FR4-GUT 3

Question 1

Explain how energy-dependent Na⁺ absorption drives passive H₂O absorption

Answer 1

• Na⁺ may be absorbed both passively and actively. When the electrochemical gradient favors movement of Na⁺ from the lumen to the blood, passive diffusion of Na1 can occur by paracellular transport between the intestinal epithelial cells through the “leaky” tight junctions into the interstitial fluid within the villus. Movement of Na⁺ through the cells is energy dependent and involves different carriers or channels at the luminal and basolateral membranes
• Na⁺ reabsorption across the kidney tubules. Na⁺ enters the epithelial cells across the luminal border either by itself passively through Na1 channels or in the company of another ion or a nutrient molecule by secondary active transport via three different carriers:
• Na⁺–Cl₂ symporter, Na⁺–H⁺ antiporter, or Na⁺–glucose (or amino acid) symporter
• Na⁺ is actively pumped out of the cell by the Na⁺–K⁺ pump at the basolateral membrane into the interstitial fluid in the lateral spaces between the cells where they are not joined by tight junctions. From the interstitial fluid, Na1 diffuses into the capillaries.
• Most H₂O absorption in the digestive tract depends on the active carrier that pumps Na1 into the lateral spaces, resulting in a concentrated area of high osmotic pressure in that localized region between the cells. This localized high osmotic pressure induces H2O to move from the lumen through the cell (and possibly from the lumen through the leaky tight junction) into the lateral space.
• Water entering the space reduces the osmotic pressure but raises the hydrostatic (fluid) pressure. The elevated hydrostatic pressure flushes H2O out of the lateral space into the interior of the villus, where it is picked up by the capillary network. Meanwhile, more Na1 is pumped into the lateral space to encourage more H2O absorption.

Question 2

Explain carbohydrate absorbtion

Answer 2

• Dietary carbohydrates are presented to the small intestine for absorption mainly in the forms of the disaccharides maltose, sucrose, and lactose (and to a lesser extent in the form of the short polysaccharide a-limit dextrins)
• The disaccharidases located in the brushborder membrane of the small intestine cells further reduce these disaccharides and polysaccharides into the absorbable monosaccharide units of glucose (mostly), galactose, and fructose. Glucose and galactose are both absorbed by secondary active transport, in which symport carriers, such as the sodium and glucose cotransporter on the luminal membrane transport both the monosaccharide and Na⁺ from the lumen into the interior of the intestinal cell
• The operation of these symporters, which do not directly use energy themselves, depends on the Na1 concentration gradient established by the energy-consuming basolateral Na⁺–K⁺pump Glucose (or galactose), having been concentrated in the cell by these symporters, leaves the cell down its concentration gradient by facilitated diffusion (passive carrier-mediated transport via the glucose transporter GLUT-2 in the basal border to enter the blood within the villus.
• In addition to glucose being absorbed through the cells by means of the symporter, recent evidence suggests that a significant amount of glucose crosses the epithelial barrier through the leaky tight junctions between the epithelial cells
• Fructose enters the epithelial cells from the lumen via GLUT-5 using facilitated diffusion. This process involves the higher concentration of luminal fructose driving the monosaccharide into the cell. Like the other monosaccharides, fructose exits via GLUT-2 and enters the blood

Question 3

Explain protein absorption

Answer 3

• Both ingested proteins and endogenous (within the body) proteins that have entered the digestive tract lumen from the following sources are digested and absorbed:
  1. Digestive enzymes, all of which are proteins, that have been secreted into the lumen
  2. Proteins within the cells that are pushed off from the villi into the lumen during the process of mucosal turnover
  3. Small amounts of plasma proteins that normally leak from the capillaries into the digestive tract lumen
• About 20 to 40 g of endogenous proteins enter the lumen each day from these three sources. This quantity can amount to more than the quantity of proteins in ingested food. All endogenous proteins must be digested and absorbed, along with the dietary proteins, to prevent depletion of the body’s protein stores.
• The amino acids absorbed from both food and endogenous proteins are used primarily to synthesize new proteins in the body. The proteins presented to the small intestine for absorption are in the form of amino acids and a few small peptide fragments. Amino acids are absorbed into the intestinal cells by symporters, similar to glucose and galactose absorption.
• The sugar symporters are distinct from the amino-acid symporters, and the amino-acid symporters are selective for different amino acids. Small peptides gain entry by means of yet another Na+-dependent carrier in a process known as tertiary active transport (tertiary meaning “third,” in reference to a third linked step ultimately being driven by energy used in the first step).
• In this case, the symporter simultaneously transports both H+ and the peptide from the lumen into the cell, driven by H+ moving down its concentration gradient and the peptide moving against its concentration gradient
• The H1 gradient is established by an antiporter in the luminal membrane that is driven by Na+ moving into the cell down its concentration gradient and H+ moving out of the cell against its concentration gradient. The Na+ concentration gradient that drives the antiporter in turn is established by the energy-dependent Na+–K+ pump at the basolateral membrane. Thus, glucose, galactose, amino acids, and small peptides all get a “free ride” in on the energy expended for Na1 transport.
• The small peptides are broken down into their constituent amino acids by the aminopeptidases in the brush-border membrane or by intracellular peptidases. Like monosaccharides, amino acids leave the intestinal cells by facilitated diffusion and enter the capillary network within the villus.

