the upper part of the small intestine may affect smooth muscle and gland activity in the stomach as well as in the lower part of the intestinal tract. In general, the myenteric plexus influences smooth muscle activity whereas the submucosal plexus influences secretory activity. muscle cells and exocrine glands—are supplied by neurons that are part of the enteric nervous system. This permits neural reflexes that are completely within the tract—that is, independent of the CNS nerve fibers from both the sympathetic and parasympathetic branches of the autonomic nervous system enter the intestinal tract and synapse with neurons in both plexuses. Via these path- ways, the CNS can influence the motility and secretory activity of the gastrointestinal tract.

transferrin

The absorbed iron that does not bind to ferritin is released on the blood side, where it circulates throughout the body bound to the plasma protein transferrin.

external anal sphincter

The anus, the exit from the rectum, is normally closed by the external anal sphincter, composed of skeletal muscle under voluntary control.

internal anal sphincter

The anus, the exit from the rectum, is normally closed by the internal anal sphincter, composed of smooth muscle

amount of stuff that enters the GI tract every day

The average American adult consumes about 500-800 g of food and 1200 mL of water per day, but this is only a fraction of the material entering the lumen of the gastrointestinal tract. An additional 7000 mL of fluid from salivary glands, gastric glands, pancreas, liver, and intestinal glands is secreted into the tract each day Of the approximately 8 L of fluid entering the tract, as much as 99% is absorbed; only about 100-200 mL is normally lost in the feces. This small amount of fluid loss represents only 4% of the total fluids lost from the body each day. Most fluid loss is via the kidneys and respiratory system. Almost all the ions in the fluids that are secreted into the GI tract are also reabsorbed into the blood. Moreover, the secreted digestive enzymes are themselves digested, and the resulting amino acids are absorbed into the blood.

daily carbohydrate intake

The average daily intake of carbohydrates is about 250 to 300 g per day in a typical American diet. This represents about half the average daily intake of calories. About two-thirds of this carbohydrate is the plant polysaccharide starch, and most of the remainder consists of the disaccharides sucrose (table sugar) and lactose Only small amounts of monosaccha- rides are normally present in the diet. Cellulose and certain other complex polysaccharides found in vegetable matter—referred to as dietary fiber (or simply fiber)—are not broken down by the enzymes in the small intestine and pass on to the large intestine, where they are partially metabolized by bacteria.

fat intake

The average daily intake of lipids is 70 to 100 g per day in a typical American diet, most of this in the form of fat (triglycerides). This represents about one-third of the average daily caloric intake.

lacteal

The center of each intestinal villus is occupied by both a single, blind-ended lymphatic vessel—a lacteal—and a capillary network most of the fat absorbed in the small intestine enters the lacteals. Material absorbed by the lacteals reaches the general circulation by eventually emptying from the lymphatic system into large veins through a structure called the thoracic duct.

mucosa

The combination of these three layers—the epithelium, lamina propria, and muscularis mucosa

concentration of acid in the gastric lumen is itself an important determinant of the rate of acid

The concentration of acid in the gastric lumen is itself an important determinant of the rate of acid secretion because H+ (acid) directly inhibits gastrin secretion. It also stimulates the release of somatostatin from D cells in the stomach wall. Somatostatin then acts on the parietal cells to inhibit acid secretion H+ also inhibits the release of gastrin and histamine The net result is a negative feedback control of acid secretion. As the contents of the gastric lumen become more acidic, the stimuli that promote acid secretion decrease.

chyme

The digestive actions of the stomach reduce food particles to a solution known as chyme contains molecular fragments of proteins and polysaccharides; droplets of fat; and salt, water, and various other small molecules ingested in the food. Virtually none of these molecules, except water, can cross the epithelium of the gastric wall, and thus little absorption of nutrients occurs in the stomach.

mucosa of the stomach

The epithelial layer lining the stomach invaginates into the mucosa, forming many tubular glands. Glands in the thin-walled upper portions of the body of the stomach secrete mucus, hydrochloric acid, and the enzyme precursor pepsinogen. The cells at the opening of the glands secrete a protective coating of mucus and HCO3- each of the three major exocrine secretions of the stomach—mucus, acid, and pepsinogen—is secreted by a different cell type.

where does secreted fat go?

The exit of these fat droplets from the cell follows the same pathway as a secreted protein. Vesicles containing the droplet pinch off the endoplasmic reticulum, are processed through the Golgi apparatus, and eventually fuse with the plasma membrane releasing the fat droplet into the interstitial fluid.

pancreatic duct

The exocrine portion of the pancreas secretes HCO3- and a number of digestive enzymes into ducts that con- verge into the pancreatic duct, joins the common bile duct from the liver just before it enters the duodenum The enzymes are secreted by gland cells at the pancreatic end of the duct system, whereas HCO3- is secreted by the epithelial cells lining the ducts

liver structure

The functional unit of the liver is the hepatic lobule Within the lobule are portal triads that are composed of branches of the bile duct, the hepatic and portal veins, and the hepatic artery (which brings oxygenated blood to the liver). Substances absorbed from the small intestine wind up in the hepatic sinusoid (blood vessel) either to reach the vena cava via the central vein or are taken up by the hepatocytes (liver cells) in which they can be modified.

G cells

The gastric glands in the antrum also contain enteroendocrine cells called G cells, which secrete gastrin.

enterogastrones

The hormones released by the intestinal tract that reflexively inhibit gastric activity. include secretin and CCK The inhibition of gastric acid secretion during the intestinal phase is mediated by short and long neural reflexes and by hormones that inhibit acid secretion by influencing the four signals that directly control acid secretion: ACh, gastrin, histamine, and somatostatin.

increasing gastric motility

The initiation of these stomach reflexes depends upon the contents of both the stomach and small intestine. All the factors that regulate acid secretion can also alter gastric motility. For example, gastrin in sufficiently high concentrations increases the force of antral smooth muscle contractions. Distension of the stomach also increases the force of antral contractions through long and short reflexes triggered by mechanoreceptors in the stomach wall. Therefore, after a large meal, the force of initial stomach contractions is greater, which results in a greater emptying per contraction.

colonic bacteria

The large intestine also absorbs some of the products formed by the bacteria colonizing this region. now recognized that the colonic bacteria make a vital metabolic contribution to health some undigested polysaccharides (fiber) are converted to short-chain fatty acids by bacteria in the large intestine and absorbed into the blood. Recent evidence suggests that these fatty acids may have important functions in immunity and cardio- vascular health. The HCO3- secreted by the large intestine helps to neutralize the increased acidity resulting from the formation of these fatty acids. These bacteria also produce small amounts of vitamins, especially vitamin K, for absorption into the blood. Although this source of vitamins generally provides only a small part of the normal daily requirement, it may make a significant contribution when dietary vitamin intake is low.

antrum

The lower portion of the stomach has a much thicker layer of smooth muscle and is responsible for mixing and grinding the stomach contents.

absorption in large intestine

The primary absorptive process in the large intestine is the active transport of Na+ from lumen to extracellular fluid, with the accompanying osmotic absorption of water If fecal material remains in the large intestine for a long time, almost all the water is absorbed, leaving behind hard fecal pellets. There is normally a net movement of K1 from blood into the large intestine lumen. Severe depletion of total-body potassium can result when large volumes of fluid are excreted in the feces. There is also a net movement of HCO3- into the lumen coupled to Cl- absorption from the lumen, and loss of this HCO3- (a base) in patients with prolonged diarrhea can cause metabolic acidosis

zymogens

The synthesis and secretion of pepsinogen, followed by its intraluminal activation to pepsin, provide an example of a process that occurs with many other secreted proteolytic enzymes in the GI tract. These enzymes are synthesized and stored intracellularly in inactive forms, collectively referred to as zymogens. zymogens do not act on proteins inside the cells that produce them; this protects the cell from proteolytic damage.

