Physiology of Liver, Biliary Tract, Exocrine Pancreas

Question 1

What are the mixing contractions of the small intestine called?

Answer 1

segmentation contractions. These are slow rhythmic waves. The duodenum and jejunum are the fastest at about 10/min.

Question 2

How long does it take to move chyme from the pylorus to the ileocecal valve?

Answer 2

3-5 hours at about 1cm/min

Question 3

What controls peristalsis?

Answer 3

• NERVOUS signals initiated by chyme entry in the duodenum (stretches duodenal wall).
• HORMONAL control: gastrin, CCK, insulin, motilin, and serotonin increase motility. Secretin and glucagon inhibit motility.

Question 4

What is peristaltic rush?

Answer 4

intense irritation of intestinal mucosa causing powerful and rapid peristalsis initiated by a combination of the autonomic nervous system, brain stem, and intrinsic enhancement of the myenteric plexus.

Question 5

Can individual fibers of the muscularis mucosa extend into intestinal villi and cause them to contract intermittently?

Answer 5

YES. Initiated by local nervous reflexes in submucosal nerve plexus.

Question 6

What is the function of the ileocecal valve?

Answer 6

• prevents backflow of fecal contents from colon into small intestine via ileocecal sphincter (normally mildly constricted).
• it is forcibly closed wen excess pressure builds up in the cecum.

Question 7

What is the gastroileal reflex?

Answer 7

after a meal, the presence of food in the stomach intensifies peristalsis in the ileum, emptying ileal contents into the cecum to make room for more food.

Question 8

What controls the degree of ileocecal sphincter contraction?

Answer 8

reflexes form the cecum and intensity of peristalsis in the terminal ileum.

Question 9

What happens when the cecum is distended?

Answer 9

contraction of the ileocecal sphincter is intensified, while ileal peristalsis is inhibited, delaying emptying into the cecum.
*irritants in the cecum also delay emptying

Question 10

What mediates the reflexes from the cecum to the ileocecal sphincter and ileum?

Answer 10

the myenteric plexus in the gut wall and extrinsic autonomic nerves (via prevertebral sympathetic ganglia)

Question 11

What are the 2 major tissue types of the pancreas?

Answer 11

1. acini (exocrine)= secrete digestive juices into the duodenum (enzymes and sodium bicarb flow through pancreatic duct that joins the hepatic duct and empties into the duodenum via the papilla of Vater, surrounded by the sphincter of Oddi).
2. islets of Langerhans (endocrine)= secrete insulin and glucagon.

Question 12

What causes pancreatic secretions in the duodenum?

Answer 12

chyme

Question 13

What are the most important pancreatic digestive enzymes?

Answer 13

trypsin, chymotrypsin, and carboxypolypeptidase
*they are synthesized as inactive zymogen forms remember (trypsinogen, chymotrypsinogen, procarboxypolypeptidase), and activated by enzymes upon secretion into the duodenum.

Question 14

What prevents premature trypsinogen activation in the pancreas?

Answer 14

trypsin inhibitor

Question 15

What pancreatic enzymes are secreted for protein digestion?

Answer 15

• trypsin and chymotrypsin= split proteins into peptides.

- carboxypolypeptidase= splits peptides into individual amino acids.

Question 16

What pancreatic enzymes are secreted for carbohydrate digestion?

Answer 16

pancreatic amylase= hydrolyzes starch, glycogen, and other carbs, forming di- and trisaccharides.

Question 17

What pancreatic enzymes are secreted for fat digestion?

Answer 17

• pancreatic lipase= hydrolyzes fat into fatty acids and monoglycerides.
• cholesterol esterase= hydrolyzes cholesterol esters.
• phospholipase= splits fatty acids from phospholipids.

Question 18

What enzyme acitivates trypsinogen to trypsin?

Answer 18

enterokinase (enteropeptidase)

Question 19

What does trypsin activate?

Answer 19

chymotrypsinogen to chymotrypsin and procarboxypolypeptidase to carboxypolypeptidase

Question 20

What is the mechanism by which the pancreas secretes Na+ bicarb into the pancreatic ductules and ducts in order to create osmotic pressure gradient for water to enter?

Answer 20

CO2 diffuses into the cell from the blood, where it combines with water via carbonic anhydrase to form carbonic acid. Carbonic acid will dissociate into H+ and HCO3- for the bicarb to be actively transported to the lumen of the duct, while H+ is exchanged for Na+ out the basolateral membrane via secondary active transport. The Na+ will then diffuse down its gradient into the lumen of the pancreatic duct to combine with HCO3-.