Question 4

Digested fat is absorbed passively and enters the lymph

Answer 4

• Fat absorption is different from carbohydrate and protein absorption because the insolubility of fat in water presents a special problem.
• Fat must be transferred from the watery chyme through the watery body fluids, even though fat is not water soluble.
• Therefore, fat must undergo a series of physical and chemical transformations to circumvent this problem during its digestion and absorption

Question 5

Describe fat emulsification and digestion

Answer 5

• When the stomach contents are emptied into the duodenum, the ingested fat is aggregated into large, oily triglyceride droplets that float in the chyme.
• Recall that through the bile salts’ detergent action in the small-intestine lumen, the large droplets are dispersed into a lipid emulsification of small droplets, exposing a greater surface area of fat for digestion by pancreatic lipase
• The products of lipase digestion (monoglycerides and free fatty acids; step 2 ) are also not very water soluble, so little of these end products of fat digestion can diffuse through the aqueous chyme to reach the absorptive lining
• However, biliary components facilitate absorption of these fatty end products by forming micelles.

Question 6

Describe fat absorption

Answer 6

• Remember that micelles are water-soluble particles that can carry the end products of fat digestion within their lipid-soluble interiors
• Once these micelles reach the luminal membranes of the epithelial cells, the monoglycerides and free fatty acids passively diffuse from the micelles through the lipid component of the epithelial cell membranes to enter the interior of these cells
• Bile salts continuously repeat their fat-solubilizing function down the length of the small intestine until all fat is absorbed. Then the bile salts themselves are reabsorbed in the terminal ileum by special active transport. This is an efficient process because relatively small amounts of bile salts can facilitate digestion and absorption of large amounts of fat, with each bile salt performing its ferrying function repeatedly before it is reabsorbed. Once within the interior of the epithelial cells, the monoglycerides and free fatty acids are resynthesized into triglycerides
• These triglycerides conglomerate into droplets and are coated with a layer of lipoprotein (synthesized by the endoplasmic reticulum of the epithelial cell), which makes the fat droplets water soluble The large, coated fat droplets, known as chylomicrons, are extruded by exocytosis from the epithelial cells into the interstitial fluid within the villus
• Chylomicrons are 75 to 500 nm in diameter, compared to micelles, which are 3 to 10 nm in diameter. The chylomicrons subsequently enter the central lacteals rather than the capillaries because of the structural differences between these two vessels Capillaries have a basement membrane (an outer layer of polysaccharides) that prevents the chylomicrons from entering, but the lymph vessels do not have this barrier. Thus, fat can be absorbed into the lymphatics but not directly into the blood
• The actual absorption of monoglycerides and free fatty acids from the chyme across the luminal membrane of the small-intestine epithelial cells is traditionally considered a passive process because the lipid-soluble fatty end products merely dissolve in and pass through the lipid part of the membrane. However, the overall sequence of events needed for fat absorption requires energy
• For example, bile salts are actively secreted by the liver, the resynthesis of triglycerides and formation of chylomicrons within the epithelial cells are active processes, and the exocytosis of chylomicrons requires energy

Question 7

Describe vitamin absorption

Answer 7

• Water-soluble vitamins are primarily absorbed passively with water, whereas fat-soluble vitamins are carried in the micelles and absorbed passively with the end products of fat digestion.
• Some of the vitamins can also be absorbed by carriers, if necessary.
• Vitamin B₁₂ is unique in that it must be in combination with gastric intrinsic factor for absorption by receptormediated endocytosis in the terminal ileum

Question 8

Describe how calcium absorption is regulated

Answer 8

• In contrast to the almost complete, unregulated absorption of other ingested electrolytes, dietary iron and calcium may not be absorbed completely because their absorption is subject to regulation, depending on the body’s needs for these electrolytes.
• Normally, only enough iron and calcium are actively absorbed into the blood to maintain the homeostasis of these electrolytes, with excess ingested quantities being lost in the feces.