sigmoid colon

The terminal portion of the descending colon is S-shaped, forming the sigmoid colon, which empties into a relatively straight segment of the large intestine, the rectum, which ends at the anus.

canaliculi (singular, canaliculus)

The unique invaginations of the apical membrane of parietal cells in the stomach increase the surface area of the parietal cells thereby maximizing secretion into the lumen of the stomach.

fundus

The uppermost part of the body of the stomach is called the fundus and is functionally part of the body (of the stomach).

chylomicrons

These 1-micron- diameter, extracellular fat droplets fat released into the interstitial fluid Chylomicrons contain not only triglycerides but other lipids (including phospholipids, cholesterol, and fat-soluble vitamins) that have been absorbed by the same process that led to fatty acid and mono- glyceride movement into the epithelial cells of the small intestine. The chylomicrons released from the epithelial cells pass into lacteals—lymphatic vessels in the intestinal villi—rather than into the blood capillaries. The chylomicrons cannot enter the blood capillaries because the basement membrane (an extracellular glycoprotein layer) at the outer surface of the capillary provides a barrier to the diffusion of large chylomicrons. In contrast, the lacteals have large pores between their endothelial cells that allow the chylomicrons to pass into the lymph. The lymph from the small intestine, as from everywhere else in the body, eventually empties into veins

cholecystokinin (CCK)

These two generalizations can be illustrated by CCK. The presence of fatty acids and amino acids in the small intes- tine triggers CCK secretion from cells in the small intestine into the blood. Circulating CCK then stimulates the pancreas to increase the secretion of digestive enzymes and causes the sphincter of Oddi to relax and the pyloric sphincter to close. CCK also causes the gallbladder to contract, delivering to the intestine the bile salts required for micelle formation. As fatty acids and amino acids are absorbed, their concentrations in the lumen decrease, removing the signal for CCK release.

defecation reflex

This conscious urge to defecate, mediated by mechanoreceptors, accompanies distension of the rectum. The sudden distension of the walls of the rectum produced by the mass movement of fecal material into it initiates the neurally mediated defecation reflex. This reflex response consists of a contraction of the rectum and relaxation of the internal anal sphincter but contraction of the external anal sphincter (initially) and increased motility in the sigmoid colon. Eventually, a pressure is reached in the rectum that triggers reflex relaxation of the external anal sphincter, allowing the feces to be expelled. Via descending pathways to somatic nerves to the external anal sphincter, however, brain centers can override the reflex sig- nals that eventually would relax the sphincter, thereby keeping the external sphincter closed and allowing a person to delay defecation. In this case, the prolonged distension of the rectum initiates a reverse movement, driving the rectal contents back into the sigmoid colon. The urge to defecate then subsides until the next mass movement again propels more feces into the rectum, increasing its volume and again initiating the defecation reflex. Voluntary control of the external anal sphincter is learned during childhood. Spinal cord damage can lead to a loss of voluntary control over defecation.

enterohepatic circulation

This recycling pathway of bile salts from the liver to the intestine and back to the liver The absorbed bile salts are returned via the portal vein to the liver, where they are once again secreted into the bile. Uptake of bile salts from portal blood into hepatocytes is driven by secondary active transport coupled to Na+ A small amount (5%) of the bile salts escapes this recycling and is lost in the feces, but the liver synthesizes new bile salts from cholesterol to replace it. During the digestion of a meal, the entire bile salt content of the body may be recycled several times via the enterohepatic circulation.

mass movement

Three to four times a day, generally following a meal, a wave of intense contraction known as a mass movement spreads rapidly over the transverse segment of the large intestine toward the rectum. The large intestine is innervated by parasympathetic and sympathetic nerves. Parasympathetic input increases segmental contractions, whereas sympathetic input decreases colonic contractions.

other symptoms that come with vomiting

Vomiting is usually preceded by increased salivation, sweating, increased heart rate, pallor, and nausea

vomiting occurs when...

Vomiting occurs when the abdominal contractions become so strong that the increased intrathoracic pressure forces the contents of the esophagus through the upper esophageal sphincter. also accompanied by strong contractions in the upper portion of the small intestine—contractions that tend to force some of the intestinal contents back into the stomach for expulsion. Thus, some bile may be present in the vomitus

water in GI tract

Water is the most abundant substance in chyme. Approximately 8000 mL of ingested and secreted water enters the small intes- tine each day, but only 1500 mL passes on to the large intestine because 80% of the fluid is absorbed in the small intestine. Small amounts of water are absorbed in the stomach, but the stomach has a much smaller surface area available for diffusion and lacks the solute-absorbing mechanisms that create the osmotic gradients necessary for net water absorption. The epithelial membranes of the small intestine are very permeable to water, and net water diffusion occurs across the epithelium whenever a water concentration difference is established by the active absorption of solutes. Na+ accounts for much of the actively transported solute because it is such an abundant solute in chyme. Na+ absorption is a primary active-transport process—using Na+/K+-ATPase pumps Cl- and HCO3- are absorbed with the Na+ and contribute another large fraction of the absorbed solute. other minerals present in smaller concentrations, such as potassium, magnesium, phosphate, and calcium ions, are also absorbed, as are trace elements such as iron, zinc, and iodine.

retropulsion

When peristaltic wave arrives to antrum and pyloric sphincter closes, Most of the antral contents are forced backward toward the body of the stomach. This backward motion of chyme, called retropulsion, generates strong shear forces that helps to disperse the food particles and improve mixing of the chyme. Recall that the lower esophageal sphincter prevents this retrograde movement of stomach contents from entering the esophagus.

motility of the gastrointestinal tract

While digestion, secretion, and absorption are taking place, contractions of smooth muscles in the gastrointestinal tract wall occur, where they serve two functions: 1) They mix the luminal contents with the various secretions, and 2) they move the contents through the tract from mouth to anus/

submucosal plexus

a network of neurons in the submucosa

muscularis mucosa

a thin layer of smooth muscle that may be involved in the movement of intestinal structures called villi

water-soluble vitamins

absorbed by diffusion or mediated transport. The exception, vitamin B12 (cya- nocobalamin), is a very large, charged molecule. To be absorbed, vitamin B12 must first bind to a protein known as intrinsic factor

Laxatives

agents that increase the frequency or ease of defecation act through a variety of mechanisms. Fiber provides a natural laxative. Some laxatives, such as mineral oil, simply lubricate the feces, making defecation easier and less painful. Others contain magnesium and aluminum salts, which are poorly absorbed and therefore lead to water retention in the intestinal tract. Still others, such as castor oil, stimulate the motility of the colon and inhibit ion transport across the wall, resulting in decreased water absorption. Excessive use of laxatives in an attempt to maintain a pre- conceived notion of regularity leads to a decreased responsiveness of the large intestine to normal defecation-promoting signals. In such cases, a long period without defecation may occur following cessation of laxative intake, appearing to confirm the necessity of taking laxatives to promote regularity.

