FR3-GUT 3

Question 1

Explain how Carbohydrate digestion continues in the body of the stomachand protein digestion begins in the antrum

Answer 1

• Two separate digestive processes take place within the stomach.
• In the body of the stomach, food remains in a semisolid mass because peristaltic contractions in this region are too weak for mixing to occur.
• Because food is not mixed with gastric secretions, little protein digestion occurs here.
• In the interior of the mass, however, carbohydrate digestion continues under the influence of salivary amylase.
• Even though acid inactivates salivary amylase, the unmixed interior of the food mass is free of acid.
• Chemical digestion by the gastric juice itself occurs in the antrum of the stomach, where the food is thoroughly mixed with pepsin and HCl via retropulsion.

Question 2

TRUE or FALSE:

No food or water is absorbed into the blood through the stomach mucosa

Question 3

Give examples of substances absorbed by the gastsrisc mucosa

Answer 3

• Alcohol is somewhat lipid soluble, so it can diffuse through the lipid membranes of the epithelial cells that line the stomach and can enter the blood through the submucosal capillaries
• acetylsalicylic acid (aspirin)

Question 4

TRUE or FALSE:

The pancreas only contains endocrine tissue

Answer 4

False

This mixed gland contains both exocrine and endocrine tissue.

Question 5

What does the exocrine parts of the pancreas consist of?

What does the endocrine part of the stomach consist of?

Answer 5

• The exocrine part of the pancreas consists of grape like clusters of secretory cells that form sacs known as acini, which connect to ducts that eventually empty into the duodenum.
• The smaller endocrine part consists of isolated islands of endocrine tissue, the islets of Langerhans, which are dispersed throughout the pancreas. The most important hormones secreted by the islet cells are insulin and glucagon

Question 6

The exocrine pancreas secretes a pancreatic juice consisting of two components:

Answer 6

(1) pancreatic enzymes actively secreted by the acinar cells that form the acini
(2) an aqueous alkaline solution actively secreted by the duct cells that line the pancreatic ducts. The aqueous (watery) alkaline component is rich in sodium bicarbonate (NaHCO₃).

Question 7

Where are pancreatic enzymes stored?

Answer 7

Like pepsinogen, pancreatic enzymes are stored within zymogen granules (secretory vesicles) after being produced and then are released by exocytosis as needed

Question 8

The acinar cells secrete three types of pancreatic enzymes capable of digesting all three categories of food-stuffs:

Answer 8

(1) proteolytic enzymes for protein digestion
(2) pancreatic amylase for carbohydrate digestion
(3) pancreatic lipase for fat digestion

Pancreatic enzymes can almost completely digest food in the absence of all other digestive secretions.

Question 9

What are the three main pancreatic proteolytic enzymes?

Are they secreted in an active or inactive form?

Answer 9

• trypsinogen
• chymotrypsinogen
• procarboxypeptidase

Question 10

Explain the activation of trypsinogen

What defence does the pancreas employ in the event of spontaneous activation of trypsinogen?

Answer 10

• When trypsinogen is secreted into the duodenal lumen, it is activated to its active enzyme form, trypsin, by enteropeptidase (formerly known as enterokinase), an enzyme embedded in the luminal membrane of the cells that line the duodenal mucosa
• Like pepsinogen, trypsinogen must remain inactive within the pancreas to prevent this proteolytic enzyme from digesting the proteins of the cells in which it is formed.
• Trypsinogen remains inactive, therefore, until it reaches the duodenal lumen, where enteropeptidase triggers the activation process. Trypsin then autocatalytically activates more trypsinogen.
• As further protection, the pancreas also produces a chemical known as trypsin inhibitor, which blocks trypsin’s actions if spontaneous activation of trypsinogen inadvertently occurs within the pancreas.

Question 11

What protects against digestion of the small-intestine wall by the activated proteolytic enzymes?

Answer 11

Mucus secreted by intestinal cells

Question 12

Describe the function of pancreatic amylase

Answer 12

• Like salivary amylase, pancreatic amylase contributes to carbohydrate digestion by converting dietary starches (amylose and amylopectin) into the disaccharide maltose and the branched polysaccharide a-limit dextrins.
• Amylase is secreted in the pancreatic juice in an active form because active amylase does not endanger the secretory cells. These cells do not contain any polysaccharides.