Question 9

Explain iron absorption

Answer 9

• Iron is essential for hemoglobin production. The normal iron intake is typically 15 to 20 mg/day, yet a man usually absorbs about 0.5 to 1 mg/day into the blood, and a woman takes up slightly more, at 1.0 to 1.5 mg/day. (Women need more iron because they periodically lose iron in menstrual blood flow.) Two main steps are involved in absorption of iron into the blood:
  (1) absorption of iron from the lumen into the smallintestine epithelial cells
  (2) absorption of iron from the epithelial cells into the blood
• Iron is actively transported from the lumen into the epithelial cells, with women having about four times more activetransport sites for iron than men
• The extent to which ingested iron is taken up by the epithelial cells depends on the type of iron consumed. Dietary iron exists in two forms: heme iron, in which iron is bound as part of a heme group found in hemoglobin and is present in meat, and inorganic iron, which is present in plants. Dietary heme is absorbed more efficiently than inorganic iron is.
• Dietary heme is absorbed more efficiently than inorganic iron is. Dietary inorganic iron exists primarily in the oxidized ferric iron (Fe3+) form, but the reduced ferrous iron (Fe2+) form is absorbed more easily. Dietary Fe31 is reduced to Fe2+ by a membrane-bound enzyme at the luminal membrane before absorption.
• The presence of other substances in the lumen can either promote or reduce iron absorption. For example, vitamin C increases iron absorption, primarily by reducing Fe3+ to Fe2+. Phosphate and oxalate, in contrast, combine with ingested iron to form insoluble iron salts that cannot be absorbed.
• lts that cannot be absorbed. Heme iron and Fe2+ are transported across the luminal membrane by separate energy-dependent carriers in the brush border: Heme iron enters the intestinal cell by heme carrier protein 1 and Fe2+ is carried in via divalent metal transporter 1, which also transports other metals that have a valence of +2. An enzyme within the cell frees iron from the heme complex

Question 10

After absorption into the small-intestine epithelial cells, iron has two possible fates:

Answer 10

1. Iron needed immediately for production of red blood cells is absorbed into the blood for delivery to the bone marrow, the site of red blood cell production.

Iron exits the smallintestine epithelial cell via a membrane iron transporter known as ferroportin.

Iron absorption is largely controlled by a recently discovered hormone, hepcidin, which is released from the liver when iron levels in the body become too high. Hepcidin prevents further iron export from the small-intestine epithelial cell into the blood by binding with ferroportin and promoting its internalization into the cell by endocytosis and its subsequent degradation by lysosomes.

Thus, hepcidin is the primary regulator of iron homeostasis. A deficiency of hepcidin leads to tissue iron overload because ferroportin continues to transfer iron into the body without control.

Iron that exits the small-intestine epithelial cell is transported in the blood by a plasma protein carrier known as transferrin. The absorbed iron is then used in the synthesis of hemoglobin for the newly produced red blood cells.

2. Iron not immediately needed is irreversibly stored within the small-intestine epithelial cells in a granular form called ferritin, which cannot be absorbed into the blood. Iron stored as ferritin is lost in the feces within three days as the epithelial cells containing these granules are sloughed off during mucosal regeneration.

Large amounts of iron in the feces give them a dark, almost black color.

Question 11

Describe calcium absorption

Answer 11

• The amount of calcium (Ca2+) absorbed is also regulated. Calcium enters the luminal membrane of the small-intestine epithelial cells down its electrochemical gradient through a specialized Ca2+ channel; is ferried within the cell by a Ca2+-binding protein, calbindin; and exits the basolateral membrane by two energy-dependent mechanisms: a primary active transport Ca2+ ATPase pump and a secondary active transport Na1–Ca2+ antiporter
• Vitamin D greatly enhances all of these steps in Ca2+absorption. Vitamin D can exert this effect only after it has been activated in the liver and kidneys, a process that is enhanced by parathyroid hormone.
• Appropriately, secretion of parathyroid hormone increases in response to a fall in Ca2+ concentration in the blood. Normally, of the average 1000 mg of Ca2+ taken in daily, only about two thirds is absorbed in the small intestine, with the rest passing out in the feces.