nontropical sprue

aka celiac disease or gluten-sensitive enteropathy due to an autoimmune-mediated loss of intestinal brush border surface area due to sensitivity to the wheat proteins collectively known as gluten. The loss of surface area can lead to decreased absorption of many nutrients, which in turn may result in a variety of health consequences. For example, it is often associated with vitamin D malabsorption, which ultimately results in a decrease in calcium ion absorption from the GI tract (and, consequently, a disruption in Ca21 homeostasis

emetic center

aka vomiting center Like swallowing, vomiting is a complex reflex coordinated by this region in the brainstem medulla oblongata Neural input to this center from receptors in many different regions of the body can initiate the vomiting reflex. For example, excessive distension of the stomach or small intestine, various substances acting upon chemoreceptors in the intestinal wall or in the brain, increased pressure within the skull, rotating movements of the head (motion sickness), intense pain, and tactile stimuli applied to the back of the throat can all initiate vomiting.

enteroendocrine cells

also at the base of the villi are enteroendocrine cells that secrete hormones that, as you will learn, control a wide variety of gastrointestinal functions, including motility and exocrine pancreatic secretions.

phospholipids

amphipathic molecules consisting of two nonpolar fatty acid chains attached to glycerol, with a charged phosphate group located on glycerol's third carbon. Bile salts are formed from cholesterol in the liver and are also amphipathic The nonpolar portions of the phospholipids and bile salts associate with the nonpolar interior of the lipid droplets, leaving the polar portions exposed at the water surface. There, they repel other lipid droplets that are similarly coated with these emulsifying agents, thereby preventing their reaggregation into larger fat droplets

pancreas

an elongated gland located behind the stomach has both endocrine and exocrine functions, but only the latter are directly involved in gastrointestinal function The exocrine portion of the pancreas secretes digestive enzymes and a fluid rich in HCO3-. The high acidity of the chyme coming from the stomach would inactivate the pancreatic enzymes in the small intestine if the acid were not neutralized by the HCO32 in the pancreatic fluid.

motilin

an increase in the plasma concentration of the intestinal hormone motilin is thought to initiate the MMC. Feeding inhibits the release of motilin; motilin stimulates MMCs via both the enteric and autonomic nervous systems.

esophageal phase of swallowing exact steps

begins with relaxation of the upper esophageal sphincter. Immediately after the food has passed, the sphincter closes, the glottis opens, and breathing resumes. Once in the esophagus, the food moves toward the stom- ach by a progressive wave of muscle contractions that proceed along the esophagus, compressing the lumen and forcing the food ahead. Such waves of contraction in the muscle layers surrounding a tube are known as peristaltic waves The lower esophageal sphincter opens and remains relaxed throughout the period of swallowing, allowing the arriving food to enter the stomach. After the food passes, the sphincter closes, resealing the junction between the esophagus and the stomach.

submucosa

beneath the mucosa a second connective-tissue layer This layer also contains a network of neurons, the submucosal plexus, and blood and lymphatic vessels whose branches penetrate into both the overlying mucosa and the underlying layers of smooth muscle called the muscularis externa.

myenteric plexus

between the inner and outer layers of the muscularis externa network of neurons There are neurons projecting from the submucosal plexus to the single layer of cells on the apical surface as well as to the myenteric plexus. The myenteric plexus is innervated by nerves from the sympathetic and parasympathetic divisions of the autonomic nervous system and has neurons that project to the submucosal plexus.

gall stone formation

bile contains not only bile salts but also cholesterol and phospholipids, which are water-insoluble and are maintained in soluble form in the bile as micelles. When the concentration of cholesterol in the bile becomes high in relation to the concentrations of phospholipid and bile salts, cholesterol crystallizes out of solution, forming gallstones. This can occur if the liver secretes excessive amounts of cholesterol or if the cholesterol becomes overly concentrated in the gallbladder as a result of ion and water absorption. Although cholesterol gallstones are the most frequently encountered gallstones in the Western world, the precipitation of bile pigments can also occasionally be responsible for gallstone formation.

motility of the small intestine

brought about by the smooth muscles in its walls, (1) mixes the luminal contents with the various secretions, (2) brings the contents into contact with the epithelial surface where absorption takes place, and (3) slowly advances the luminal material toward the large intes- tine, the next segment of the alimentary canal.

neural and hormonal control of the digestive system is divisible into three phases:

cephalic, gastric, and intestinal according to where the stimulus is perceived. Each phase is characterized by efferent output to virtually all organs in the gastrointestinal tract. Also, these phases do not occur in temporal sequence except at the very beginning of a meal. Rather, during ingestion and the much longer absorptive period, reflexes characteristic of all three phases may be occurring simultaneously.

Diarrhea

characterized by large, frequent, watery stools. Diarrhea can result from decreased fluid absorption, increased fluid secretion, or both. The increased motility that accompanies diarrhea probably does not cause most cases of diarrhea (by decreasing the time available for fluid absorption) but, rather, results from the distension produced by increased luminal fluid. A number of bacterial, protozoan, and viral diseases of the intestinal tract cause secretory diarrhea In addition to decreased blood volume due to ion and water loss, other consequences of severe diarrhea are K+ depletion and metabolic acidosis resulting from the excessive fecal loss of K+ and HCO3-

emetics

chemicals that can stimulate vomiting via receptors in the stomach, duodenum, or brain.

colipase

coating of the lipid droplets with these emulsifying agents, however, impairs the accessibility of the water-soluble pancreatic lipase to its lipid substrate. To overcome this problem, the pancreas secretes a protein known as colipase, which is amphipathic and lodges on the lipid droplet surface. Colipase binds the lipase enzyme, holding it on the surface of the lipid droplet.

Swallowing reflex

complex reflex initiated when pressure receptors in the walls of the pharynx are stimulated by food or drink forced into the rear of the mouth by the tongue receptors send afferent impulses to the swallowing center in the medulla oblongata of the brainstem As the ingested material moves into the pharynx, the soft palate elevates and lodges against the back wall of the pharynx, preventing food from entering the nasal cavity

contractions in large intestine

contractions of the circular smooth muscle in the large intestine produce a segmentation motion with a rhythm considerably slower (one every 30 min) than that in the small intestine. Because of the slow propulsion of the large-intestine contents, material entering the large intestine from the small intestine remains for about 18 to 24 h. This provides time for bacteria to grow and multiply.

secretion of saliva

controlled by both sympathetic and parasympathetic neurons. Unlike their antagonistic activity in most organs, both systems stimulate salivary secretion, with the parasympathetic neurons producing the greater response. There is no hormonal regulation of salivary secretion. In the absence of ingested material, a low rate of salivary secretion keeps the mouth moist. The smell or sight of food induces a cephalic phase of salivary secretion. This reflex can be conditioned to other cues, a phenomenon made famous by Pavlov. Salivary secretion can increase markedly in response to a meal. This reflex is initiated by chemoreceptors (acidic foods are particularly strong stimuli) and pressure receptors in the walls of the mouth and on the tongue. Increased saliva secretion is accomplished by a large increase in blood flow to the salivary glands, which is mediated primarily by an increase in parasympathetic neural activity. The volume of saliva secreted per gram of tissue is the largest secretion of any of the body's exocrine glands.