Question 13

Describe the function of pancreatic lipase

Answer 13

• Pancreatic lipase is extremely important because it is the only enzyme secreted throughout the entire digestive system that can digest fat. (In humans, insignificant amounts of lipase are secreted in the saliva and gastric juice—lingual lipase and gastric lipase.)
• Pancreatic lipase hydrolyzes dietary triglycerides into monoglycerides and free fatty acids, which are the absorbable units of fat
• Like amylase, lipase is secreted in its active form because there is no risk of pancreatic self-digestion by lipase. Triglycerides are not a structural component of pancreatic cells.

Question 14

What happens when pancreatic enzymes are deficient?

Answer 14

• When pancreatic enzymes are deficient, digestion of food is incomplete. Because the pancreas is the only significant source of lipase, pancreatic enzyme deficiency results in serious maldigestion and malabsorption of dietary fat.
• The main clinical manifestation of pancreatic exocrine insufficiency is steatorrhea, or excessive undigested fat in the feces. Up to 60% to 70% of the ingested fat may be excreted in the feces.
• Digestion of protein and carbohydrates is impaired to a lesser degree because salivary, gastric, and small-intestinal enzymes contribute to the digestion of these two foodstuffs.

Question 15

Pancreatic enzymes function best in which environments?

Answer 15

• a neutral or slightly alkaline environment, yet the highly acidic gastric contents empty into the duodenum in the vicinity of pancreatic enzyme entry into the duodenum.
• This acidic chyme must be neutralized quickly in the duodenal lumen, not only to allow optimal functioning of the pancreatic enzymes but also to prevent acid damage to the duodenal mucosa. The alkaline (NaHCO3- rich) fluid secreted by the pancreatic duct cells into the duodenum serves the important function of neutralizing the acidic chyme that empties into the duodenum from the stomach.
• This aqueous NaHCO3 secretion is by far the largest component of pancreatic secretion.
• The volume of pancreatic secretion ranges between 1 and 2 liters per day, depending on the type and degree of stimulation.

Question 16

Desccribe the mechnaism of NaHCO₃ secretion

Answer 16

• Most of the to-be-secreted HCO₃^- enters the pancreatic duct cell by means of a Na⁺–HCO₃ symporter in the basolateral membrane, but some is generated within the duct cell through carbonic-anydrase catalyzed formation of HCO₃^- and H⁺ from H₂O and CO₂.
• HCO₃ ^- is secreted from the pancreatic duct cell into the pancreatic duct lumen by two avenues: via a HCO₃^-–Cl^- antiporter and through a CFTR channel, both in the luminal membrane. Na1 diffuses down its electrochemical gradient through “leaky” tight junctions between the pancreatic duct cells to complete NaHCO3 secretion.

Question 17

Pancreatic exocrine secretion is regulated by

Answer 17

secretin and CCK

The release of the two major enterogastrones, secretin and CCK, in response to chyme in the duodenum plays the central role in controlling pancreatic exocrine secretion

Question 18

Explain the role of Secretin in Pancreatic Secretion

Answer 18

• The primary stimulus specifically for secretin release into the blood from the duodenal mucosa is acid in the duodenal lumen. The blood carries secretin to the pancreas, where it stimulates the duct cells to markedly increase their secretion of a NaHCO₃^- rich aqueous fluid into the duodenum.
• It is appropriate that the most potent stimulus for secretin release is acid in the small intestine lumen because the resulting alkaline pancreatic secretion neutralizes the acid.
• The amount of secretin released is proportional to the amount of acid that enters the duodenum, so the amount of NaHCO₃ secreted parallels duodenal acidity

Question 19

Explain the role of CCK in Pancreatic Secretion

Answer 19

• CCK is important in regulating pancreatic digestive enzyme secretion.
• The main stimulus for CCK release into the blood from the duodenal mucosa is the presence of fat and, to a lesser extent, products of protein digestion in the lumen.
• The blood transports CCK to the pancreas where it stimulates the pancreatic acinar cells to increase digestive enzyme secretion. Among these enzymes are pancreatic lipase and the proteolytic enzymes, which appropriately further digest the fat and protein that initiated the response.
• In contrast to fat and protein, carbohydrate does not directly influence pancreatic digestive enzyme secretion

Question 20

How are all the pancreatic enzymes are released together during exocytosis?