Question 12

Explain how extensive absorption by the small intestine keeps pace with secretion

Answer 12

• The small intestine normally absorbs about 9 liters of fluid per day in the form of H₂O and solutes, including the absorbable units of nutrients, vitamins, and electrolytes. How can that be, when humans normally ingest only about 1250 mL of fluid and consume 1250 g of solid food (80% of which is H₂O) per day
• Each day, about 9500 mL of H2O and solutes enter the small intestine. Note that of this 9500 mL, only 2500 mL are ingested from the external environment. The remaining 7000 mL (7 liters) of fluid are digestive juices derived from the plasma. Recall that plasma is the ultimate source of digestive secretions because the secretory cells extract from the plasma the necessary raw materials for their secretory product.
• Considering that the entire plasma volume is only about 2.75 liters, absorption must closely parallel secretion to keep the plasma volume from falling sharply. Of the 9500 mL of fluid entering the small-intestine lumen per day, about 95%, or 9000 mL of fluid, is normally absorbed by the small intestine back into the plasma, with only 500 mL of the small-intestine contents passing on into the colon.
• Thus, the body normally does not lose the digestive juices. After the constituents of the juices are secreted into the digestive tract lumen and perform their function, they are returned to the plasma. The only secretory product that escapes from the body is bilirubin, a waste product that must be eliminated.

Question 13

The causes of diarrhea are as follows:

Answer 13

1. The most common cause of diarrhea is excessive smallintestinal motility, which arises either from local irritation of the gut wall by bacterial or viral infection of the small intestine or from emotional stress. Rapid transit of the small-intestine contents does not allow enough time for adequate absorption of fluid to occur.
2. Diarrhea also occurs when excess osmotically active particles, such as those found in lactase deficiency, are present in the digestive tract lumen. These particles cause excessive fluid to enter and be retained in the lumen, thus increasing the fluidity of the feces.
3. Toxins of the bacterium Vibrio cholera (the causative agent of cholera) and certain other microorganisms promote the secretion of excessive amounts of fluid by the small-intestine mucosa, resulting in profuse diarrhea. Diarrhea produced in response to toxins from infectious agents is the leading cause of death of small children in developing nations.

Question 14

The large intestine consists of

Answer 14

• the colon, cecum, appendix, and rectum
• The cecum forms a blind-ended pouch below the junction of the small and large intestines at the ileocecal valve. The small, fingerlike projection at the bottom of the cecum is the appendix, a lymphoid tissue that houses lymphocytes
• The colon, which makes up most of the large intestine, is not coiled like the small intestine but consists of three relatively straight parts—the ascending colon, the transverse colon, and the descending colon
• ding colon, the transverse colon, and the descending colon. The end part of the descending colon becomes S shaped, forming the sigmoid colon (sigmoid means “S shaped”), and then straightens out to form the rectum (meaning “straight”)

Question 15

The large intestine is primarily a drying and storage organ

Answer 15

• The colon normally receives about 500 mL of chyme from the small intestine each day.
• Because most digestion and absorption have been accomplished in the small intestine, the contents delivered to the colon consist of indigestible food residues (such as cellulose), unabsorbed biliary components, and the remaining fluid.
• The colon extracts more H₂O and salt, drying and compacting the contents to form a firm mass known as feces for elimination from the body.
• The primary function of the large intestine is to store feces before defecation.
• Cellulose and other indigestible substances in the diet provide bulk and help maintain regular bowel movements by contributing to the volume of the colonic contents.

Question 16

Haustral contractions slowly shuffle the colonic contents back and forth

Answer 16

• The outer longitudinal smooth muscle layer does not completely surround the large intestine. Instead, it consists only of three separate, conspicuous, longitudinal bands of muscle, the taeniae coli, which run the length of the large intestine.
• These taeniae coli are shorter than the underlying circular smooth muscle and mucosal layers would be if these layers were stretched out flat. Because of this, the underlying layers are gathered into pouches or sacs called haustra, much as the material of a full skirt is gathered at the narrower waistband.
• The haustra are not merely passive permanent gathers, however; they actively change location as a result of contraction of the circular smooth muscle layer. Most of the time, movements of the large intestine are slow and nonpropulsive, as is appropriate for its absorptive and storage functions.
• These contractions, which throw the large intestine into haustra, are oscillating ringlike contractions similar to small-intestine segmentations but occur less frequently. Thirty minutes may elapse between haustral contractions, whereas segmentation contractions in the small intestine occur at rates of between 9 and 12 per minute.
• The location of the haustral sacs gradually changes as a relaxed segment that has formed a sac slowly contracts while a previously contracted area simultaneously relaxes to form a new sac.
• These movements are nonpropulsive; they slowly shuffle the contents in a back-and-forth mixing movement that exposes the colonic contents to the absorptive mucosa. Haustral contractions are largely controlled by locally mediated reflexes involving the intrinsic plexuses.