chewing

controlled by the somatic nerves to the skeletal muscles of the mouth and jaw. in addition to voluntary control of these muscles, rhythmic chewing motions are reflexively activated by the pressure of food against the gums, hard palate at the roof of the mouth, and tongue. Activation of these mechanoreceptors leads to reflexive inhibition of the muscles holding the jaw closed. The resulting relaxation of the jaw reduces the pressure on the various mechanoreceptors, leading to a new cycle of contraction and relaxation. Chewing prolongs the subjective pleasure of taste. Chew- ing also breaks up food particles, creating a bolus that is easier to swallow and, possibly, digest. Attempting to swallow a large particle of food can lead to choking if the particle lodges over the trachea, blocking the entry of air into the lungs.

bile canaliculi,

converge to form the common hepatic bile duct

villi

cover circular folds The circular folds (mucosa and submucosa) are covered with fingerlike projections called villi The surface of each villus is covered with a layer of epithelial cells whose surface membranes form small projections called microvilli (singular, microvillus; also known collectively as the brush border) Epithelial surfaces in the GI tract are continuously being replaced by new epithelial cells. In the small intestine, for example, new cells arise by cell division from cells at the base of the villi. These cells differentiate as they migrate to the top of the villus, replacing older cells that die and are discharged into the intestinal lumen. These dead cells release their intracellular enzymes into the lumen, which then contribute to the digestive process. About 17 billion epithelial cells are replaced each day, and the entire epithelium of the small intestine is replaced approximately every 5 days. It is because of this rapid cell turnover that the lining of the intestinal tract is so susceptible to damage by treatments that inhibit cell divi- sion, such as anticancer drugs and radiation therapy.

aspiration

dangerous situation in which food travels down the trachea and can cause choking, or when regurgitated stomach contents are allowed into the lungs causing damage.

secretion

digestion and is accomplished by the action of hydrochloric acid in the stomach, bile from the liver, and a variety of digestive enzymes released by the system's exocrine glands. Each of these substances is released into the lumen of the GI tract through the process of secretion some digestive enzymes are located on the apical membranes of the intestinal epithelium.

four processes of the digestive tract

digestion, secretion, absorption, and motility

lactase

digests lactose embedded in the apical plasma membranes of intestinal epithelial cells. Lactase is usually present at birth and allows the nursing infant to digest the lactose in breast milk. Because the only dietary source of lactose is from milk and milk products, all mammals—including most humans—lose the ability to digest this disaccharide around the time of weaning.

emulsification

division of the large lipid droplets into many very small droplets, each about 1 mm in diameter, thereby increasing their surface area and accessibility to lipase action. pancreatic lipase is a water- soluble enzyme, its digestive action in the small intestine can take place only at the surface of a lipid droplet. Therefore, if most of the ingested fat remained in large lipid droplets, the rate of triglyceride digestion would be very slow because of the small surface-area- to-volume ratio of these big fat droplets. The rate of digestion is, however, substantially increased bc of this enzyme

feces

elimination of the GI tract The material known as feces leaves the system via the anus at the end of the gastrointestinal tract. Feces consist almost entirely of bacteria and ingested material that was neither digested nor absorbed, and therefore was never actually absorbed into the internal environment.

Cholera

endemic in many parts of the world is caused by a bacterium that releases a toxin that stimulates the production of cyclic AMP in the secretory cells at the base of the intestinal villi. This leads to an increased frequency in the opening of the Cl- channels in the apical membrane and, hence, increased secretion of Cl-. An accompanying osmotic flow of water into the intestinal lumen occurs, resulting in massive diarrhea that can be life threatening due to dehydration and decreased blood volume that leads to circulatory shock. The ions and water lost by this severe form of diarrhea can be balanced by ingesting a simple solution containing salt and glucose. The active absorption of these solutes is accompanied by absorption of water, which replaces the fluid lost by diarrhea.

function of pancreatic enzymes

enzymes the pancreas secretes digest fat, polysaccharides, proteins, and nucleic acids to fatty acids and monoglycerides, sugars, amino acids, and nucleotides, respectively. proteolytic enzymes are secreted in inactive forms (zymogens) amd activated in the duodenum by other enzymes. Like pepsinogen, the secretion of zymogens protects pancreatic cells from autodigestion. The nonproteolytic enzymes secreted by the pancreas (e.g., amy- lase and lipase) are released in fully active form.

potentiation

ex. of when variety of inputs that can affect the cell's response exemplified by the interaction between secretin and CCK. Secretin strongly stimulates pancreatic HCO3- secretion, whereas CCK is a weak stimulus of HCO3- secretion. Both hormones together, however, stimulate pancreatic HCO3- secretion more strongly than would be predicted by the sum of their individual stimulatory effects. This is because CCK amplifies the response to secretin. One of the consequences of potentiation is that small changes in the plasma concentration of one gastrointestinal hormone can have large effects on the actions of other gastrointestinal hormones. In addition to their stimulation (or, in some cases, inhibition) of effector cell functions, the gastrointestinal hormones also have trophic (growth-promoting) effects on various tissues, including the gastric and intestinal mucosa and the exocrine portions of the pancreas. Finally, many additional GI hormones have been described, some of which are involved in the control of blood glucose by serving as a feedforward signal from the GI tract to the endocrine pancreas; others may regulate appetite.

bile

excreted by the liver Bile contains six major ingredients: (1) bile salts, (2) lecithin (a phospholipid), (3) HCO3- and other ions, (4) cholesterol, (5) bile pigments and small amounts of other metabolic end products, and (6) trace metals. The HCO3-, like that from the pan- creas, helps neutralize acid from the stomach, whereas the bile salts, solubilize dietary fat. These fats would otherwise be insoluble in water, and their solubilization increases the rates at which they are digested and absorbed. Bile salts and lecithin are synthesized in the liver and help solubilize fat in the small intestine. HCO3- neutralizes acid in the duodenum, and the last three ingredients represent substances extracted from the blood by the liver and excreted via the bile. Bile is secreted by the liver into small ducts that join to form the common hepatic duct.

triglyceride digestions

fat occurs to a limited extent in the mouth and stomach, but it predominantly occurs in the small intestine.

malabsorption

fat soluble vitamins are solubilized in micelles; thus, any interference with the secretion of bile or the action of bile salts in the intestine decreases the absorption of the fat-soluble vitamins, a pathological condition called malabsorption Malabsorption syndromes can lead to deficiency of fat-soluble vitamins

cecum

first portion of the large intetine

trypsinogen

forms trypsin by enterokinase Trypsin is also a proteolytic enzyme; once activated, it activates the other pancreatic zymogens by splitting off peptide fragments This activating function is in addition to the function of trypsin in digesting ingested protein.

gastric phase

four stimuli in the stomach initiate the reflexes that con- stitute the gastric phase of regulation: distension, acidity, amino acids, and peptides formed during the partial digestion of ingested protein The responses to these stimuli are mediated by short and long neural reflexes and by release of the hormone gastrin.

decreasing gastric motility

gastric emptying is inhibited by distension of the duodenum, the presence of fat, high acidity (low pH), or hyper- tonic solutions in the lumen of the duodenum These are the same factors that inhibit acid and pepsin secretion in the stomach. Fat is the most potent of these chemical stimuli. This prevents overfilling of the duodenum. The rate of gastric emptying has significant clinical implications particularly when considering what food type is eaten with oral medications. A meal rich in fat content tends to slow oral drug absorption due to a delay of the drug entering the small intestine through the pyloric sphincter. a hypertonic solution in the duodenum is one of the stimuli inhibiting gastric emptying. This reflex prevents the fluid in the duodenum from becoming too hypertonic. It does so by slowing the rate of entry of chyme and thereby the delivery of large molecules that can rapidly be broken down into many small molecules by enzymes in the small intestine. Autonomic nerve fibers to the stomach can be activated by the CNS independently of the reflexes originating in the stomach and duodenum and can influence gastric motility. An increase in parasympathetic activity increases gastric motility, whereas an increase in sympathetic activity decreases motility. Via these pathways, pain and emotions can alter motility; however, different people show different GI responses to apparently similar emotional states.