Answer 20

• All three types of pancreatic digestive enzymes are packaged together in the zymogen granules, so all the pancreatic enzymes are released together during exocytosis.
• Therefore, even though the total amount of enzymes released varies depending on the type of meal consumed (the most being secreted in response to fat), the proportion of enzymes released does not vary on a meal-to-meal basis.
• That is, a high-protein meal does not cause the release of a greater proportion of proteolytic enzymes

Question 21

Besides pancreatic juice, what is the other secretory product emptied into the duodenal lumen?

Answer 21

Bile

Question 22

What does the biliary system include?

Answer 22

The liver, the gallbladder, and associated ducts.

Question 23

Describe how Bile is continuously secreted by the liver and is diverted to the gallbladder between meals

Answer 23

• The liver continuously secretes bile, even between meals. The opening of the bile duct into the duodenum is guarded by the sphincter of Oddi, which prevents bile from entering the duodenum except during digestion of meals
• When this sphincter is closed, bile secreted by the liver hits the closed sphincter and is diverted back up into the gallbladder, a small, saclike structure tucked beneath but not directly connected to the liver. Thus, bile is not transported directly from the liver to the gallbladder. Bile is subsequently stored and concentrated in the gallbladder between meals
• After a meal, bile enters the duodenum as a result of the combined effects of relaxation of the sphincter of Oddi, gallbladder contraction, and increased bile secretion by the liver. The amount of bile secreted per day ranges from 250 mL to 1 liter, depending on the degree of stimulation.

Question 24

Bile salts are recycled through

Answer 24

the enterohepatic circulation

Question 25

How do Bile salts aid fat digestion?

Answer 25

• Through their detergent action (emulsification) and facilitate fat absorption by participating in the formation of micelles.
• Both functions are related to the structure of bile salts.

Question 26

Describe the Detergent Action of Bile Salts

Answer 26

• The term detergent action refers to bile salts’ ability to convert large fat globules into a lipid emulsion consisting of many small fat droplets suspended in the aqueous chyme.
• Breaking up the large fat globule into small, stabilized droplets increases the surface area available for attack by pancreatic lipase.
• To digest fat, lipase must come into direct contact with the triglyceride molecule.
• Because triglycerides are not soluble in water, they tend to aggregate into large droplets in the watery environment of the small-intestine lumen.
• If bile salts did not emulsify these large droplets, lipase could act on the triglyceride molecules only at the surface of the large droplets, and fat digestion would be greatly prolonged.

Question 27

Explain the structure of a bile salt molecule

Answer 27

A bile salt molecule contains a lipid-soluble part (a steroid derived from cholesterol) plus a negatively charged, water-soluble part.

Question 28

How do Bile salts adsorb on the surface of a fat droplet?

Answer 28

• the lipidsoluble part of the bile salt dissolves in the fat droplet, leaving the charged water-soluble part projecting from the surface of the droplet
• Intestinal mixing movements break up large fat droplets into smaller ones. These small droplets would quickly recoalesce were it not for bile salts adsorbing on their surface and creating a shell of water-soluble negative charges on the surface of each little droplet.
• Because like charges repel, these negatively charged groups on the droplet surfaces cause the small fat droplets to repel one another

Question 29

Explain the formation of micelles

Answer 29

• Bile salts— along with cholesterol and lecithin, which are also constituents of bile— play an important role in facilitating fat absorption through formation of micelles. Like bile salts, lecithin (a phospholipid similar to the ones in the lipid bilayer of the plasma membrane) has both a lipid-soluble and a water-soluble part, whereas cholesterol is almost totally insoluble in water.
• In a micelle, the bile salts and lecithin aggregate in small clusters with their fatsoluble parts huddled together in the middle to form a hydrophobic (“water-fearing”) core, while their water-soluble parts form an outer hydrophilic (“water-loving”) shell

Question 30

What is the average size of micelles?

Answer 30

. A micelle is 3 to 10 nm in diameter, compared to an average diameter of 1000 nm for an emulsified lipid droplet.

Question 31

What is any substance that increases bile secretion is called?