Question 17

Describe mass movements propel feces long distances

Answer 17

• Three to four times a day, generally after meals, a marked increase in motility takes place during which large segments of the ascending and transverse colon contract simultaneously, driving the feces one third to three fourths of the length of the colon in a few seconds.
• These massive contractions, appropriately called mass movements, drive the colonic contents into the distal part of the large intestine, where material is stored until defecation occurs.
• When food enters the stomach, mass movements are triggered in the colon primarily by the gastrocolic reflex, which is mediated from the stomach to the colon by gastrin and by parasympathetic innervation.
• In many people, this reflex is most evident after the first meal of the day and is often followed by the urge to defecate
• The gastroileal reflex moves the remaining small-intestine contents into the large intestine, and the gastrocolic reflex pushes the colonic contents into the rectum, triggering the defecation reflex

Question 18

Explain how feces are eliminated by the defecation reflex.

Answer 18

• When mass movements move feces into the rectum, the resultant distension of the rectum stimulates stretch receptors in the rectal wall, initiating the defecation reflex.
• This reflex causes the internal anal sphincter (which is smooth muscle) to relax and the rectum and sigmoid colon to contract more vigorously.
• If the external anal sphincter (which is skeletal muscle) is also relaxed, defecation occurs.
• Being skeletal muscle, the external anal sphincter is under voluntary control.

Question 19

Explain why constipation occurs when the feces become too dry.

Answer 19

• If defecation is delayed too long, constipation may result. When colonic contents are retained for longer periods than normal, more than the usual amount of H₂O is absorbed from the feces, so they become hard and dry.
• Normal variations in frequency of defecation among individuals range from after every meal to up to once a week. When the frequency is delayed beyond what is normal for a particular person, constipation and its attendant symptoms may occur.
• Possible causes for delayed defecation that might lead to constipation include:
• (1) ignoring the urge to defecate; (2) decreased colon motility accompanying aging, emotion, or a low-bulk diet; (3) obstruction of fecal movement in the large bowel caused by a local tumor or colonic spasm; and (4) impairment of the defecation reflex, such as through injury of the nerve pathways involved.
• If hardened fecal material becomes lodged in the appendix, it may obstruct normal circulation and mucus secretion in this narrow, blind-ended appendage

Question 20

Occasionally, instead of feces passing from the anus, intestinal gas, or flatus, passes out. This gas is derived from two sources:

Answer 20

(1) swallowed air (as much as 500 mL of air may be swallowed during a meal) and (2) gas produced by bacterial fermentation in the colon.

Question 21

Explain why the colon contains myriad beneficial bacteria

Answer 21

• Because of slow colonic movement, bacteria have time to grow and accumulate in the large intestine.
• Not all ingested bacteria are destroyed by antimicrobial agents earlier in the digestive tract; the surviving bacteria continue to thrive in the large intestine.
• About 10 times more bacteria live in the human colon than the human body has cells.
• Collectively, the community of microbes that coexist peacefully and usefully with their human host is called the microbiota, and the aggregate collection of genomes of our microbiota is known as the microbiome.

Question 22

The indigenous colonic microbes:

Answer 22

1. Promote colonic motility
2. Help maintain colonic mucosal integrity
3. Aid immune function
4. Compete with potentially pathogenic microbes for nutrients and space
5. Help digest food and make nutritional contributions
6. Influence the brain and behavior

Question 23

Explain how the large intestine absorbs salt and water, converting the luminal contents into feces.

Answer 23

• Some absorption takes place within the colon but not to the same extent as in the small intestine. Because the luminal surface of the colon is fairly smooth, it has considerably less absorptive surface area than the small intestine. Furthermore, the colon is not equipped with extensive specialized transport mechanisms like the small intestine.
• When excessive smallintestine motility delivers the contents to the colon before absorption of nutrients has been completed, the colon cannot absorb most of these materials and they are lost in diarrhea. The colon normally absorbs salt and H₂O. Na+ is actively absorbed, Cl₂ follows passively down the electrical gradient, and H₂O follows osmotically.
• The colon absorbs token amounts of other electrolytes, as well as the short-chain fatty acids and vitamins produced by colonic bacteria. Through absorption of salt and H₂O, a firm fecal mass is formed
• This fecal material normally consists of 100 g of H₂O and 50 g of solid, including undigested cellulose, bilirubin, bacteria, and small amounts of salt.
• Thus, contrary to popular thinking, the digestive tract is not a major excretory passageway for eliminating wastes from the body. The main waste product excreted in the feces is bilirubin
• The other fecal constituents are unabsorbed food residues and bacteria, which were never actually a part of the body. Bacteria account for nearly one third of the dry weight of feces