sodium-glucose cotransporter (SGLT)

glucose and galactose undergo secondary active transport coupled to Na+ via the sodium-glucose cotransporter (SGLT) These monosaccharides then leave the epithelial cells and enter the interstitial fluid by way of facilitated diffusion via various GLUT proteins in the basolateral membranes of the epithelial cells. From there, the monosaccharides diffuse into the blood through capillary pores. Most ingested carbohydrates are digested and absorbed within the first 20% of the small intestine.

constipation

headache, loss of appetite, nausea, and abdominal distension—may arise when defecation has not occurred for several days or longer symptoms not caused by toxins but by distension of the rectum longer that fecal mate- rial remains in the large intestine, the more water is absorbed and the harder and drier the feces become, making defecation more difficult and sometimes painful. Decreased motility of the large intestine is the primary factor causing constipation. This often occurs in elderly people, or it may result from damage to the colon's enteric nervous system or from emotional stress. One of the factors increasing motility in the large intestine— and thus opposing the development of constipation—is distension. As noted earlier, dietary fiber (cellulose and other complex poly- saccharides) is not digested by the enzymes in the small intestine and is passed on to the large intestine, where its bulk produces distension and thereby increases motility. Bran, most fruits, and vegetables are examples of foods that have a relatively high fiber content.

hormonal regulation

hormones that control the gastrointestinal system are secreted mainly by enteroendocrine cells scattered throughout the epithelium of the stomach and small intestine cells are not clustered into discrete endocrine glands like the thyroid or adrenal glands. One surface of each endocrine cell is exposed to the lumen of the GI tract. At this surface, various chemical substances in the chyme stimulate the cell to secrete its hormones from the opposite side of the cell into the blood. The gastrointestinal hormones reach their target cells via the circulation. In many cases, a single effector cell contains recep- tors for more than one hormone, as well as receptors for neu- rotransmitters and paracrine substances. The result is a variety of inputs that can affect the cell's response.

secondary peristalsis

if a large food bolus does not reach the stomach during the initial peristaltic wave, the maintained distension of the esophagus by the bolus activates receptors that initiate reflexes, causing repeated waves of peristaltic activity The lower esophageal sphincter undergoes brief periods of relaxation not only during a swallow but also in the absence of a swallow. During these periods of relaxation, small amounts of the acid contents from the stomach normally reflux into the esophagus. The acid in the esophagus triggers a secondary peristaltic wave and also stimulates increased salivary secretion, which helps to neutralize the acid and clear it from the esophagus.

Sjögren's syndrome

immune disorder in which many different exocrine glands are rendered nonfunctional by the infiltration of white blood cells and immune complexes. loss of salivary gland function, which frequently occurs in this syndrome, can be treated by taking frequent sips of water and with oral fluoride treatment to prevent tooth decay. In addition, these patients—mostly women—can have an impaired sense of taste, difficulty chewing, and even ulcers (holes) in the mucosa of the mouth.

amylase

in saliva partially digests polysaccharides (complex carbohydrates).

peristalsis

in some cases, muscular movements travel in a wavelike fashion in one direction along the length of a part of the tract, a process called peristalsis.

digestive system

includes the gastrointestinal (GI) tract (or alimentary canal), consisting of the mouth, pharynx, esophagus, stomach, small intestine, and large intestine; and the accessory organs and tissues, consisting of the salivary glands, liver, gallbladder, and exocrine pancreas. overall function of the digestive system is to process ingested foods into molecular forms that are then transferred, along with small molecules, ions, and water, to the body's internal environment, where the circulatory system can distribute them to cells under the local neural control of the enteric nervous system and also of the central nervous system.

increasing the protein content of a meal:

increases acid secretion. This occurs for two reasons: First, protein ingestion increases the concentration of peptides in the lumen of the stomach. These peptides, as we have seen, stimulate acid secretion through their actions on gastrin. The second reason is more complicated and reflects the effects of proteins on luminal acidity. During the cephalic phase, before food enters the stomach, the H+ concentration in the lumen increases because there are few buffers present to bind any secreted H+. Thereafter, the rate of acid secretion soon decreases because high acidity reflexively inhibits acid secretion. The protein in food is an excellent buffer, how- ever, so as it enters the stomach, the H+ concentration decreases as H+ binds to proteins and begins to denature them. This decrease in acidity removes the inhibition of acid secretion. The more protein in a meal, the greater the buffering of acid and the more acid secreted.

intestinal phase

initiated by stimuli in the small intestine including distension, acidity, osmolarity, and various digestive products. The intestinal phase is mediated by both short and long neural reflexes and by the hormones secretin, CCK, and GIP, all of which are secreted by enteroendocrine cells of the small intestine.

cephalic phase

initiated when sensory receptors in the head are stimulated by sight, smell, taste, and chewing. Various emotional states can also initiate this phase. The efferent pathways for these reflexes are primarily mediated by parasympathetic fibers carried in the vagus nerves. These fibers activate neurons in the gastrointestinal nerve plexuses, which in turn affect secretory and contractile activity.

Ulcer formation

involves breaking the mucosal barrier and exposing the underlying tissue to the corrosive action of acid and pepsin it is not always clear what produces the initial damage to the barrier. Although acid is essential for ulcer formation, it is not necessarily the primary factor; many patients with ulcers have normal or even subnormal rates of acid secretion. Many factors, including genetic susceptibility, drugs, alcohol, bile salts, and an excessive secretion of acid and pepsin, may contribute to ulcer formation. One major factor, however, is the presence of a bacterium, Helicobacter pylori, that is present in the stomachs of many patients with ulcers or gastritis (stomach inflammation) Suppression of these bacteria with antibiot- ics usually helps heal the damaged mucosa. contribution of stress in producing ulcers remains unclear. Once the ulcer has been formed, however, emotional stress can aggravate it by increasing acid secretion and also decreasing appetite and food intake.

lamina propria

just below the epithelium a layer of loose connective tissue through which pass small blood vessels, nerve fibers, and lymphatic vessels. separated from underlying tissues by the muscularis mucosa

enterokinase

key enzyme in activation of zymogens in the pancreas embedded in the apical plasma membranes of the intestinal epithelial cells. Enterokinase is a proteolytic enzyme that splits off a peptide from pancreatic trypsinogen, forming the active enzyme trypsin

liver

large organ located in the upper-right portion of the abdomen We will be concerned in this chapter primarily with the liver's exocrine functions that are directly related to the secretion of bile.

colon

large part of large intestine, before the rectum consists of three relatively straight segments—the ascending, transverse, and descending portions

muscularis externa

layers of smooth muscle that lie below the submucosa Contractions of these muscles provide the forces for moving and mixing the gastrointestinal contents. Except in the stomach, which has three layers, elsewhere the muscularis externa has two layers: (1) a relatively thick inner layer of circular muscle, the fibers of which are oriented in a circular pattern around the tube so that con- traction produces a narrowing of the lumen; and (2) a thinner outer layer of longitudinal muscle, the contraction of which shortens the tube.