Answer 31

A choleretic.

The most potent choleretic is bile salts themselves.

Question 32

What triggers the release of CCK?

Answer 32

When chyme reaches the small intestine, the presence especially of fat products in the duodenal lumen triggers release of CCK. This hormone stimulates contraction of the gallbladder and relaxation of the sphincter of Oddi, so bile is discharged into the duodenum, where it appropriately aids in the digestion and absorption of the fat that initiated CCK release.

Question 33

What is Bilirubin?

Answer 33

• Bilirubin is a waste product excreted in the bile.
• Bilirubin, the other major constituent of bile, does not play a role in digestion but instead is a waste product excreted in the bile. Bilirubin is the primary bile pigment derived from the breakdown of worn-out red blood cells, which are removed from the blood by the macrophages that line the liver sinusoids and reside in other areas in the body.
• Bilirubin is the end product from degradation of the heme (iron-containing) part of the hemoglobin contained within these old red blood cells.
• Hepatocytes take up bilirubin from the plasma, slightly modify the pigment to increase its solubility, then actively excrete it into the bile
• Bilirubin is a yellow pigment that gives bile its color.

Question 34

What site does most digestion and absorption take place?

Answer 34

The small intestine

Question 35

The small intestine is arbitrarily divided into which three segments?

Answer 35

duodenum,the jejunum, and the ileum

Question 36

What is the function of segmentaion contractions?

Answer 36

• Segmentation contractions mix and slowly propel the chyme.
• Segmentation, the small intestine’s primary motility during digestion of a meal, both mixes and slowly propels the chyme. Segmentation consists of oscillating, ringlike contractions of the circular smooth muscle along the small intestine’s length; between the contracted segments are relaxed areas containing a small bolus of chyme.
• The contractile rings occur every few centimeters, dividing the small intestine into segments. These contractile rings do not sweep along the length of the intestine as peristaltic waves do. Rather, after a brief period, the contracted segments relax, and ringlike contractions appear in the previously relaxed areas.
• The new contraction forces the chyme in a previously relaxed segment to move in both directions into the now relaxed adjacent segments. A newly relaxed segment therefore receives chyme from both the contracting segment immediately ahead of it and the one immediately behind it. Shortly thereafter, the areas of contraction and relaxation alternate again. In this way, the chyme is chopped, churned, and thoroughly mixed

Question 37

The mixing accomplished by segmentation serves the dual functions of:

Answer 37

(1) mixing the chyme with the digestive juices secreted into the small-intestine lumen and (2) exposing all the chyme to the absorptive surfaces of the small-intestine mucosa.

Question 38

Segmentation not only accomplishes mixing but also slowly moves chyme through the small intestine. How can this be, when each segmental contraction propels chyme both forward and backward?

Answer 38

• The chyme slowly progresses forward because the frequency of segmentation declines along the length of the small intestine.
• The pacemaker cells in the duodenum spontaneously depolarize faster than those farther down the tract, with segmentation contractions occurring in the duodenum at a rate of 12 per minute, compared to only 9 per minute in the terminal ileum.
• Because segmentation occurs with greater frequency in the upper part of the small intestine than in the lower part, more chyme, on average, is pushed forward than is pushed backward.
• As a result, chyme is moved slowly from the upper to the lower part of the small intestine

Question 39

During periods of short fasting, when most of the meal has been absorbed, the stomach and small intestine exhibit a unique motor activity. What is it?

Answer 39

Intestinal segmentation contractions cease and are replaced by the migrating motility complex (MMC), or “intestinal housekeeper” activity

Question 40

The MMC cycles through the following phases in a repetitive pattern about every 1.5 hours as long as a person is fasting:

Answer 40

1. Phase I: A long period lasting about 40 to 60 minutes of relative quiet with very few contractions
2. Phase II: A 20- to 30-minute period with some peristaltic contractions, with the time varying between contractions 3. Phase III: The shortest phase, where intense peristaltic contractions begin in the upper stomach and propagate (migrate) through to the end of the small intestine. The contractions rhythmically repeat for 5 to 10 minutes. During this period, the pyloric sphincter relaxes and opens completely

Question 41

What is the motor activity of the MMC is thought to do?

The MMC is regulated between meals by which hormone?