Question 24

State the three major GI hormones:

Answer 24

• gastrin, secretin, and CCK
• They perform their functions by binding to G-protein-coupled receptors on the plasma membrane of their target cells, thereby activating second-messenger pathways that bring about the desired responses

Question 25

What is GIP?

How does it function?

Answer 25

A more recently identified GI hormone, glucose-dependent insulinotropic peptide (GIP)

By binding to G-protein-coupled receptors on the plasma membrane of their target cells, thereby activating second-messenger pathways that bring about the desired responses

Question 26

Gastrin Protein in the stomach stimulates the release of gastrin, which performs the following functions

Answer 26

1. It acts in multiple ways to increase secretion of HCl and pepsinogen, two substances of primary importance in initiating digestion of the protein that promoted their secretion.
2. It enhances gastric motility, stimulates ileal motility, relaxes the ileocecal sphincter, and induces mass movements in the colon—all functions aimed at keeping the contents moving through the tract on arrival of a new meal.
3. It also is trophic to both the stomach mucosa and the small-intestine mucosa, helping maintain a well-developed, functionally viable digestive tract lining

Predictably, gastrin secretion is inhibited by an accumulation of acid in the stomach and by the presence in the duodenal lumen of acid and other constituents that necessitate a delay in gastric secretion

Question 27

Secretin As the stomach empties into the duodenum, the presence of acid in the duodenum stimulates the release of secretin, which performs the following interrelated functions:

Answer 27

1. It inhibits gastric emptying to prevent further acid from entering the duodenum until the acid already present is neutralized.
2. It inhibits gastric secretion to reduce the amount of acid being produced.
3. It stimulates the pancreatic duct cells to produce a large volume of aqueous NaHCO3 secretion, which is emptied into the duodenum to neutralize the acid. Neutralization of the acidic chyme in the duodenum helps prevent damage to the duodenal walls and provides a suitable environment for optimal functioning of the pancreatic digestive enzymes, which are inhibited by acid.
4. Secretin and CCK are both trophic to the exocrine pancreas

Question 28

As chyme empties from the stomach, fat and other nutrients enter the duodenum. These nutrients—especially fat and, to a lesser extent, protein products—cause the release of CCK, which performs the following interrelated functions:

Answer 28

1. It inhibits gastric motility and secretion, thereby allowing adequate time for the nutrients already in the duodenum to be digested and absorbed.
2. It stimulates the pancreatic acinar cells to increase secretion of pancreatic enzymes, which continue the digestion of these nutrients in the duodenal lumen (this action is especially important for fat digestion because pancreatic lipase is the only enzyme that digests fat).
3. It causes contraction of the gallbladder and relaxation of the sphincter of Oddi so that bile is emptied into the duodenum to aid fat digestion and absorption. Bile salts’ detergent action is particularly important in enabling pancreatic lipase to perform its digestive task. Again, the multiple effects of CCK are remarkably well adapted to dealing with the fat whose presence in the duodenum triggered this hormone’s release.
4. Besides facilitating digestion of ingested nutrients, CCK is an important regulator of food intake. It plays a key role in satiety, the sensation of having had enough to eat

Question 29

GIP A more recently recognized hormone released by the duodenum, GIP, helps promote metabolic processing of the nutrients once they are absorbed

Answer 29

• This hormone was originally named gastric inhibitory peptide (GIP) for its presumed role as an enterogastrone.
• It was believed to inhibit gastric motility and secretion, similar to secretin and CCK.
• Its contribution in this regard is now considered minimal. Instead, this hormone stimulates insulin release by the pancreas, so it is now called glucose-dependent insulinotropic peptide (again, GIP).
• This action is remarkably adaptive. As soon as the meal is absorbed, the body has to shift its metabolic gears to use and store the newly arriving nutrients.
• The metabolic activities of this absorptive phase are largely under the control of insulin.
• Stimulated by the presence of a meal, especially glucose, in the digestive tract, GIP initiates the release of insulin in anticipation of absorption of the meal, in a feedforward fashion. Insulin is especially important in promoting the uptake and storage of glucose.