glucose-dependent insulinotropic peptide (gip)

like others, just works on glucose and fat in the small intestine

hepatocytes

liver cells can rid the body of substances by secretion into the bile canaliculi, The bile salts, cholesterol, lecithin, and bile pigments are secreted by hepatocytes (whereas most of the HCO3- -rich solution is secreted by the epithelial cells lining the bile ducts, which is stimulated by secretin in response to the presence of acid in the duodenum.)

apical surface

lumen surface of the GI tract highly convoluted, a feature that greatly increases the surface area available for absorption. From the stomach on, this surface is covered by a single layer of epithelial cells linked together along the edges of their apical surfaces by tight junctions. Invaginations of the epithelium into the underlying tis- sue form exocrine glands that secrete acid, enzymes, water, ions, and mucus into the lumen. Other cells in the epithelium secrete hormones into the blood that are important in regulating various aspects of digestion and appetite.

circular folds

macro- and microscopic structure of the wall of the small intestine is particularly elaborate circular folds (mucosa and submucosa) are covered with fingerlike projections called villi The combination of circular folds, villi, and microvilli increases the small intestine's surface area about 600-fold over that of a flat- surfaced tube having the same length and diameter. The human small intestine's total surface area is about 250 to 300 square meters, roughly the area of a tennis court.

Lactose

major carbohydrate in milk. It cannot be absorbed directly but must first be digested into its components, glucose and galactose, which are readily absorbed by secondary active transport and facilitated diffusion Lactose is a disaccharide

pancreatic lipase

major digestive enzyme in fat digestion (triglycerides) catalyzes the splitting of bonds linking fatty acids to the first and third carbon atoms of glycerol, producing two free fatty acids and a monoglyceride as products: Triglyceride ⎯→ Monoglyceride+ 2 Fatty acids

body of the stomach

middle part

absorption

molecules produced by digestion, along with water and small nutrients that do not require digestion, move from the lumen of the gastrointestinal tract across a layer of epithelial cells and enter the blood or lymph. Within fairly wide limits, the digestive system will absorb as much of any particular substance that is ingested, with a few important exceptions. Therefore, the digestive system does not regulate the total amount of nutrients absorbed or their concentrations in the internal environment. The plasma concentration and distribution of the absorbed nutrients throughout the body are primarily controlled by hormones from a number of endocrine glands and by the kidneys.

bile salts

most important digestive components of bile are the bile salts. Bile salts and lecithin are synthesized in the liver and, as we have seen, help solubilize fat in the small intestine. During the digestion of a fatty meal, most of the bile salts entering the intestinal tract via the bile are absorbed by specific Na+-coupled transporters in the ileum

iron

necessary for normal health because it is the O2 binding component of hemoglobin, and it is also a key component of many enzymes. Only about 10% of ingested iron is absorbed into the blood each day. Iron ions are actively transported into intestinal epithelial cells, where most of them are incorporated into ferritin, the protein-iron complex that functions as an intracellular iron store The absorbed iron that does not bind to ferritin is released on the blood side, where it circulates throughout the body bound to the plasma protein transferrin.

area postrema

nucleus in the medulla oblon- gata and is outside the blood-brain barrier, which allows it to be sensitive to toxins in the blood and to initiate vomiting

long reflexes

one of two types of neural-reflex arc from receptors in the tract to the CNS by way of afferent nerves, and back to the nerve plexuses and effector cells by way of autonomic nerve fibers.

short reflexes

one of two types of neural-reflex arc from receptors through the nerve plexuses to effector cells all within the GI tract

HCO3- pancreatic duct

pancreatic duct cells secrete HCO3- (produced from CO2 and water) into the duct lumen via an apical membrane Cl-/HCO3- exchanger H+ produced is exchanged for extracellular Na+ on the basolateral side of the cell (blood) The H+ enters the pancreatic capillaries to eventually meet up in portal vein blood with the HCO3- produced by the stomach during the generation of luminal H+ As with many transport systems, the energy for secretion of HCO3- is ultimately provided by Na+/K+-ATPase pumps on the basolateral membrane. Cl- normally does not accumulate within the cell because these ions are recycled into the lumen through the cystic fibrosis transmembrane conductance regulator (CFTR) Via a paracellular route, Na+ and water move into the pancreatic ducts due to the electrochemical gradient established by chloride movement through the CFTR. This dependence on Cl- explains why mutations in the CFTR that cause cystic fibrosis result in decreased pancreatic HCO3- secretion lack of normal water movement into the lumen (bc it, along with Na+, usually follows Cl-) leads to a thickening of pancreatic secretions; this can lead to clogging of the pancreatic ducts and pancreatic damage. In fact, the cystic and fibrotic (scarring) appearance of the diseased pancreas was the origin of the name of this disease.

three pairs of salivary glands

parotid, sublingual, and submandibular glands

accessory organs

part of the digestive system consisting of the salivary glands, liver, gallbladder, and exocrine pancreas. accessory organs are not part of the tract but secrete substances into it via connect- ing ducts.

pepsinogen

pepsin is produced from an inactive precursor pepsinogen

pepsin secretion

pepsin is secreted by chief cells in the form of an inactive precursor called pepsinogen Exposure to low pH in the lumen of the stomach activates a very rapid, autocatalytic process in which pepsin is produced from pepsinogen. Pepsin is active only in the presence of a high H+ concentration (low pH). It is inactivated when it enters the small intestine, where the HCO3- secreted into the small intestine neutralizes the H1. The primary pathway for stimulating pepsinogen secretion is input to the chief cells from the enteric nervous system. During the cephalic, gastric, and intestinal phases, most of the factors that stimulate or inhibit acid secretion exert the same effect on pepsinogen secretion. Thus, pepsinogen secretion parallels acid secretion.

aminopeptidases

peptide fragments can be absorbed if they are small enough or are further digested to free amino acids by aminopeptidases located on the apical membranes of the small-intestine epithelial cells split off amino acids from the amino ends of peptide fragments

carboxypeptidases

peptide fragments can be absorbed if they are small enough or are further digested to free amino acids by carboxypeptidases additional proteases secreted by the pancreas split off amino acids from the carboxyl ends of peptide fragments

fxn of large intestine

primary fxn= store and concentrate fecal material before defecation. The secretions of the large intestine are scanty, lack digestive enzymes, and consist mostly of mucus and fluid containing HCO3- and K+. About 1500 mL of chyme enters the large intestine from the small intestine each day. This material is derived largely from the secretions of the lower small intestine because most of the ingested food is absorbed before reaching the large intestine. Fluid absorption by the large intestine normally accounts for only a small fraction of the fluid absorbed by the GI tract each day.