Answer 41

• sweep any remnants of the preceding meal plus mucosal debris and bacteria forward toward the colon, just like a good “intestinal housekeeper.” If a person continues to fast, the MMC motor activity repeats itself, beginning again at Phase I.
• The MMC is regulated between meals by the hormone motilin

Question 42

What prevents contamination of the small intestine by colonic bacteria?

Answer 42

The ileocecal juncture

Question 43

TRUE or FALSE:

Small-intestine secretions do not contain any digestive enzymes

Answer 43

True

No digestive enzymes are secreted into this intestinal juice. The small intestine does synthesize digestive enzymes, but they act intracellularly within the brush-border membrane of the epithelial cells that line the lumen instead of being secreted directly into the lumen

Question 45

Pancreatic enzymes are responsible for most of the digestion within the small-intestine lumen. What is the result of pancreatic enzyme activity for each macronutrient?

Answer 45

• Fats are completely reduced to their absorbable units of monoglycerides and free fatty acids
• proteins are broken down into small peptide fragments and some amino acids
• carbohydrates are reduced to disaccharides, a-limit dextrins, and some monosaccharides

Fat digestion is completed within the small-intestine lumen, but carbohydrate and protein digestion have not been brought to completion

Question 46

The brushborder plasma membrane contains three categories of membranespanning proteins that function as membrane-bound enzymes:

Answer 46

1. Enteropeptidase, which activates the pancreatic proteolytic enzyme trypsinogen
2. The disaccharidases (maltase, sucrase-isomaltase, and lactase), which target maltose, a-limit dextrins, and dietary disaccharides.
3. The aminopeptidases, which hydrolyze most of the small peptide fragments into their amino acid components,

Question 47

Which parts of the small intestine does most absorption occur?

Answer 47

Most absorption occurs in the duodenum and jejunum; very little occurs in the ileum, not because the ileum does not have absorptive capacity but because most absorption has already been accomplished before the intestinal contents reach the ileum.

Question 48

The mucous lining of the small intestine is remarkably well adapted for its special absorptive function for two reasons:

Answer 48

(1) it has a large surface area, and (2) the epithelial cells in this lining have a variety of specialized transport mechanisms.

Question 49

Describe adaptations that Increase the Small Intestine’s Surface Area

Answer 49

■ The inner surface of the small intestine is thrown into permanent circular folds that are visible to the naked eye and increase the surface area threefold.

■ Extending from this folded surface are microscopic, fingerlike projections known as villi, which give the lining a velvety appearance and increase the surface area another 10 times

■ Even smaller hairlike projections, the microvilli or brush border, arise from the luminal surface of these epithelial cells, increasing the surface area another 20-fold.

Question 50

Each villus has the following major components:

Answer 50

■ Epithelial cells that cover the surface of the villus. The epithelial cells are joined at their lateral borders by tight junctions, which limit passage of luminal contents between the cells, although the tight junctions in the small intestine are leakier than those in the stomach. Within their luminal brush borders, these epithelial cells have carriers for absorption of specific nutrients and electrolytes from the lumen, and the membrane-bound digestive enzymes that complete carbohydrate and protein digestion.

■ A connective tissue core. This core is formed by the lamina propria.

■ A capillary network. Each villus is supplied by an arteriole that breaks up into a capillary network within the villus core. The capillaries rejoin to form a venule that drains away from the villus.

■ A terminal lymphatic vessel. Each villus is supplied by a single blind-ended lymphatic vessel known as the central lacteal, which occupies the center of the villus core.

Question 51

During the process of absorption, where do digested substances enter?

To be absorbed where must a substance pass?

Answer 51

• During the process of absorption, digested substances enter the capillary network or the central lacteal.
• To be absorbed, a substance must pass completely through the epithelial cell, diffuse through the interstitial fluid within the connective tissue core of the villus, and then cross the wall of a capillary or lymph vessel.

Question 52

What are the crypts of Lieberkühn?

Answer 52

• Dipping down into the mucosal surface between the villi are shallow invaginations known as the crypts of Lieberkühn
• Unlike the gastric pits, these intestinal crypts do not secrete digestive enzymes, but they do secrete water and salt, which, along with the mucus secreted by the cells on the villus surface, constitute the succus entericus