Question 21

What are the 3 phases of pancreatic secretion (similar to that of gastric secretion)?

Answer 21

1. CEPHALIC phase= Ach release by vagal nerve endings in pancreas.
2. GASTRIC phase= nervous stimulation of enzyme secretion continues.
3. INTESTINAL phase= after chyme leaves stomach and enters small intestine (secretion is copious) mainly in response to SECRETIN.

Question 22

What is secretin?

Answer 22

hormone secreted from S cells in mucosa of duodenum and jejunum into the blood that stimulates copious secretion of sodium-bicarb ions from pancreas to neutralize acidic stomach chyme.

Question 23

At what pH do pancreatic enzymes function?

Answer 23

7-8

Question 24

What is cholecystokinin (CCK)?

Answer 24

hormone secreted by I cells of duodenum and jejunum into the blood that induces secretion of more pancreatic digestive enzymes (by acinar cells remember) due to presence of partially digested protein and long chain fatty acids in chyme.

Question 25

What is the largest internal organ?

Answer 25

liver and it can regenerate

*functional unit is liver lobule (50k-100k)

Question 26

What are the 5 functions of the liver?

Answer 26

1. filtration and storage of blood.
2. metabolism of carbohydrates, proteins, fats, hormones, and foreign chemicals.
3. storage of vitamins and iron.
4. formation of coagulation factors.
5. FORMATION OF BILE (600-1000 mL/day).

Question 27

What is the role of bile?

Answer 27

• emulsifies fats (increasing surface area for lipase to work) and aids in absorption of digested fat through intestinal mucosal membrane.
• it also serves as a means for excretion of blood waste products (bilirubin= end product of hemoglobin destruction, which causes brown color of feces) and cholesterol.

Question 28

What are the 2 stages of liver biliary secretion?

Answer 28

1. principal hepatocytes secrete large amounts of bile acids, cholesterol and other organic constituents into minute bile canaliculi (between hepatocytes). Bile flows toward interlobular setpa where it empties into terminal bile ducts and then the hepatic and common bile duct. Bile empties directly into duodenum or is diverted for minutes to hours thorugh the cystic duct into the gallbladder.
2. within bile ducts, a second portion of bile secretion is added to initial bile (water solution of Na+ and bicarb) by epithelial cells that line the bile ductules and ducts. This is stimulated by SECRETIN.

Question 29

What is the max volume of bile that the gallbladder can hold?

Answer 29

30-60 mL

Question 30

What is continually absorbed out through the gallbladder mucosa?

Answer 30

water, NaCl (Na+ actively transported out while Cl- follows), and other electrolytes, thus concentrating the remaining bile.

Question 31

What are the contents of bile?

Answer 31

bile salts, bilirubin, cholesterol, fatty acids, lecithin, Na+, K+, Ca2+, Cl-, and HCO3-

Question 32

How does the gall bladder empty itself?

Answer 32

as food begins digestion in the upper GI system, the gallbladder empties via rhythmical contractions of its walls with simultaneous relaxation of the sphincter of Oddi.
*CCK is the strongest stimulus for gall bladder emptying, along with Ach secreted by nerve fibers from both the vagus nerve and intestinal enteric nervous system.

Question 33

How are bile salts formed?

Answer 33

via cholesterol, which is converted to cholic acid or chenodoexycholic acid. The acid combines with glycine and taruine to form glycol- and taruo- conjugated bile acids. Salts of these acids (mainly Na+ salts) are secreted in bile.

Question 34

What are the 2 functions of bile salts?

Answer 34

1. fat digestion acting as a detergent on fat particles in food (emulsifying).
2. aids absorption of lipids (fatty acids, monoglycerides, cholesterol) by forming small complexes with lipids (micelles) to be ferried to intestinal mucosa.

Question 35

What percentage of bile salts are reabsorbed into the portal blood form the small intestine?

Answer 35

94% recycled about 17 times (half by diffusion and half by active transport).
*thus 6% is newly generated bile in the liver.

Question 36

How are gallstones formed (cholelithiasis)?

Answer 36

due to cholesterol precipitation in the gallbladder (related to amount of fat in diet). Inflammation of the gallbladder epithelium may change absorptive characteristics of the gallbladder mucosa leading to excessive absorption of water or bile acids, leaving behind cholesterol.

Question 37

What are some disorders of the exocrine pancreas?

Answer 37

pancreatitis, cysts, tumors, carcinomas

Question 38

What are some disorders of the gallbladder and biliary tract?

Answer 38

gallstone formation (cholelithiasis), bile duct stones (choledocholothiasis), infection/closure of ducts, carcinoma