Traveler's diarrhea

produced by several species of bacteria, produces a secretory diarrhea by the same mechanism as the cholera bacterium but is usually less severe.

ferritin

protein-iron complex that functions as an intracellular iron store Most of the iron bound to ferritin in the epithelial cells is released back into the intestinal lumen when the cells at the tips of the villi disintegrate, and the iron is then excreted in the feces. Iron absorption depends on the body's iron content. When body stores are ample, the increased concentration of free iron in the plasma and intestinal epithelial cells leads to an increased transcription of the gene encoding the ferritin protein and, as a consequence, an increased synthesis of ferritin. This results in the increased binding of iron in the intestinal epithelial cells and a reduction in the amount of iron released into the blood. When body stores of iron decrease (for example, after a loss of blood), the production of intestinal ferritin decreases. This leads to a decrease in the amount of iron bound to ferritin, thereby increasing the unbound iron released into the blood.

peristaltic waves

push food down the esophagus One esophageal peristaltic wave takes about 9 seconds to reach the stomach. Swallowing can occur even when a person is upside down or in zero gravity (outer space) because it is not primarily gravity but the peristaltic wave that moves the food to the stomach.

elimination

relatively minor function of the GI tract Small amounts of certain metabolic end products are excreted via the gastrointestinal tract, primarily by way of the bile. (feces) the lungs and kidneys are usually responsible for the elimination of most of the body's waste products, such as CO2.

enterochromaffin-like (ECL) cells

release the paracrine substance histamine are scattered throughout the tubular glands or in surrounding tissue contribute to the regulation of acid secretion by the stomach.

adaptive value of emetic reflex

removal of ingested toxic substances before they can be absorbed is beneficial. Moreover, the nausea that usually accompanies vomiting may have the adaptive value of conditioning the individual to avoid the future ingestion of foods containing such toxic substances. Why other types of stimuli, such as those producing motion sickness, have become linked to the vomiting center is not clear.

basic electrical rhythm of the stomach

responsible for producing gastric peristaltic waves rhythm (three per minute) is generated by pace- maker cells in the longitudinal smooth muscle layer. These smooth muscle cells undergo spontaneous depolarization- repolarization cycles (slow waves) known as the basic electrical rhythm of the stomach. These slow waves are conducted through gap junctions along the stomach's longitudinal muscle layer and also induce similar slow waves in the overlying circular muscle layer. In the absence of neural or hormonal input, however, these depolarizations are too small to cause significant contractions. Excitatory neurotransmitters and hormones act upon the smooth muscle to further depolarize the membrane, thereby bringing it closer to threshold. Action potentials may be generated at the peak of the slow-wave cycle if threshold is reached, causing larger contractions The number of spikes fired with each wave determines the strength of the muscle contraction. Therefore, whereas the frequency of con- traction is determined by the intrinsic basic electrical rhythm and remains essentially constant, the force of contraction—and, consequently, the amount of gastric emptying per contraction— is determined reflexively by neural and hormonal input to the antral smooth muscle.

segmentation

rhythmic contraction and relaxation of small intestine most common motion in the small intestine during digestion of a meal is a stationary contraction and relaxation of intestinal segments, with little apparent net movement toward the large intestine Each contracting segment is only a few centimeters long, and the contraction lasts a few seconds. The chyme in the lumen of a contracting segment is forced both up and down the intestine. produces a continuous division and subdivision of the intestinal contents, thoroughly mixing the chyme in the lumen and bringing it into contact with the intestinal wall. segmenting movements are initiated by electrical activity generated by pacemaker cells in the circular smooth muscle layer intensity of segmentation can be altered by hormones, the enteric nervous system, and autonomic nerves; parasympathetic activity increases the force of contraction, and sympathetic stimulation decreases it. cephalic phase stimuli, as well as emotional states, can alter intestinal motility. As is true for the stomach, these inputs produce changes in the force of smooth muscle contraction but do not significantly change the frequencies of the basic electrical rhythms.

stomach

saclike organ located between the esophagus and the small intestine. Its functions are to store, dissolve, and partially digest the macromolecules in food and to regulate the rate at which its contents empty into the small intestine. The acidic environment in the gastric (adjective for "stomach") lumen alters the ionization of polar molecules, leading to denaturation of protein. This exposes more sites for digestive enzymes to break down the proteins, and disrupts the extracellular network of connective-tissue proteins that form the structural framework of the tissues in food. Polysaccharides and fat are major food components that are not dissolved to a significant extent by acid. The low pH also kills most of the bacteria that enter along with food. This process is not completely effective, and some bacteria survive to colonize and multiply in the remainder of the GI tract, particularly the large intestine. The digestive actions of the stomach reduce food particles to a solution known as chyme

pancreas and liver major function

secrete sub- stances that flow via ducts into the duodenum

D cells

secrete the polypeptide somatostatin (inhibits acid secretion) are scattered throughout the tubular glands or in surrounding tissue contribute to the regulation of acid secretion by the stomach.

Saliva

secreted by three pairs of exocrine salivary glands located in the head ( drains into the mouth through a series of short ducts. contains mucus and HCO3-, moistens and lubricates the food particles, thereby facilitating swallowing. contains the enzyme amylase, which partially digests polysaccharides (complex carbohydrates). A third function of saliva is to dissolve some of the food molecules. Only in the dissolved state can these molecules react with chemoreceptors in the mouth, giving rise to the sensation of taste Finally, saliva has antipathogenic properties.

four best-understood GI hormones

secretin, cholecystokinin (CCK), gastrin, and glucose-dependent insulinotropic peptide (GIP) (1) Most of the hormones participate in a feed- back control system that regulates some aspect of the GI lumi- nal environment, and (2) most GI hormones affect more than one type of target cell.

pharynx and esophagus

segments of the GI tract after the mouth/saliva do not contribute to digestion but provide the pathway for ingested materials to reach the stomach. The muscles in the walls of these segments control swallowing.

appendix

small, fingerlike projection that extends from the cecum and may participate in immune function but is not essential

gastroesophageal reflux, or simply acid reflux

some of the gastric contents forced up into the esophagus The hydrochloric acid from the stomach irritates the esophageal walls, producing pain known as heartburn (because the pain appears to be located in the area of the heart). Some people have less efficient lower esophageal sphincters, resulting in repeated episodes of gastric contents refluxing into the esophagus. In extreme cases, ulcer- ation, scarring, obstruction, or perforations (holes) of the lower esophagus may occur. Gastroesophageal reflux can also occur after a large meal, which can sufficiently increase the pressure in the stomach to force acid into the esophagus. It can also cause coughing and irritation of the larynx in the absence of any esophageal symptoms, and it has even been implicated in the onset of asthmatic symptoms in susceptible individuals.

ileocecal valve (or ileocecal sphincter)

sphincter between the ileum (of the small intestine) and the cecum (of the large intestine) composed primarily of circular smooth muscle innervated by sympathetic nerves. The circular muscle contracts with distension of the colon and limits the movement of colonic contents backward into the ileum This prevents bacteria from the large intestine from colonizing the final part of the small intestine.

peristaltic waves in stomach

stomach produces peristaltic waves in response to the arriving food. Each wave begins in the body of the stomach and produces only a ripple as it proceeds toward the antrum; this contraction is too weak to produce much mixing of the luminal contents with acid and pepsin. As the wave approaches the larger mass of wall muscle surrounding the antrum, however, it produces a more powerful contraction, which both mixes the luminal contents and closes the pyloric sphincter. The pyloric sphincter muscles contract upon arrival of a peristaltic wave. As a consequence of the sphincter closing, only a small amount of chyme is expelled into the duodenum with each wave. Most of the antral contents are forced backward toward the body of the stomach. This backward motion of chyme, called retropulsion, generates strong shear forces that helps to disperse the food particles and improve mixing of the chyme.

bile pigments

substances formed from the heme portion of hemoglobin when old or damaged erythrocytes are broken down in the spleen and liver. The predominant bile pigment is bilirubin, which is extracted from the blood by hepatocytes and actively secreted into the bile.

medical response to gall stone

surgery may be necessary to remove an inflamed gallbladder (cholecystectomy) or stones from an obstructed duct, newer techniques use drugs to dissolve gallstones. Patients who have had a cholecystectomy still make bile and transport it to the small intestine via the bile duct. Therefore, fat digestion and absorption can be maintained, but bile secretion and fat intake in the diet are no longer coupled. Thus, large, fatty meals are difficult to digest because of the absence of a large pool of bile normally released from the gallbladder in response to CCK. A diet low in fat content is usually advisable.

serosa

surrounding the outer surface of the tube is a thin layer of connective tissue called the serosa. Thin sheets of connective tissue connect the serosa to the abdominal wall and sup- port the GI tract in the abdominal cavity.

glottis

the area around the vocal cords and the space between them

starch digestion

the digestion of starch by salivary amylase begins in the mouth but accounts for only a small fraction of total starch digestion. It continues very briefly in the upper part of the stomach before gastric acid inactivates the amylase. Most (∼95% or more) starch digestion is completed in the small intestine by pancreatic amylase

rectum

the final segment of the large intestine contractions allow defecation

Vomiting

the forceful expulsion of the contents of the stomach and upper intestinal tract through the mouth.

ulcer drugs

the inhibition of acid secretion can remove the constant irritation and allow the ulcer to heal. Two classes of drugs are potent inhibitors of acid secretion. One class of inhibitors acts by blocking a specific class of histamine receptors (H2) found on parietal cells, which stimulate acid secretion. An example of a commonly used H2 receptor antagonist is cimetidine. The second class of drugs directly inhibits the H+/ K+-ATPase pump in parietal cells. Examples of these so-called proton-pump inhibitors are omeprazole and lansoprazole.

retching

the initial sequence of events of vomiting, can occur repeatedly without expulsion via the mouth begin with a deep breath, closure of the glottis, and elevation of the soft palate. The abdominal muscles then contract, thereby increasing the abdominal pressure, which is transmitted to the stomach's contents. The lower esophageal sphincter relaxes, and the high abdominal pressure forces the contents of the stomach into the esophagus.

bilirubin

the predominant bile pigment extracted from the blood by hepatocytes and actively secreted into the bile. During their passage through the intestinal tract, some of the bile pigments are absorbed into the blood and are eventually excreted in the urine, giving urine its yellow color After entering the intestinal tract, some bilirubin is modified by bacterial enzymes to form the brown pigments that give feces their characteristic color.

lymphatic nodules

the small intestine has regions of immune tissue called lymphatic nodules that contain immune cells these cells secrete factors that alter intestinal motility and kill microorganisms.

lower esophageal sphincter

the smooth muscle in the last portion of the esophagus The ability of the lower esophageal sphincter to maintain a barrier between the stomach and the esophagus when swallowing is not taking place is aided by the fact that the last portion of the esophagus lies below the diaphragm and is subject to the same abdominal pressures as the stomach. In other words, if the pres- sure in the abdominal cavity increases, for example, during cycles of respiration or contraction of the abdominal muscles, the pres- sures on both the gastric contents and the terminal segment of the esophagus are increased together. This prevents the formation of a pressure gradient between the stomach and esophagus that could force the stomach's contents into the esophagus. During pregnancy, the growth of the fetus not only increases the pressure on the abdominal contents but also can push the terminal segment of the esophagus through the diaphragm into the thoracic cavity. The sphincter is therefore no longer assisted by changes in abdominal pressure. Consequently, during the last half of pregnancy, increased abdominal pressure tends to force some of the gastric contents up into the esophagus. subsides in the last weeks of pregnancy prior to delivery, as the uterus descends lower into the pelvis, decreasing the pressure on the stomach.

stomach HCl production

the stomach secretes about 2 L of hydrochloric acid per day. The concentration of H+ in the lumen of the stomach may reach >150 mM, which is 1 to 3 million times greater than the concentration in the blood. This requires an efficient production mechanism to generate large numbers of hydrogen ions. The origin of the hydrogen ions is CO2 in the parietal cell, which contains the enzyme carbonic anhydrase. carbonic anhydrase catalyzes the reaction between CO2 with water to produce carbonic acid, which dissociates to H+ and HCO3- Primary H+/K+ -ATPases in the apical membrane of the parietal cells pump these hydrogen ions into the lumen of the stomach This primary active transporter also pumps K+ into the cell, which then leaks back into the lumen through K+ channels. As H+ is secreted into the lumen, HCO3- is secreted on the opposite side of the cell in exchange for Cl-, which maintains electroneutrality. Removal of the end products (H+ and HCO3-) of this reaction enhances the rate of the reaction by the law of mass action. In this way, production and secretion of H+ are coupled.

esophageal swallowing

the tube that passes through the thoracic cavity, penetrates the diaphragm (which separates the thoracic cavity from the abdominal cavity), and joins the stomach a few centimeters below the diaphragm. Skeletal muscle surrounds the upper third of the esophagus, and smooth muscle surrounds the lower two-thirds. the pressure in the thoracic cavity can be negative relative to atmospheric pressure, and this subatmospheric pressure is transmitted across the thin wall of the intrathoracic portion of the esophagus to the lumen In contrast, the luminal pressure in the pharynx at the opening to the esophagus is equal to atmospheric pressure, and the pressure at the opposite end of the esophagus in the stomach is slightly greater than atmospheric pressure. Therefore, pressure differences could tend to force both air (from above) and gastric contents (from below) into the esophagus. This does not occur, however, because both ends of the esophagus are normally closed by the contraction of sphincter muscles.

lactose intolerance

the vast majority of people undergo a total or partial decline in lactase production beginning at about 2 years of age. a normal condition characterized by inability to completely digest lactose such that its concentration remains high in the small intestine after ingestion. Current hypotheses for why certain populations of people retained the ability to express lactase relate to a mutation in the regulatory region of the lactase gene that occurred around the time certain groups of neolithic humans domesticated cattle as a food source.

large intestine shape/structure

tube about 6.5 cm (2.5 inches) in diameter and about 1.5 m (5 feet) long. Although the large intestine has a greater diameter than the small intestine, its epithelial surface area is far smaller because the large intestine is shorter than the small intestine, its surface is not convoluted, and its mucosa lacks villi found in the small intestine is made up by the colon and rectum

swallowing center

when food pushed to bakc of throat, receives impulses in the medulla oblongata of the brainstem center then elicits swallowing via efferent fibers to the muscles in the pharynx and esophagus as well as to the respiratory muscles. Impulses from the swallowing center inhibit respiration, raise the larynx, and close the glottis, keeping food from moving into the trachea. As the tongue forces the food farther back into the pharynx, the food tilts a flap of tissue, the epiglottis, backward to cover the glottis to prevent aspiration . Both skeletal and smooth muscles are involved, so the swallowing center must direct efferent activity in both somatic nerves (to skeletal muscle) and autonomic nerves (to smooth muscle).

small intestine

where most digestion and absorption occurs a tube about 2.4 cm in diameter and 3 m in length, which leads from the stomach to the large intestine. (The small intestine is almost twice as long if removed from the abdomen because the muscular wall loses its tone.) Hydrolytic enzymes in the small intestine break down molecules of intact or partially digested carbohydrates, fats, proteins, and nucleic acids into monosaccharides, fatty acids, amino acids, and nucleotides. Some of these enzymes are on the apical membranes of the intestinal lining cells, whereas others are secreted by the pancreas and enter the intestinal lumen. The products of digestion are absorbed across the epithelial cells and enter the blood and/or lymph. Vita- mins, minerals, and water, which do not require enzymatic diges- tion, are also absorbed in the small intestine.