Module 6 Section 4

Question 1

exocrine pancreas

Answer 1

• a major physiological role of the pancreas is to secret pancreatic juice: a mixture of pancreatic enymes and an aqueous alkaline secretion

Question 2

pancreatic enzymes

Answer 2

• the exocrine part of the pancreas is predominant and consists of grape-like clusters of secretory cells called acini, whcih connect to ducts that empty into the duodenum
• pancreatic acinar cells secrete 3 different types of pancreatic enzymes

Question 3

proteolytic enzymes

Answer 3

(all secreted in their inactive form)
1. trypsinogen: converted to the active trypsin by enterokinase, whihc is embedded int he luminal border of the cells lining the duodenal mucusa. within the secretory vesicles containing trypsinogen is trypsin inhibitor to protect the pancreas in case trypsinogen is accidnetly acitvated
2. chymotripsinogen: converted to chymotrypsin by trypsin
3. procarboxypeptidase: converted to carboxypeptidase by trypsin

Question 4

pancreatic amylase

Answer 4

• similar to salivary amylase, pancreatic amylase converts polysaccharides into the disaccharide maltose
• it hydrolyzes starches, glycogen, and most other carbs
• the excpetion is cellulose

Question 5

pancreatic lipase

Answer 5

• the only enzyme within the entire digestive system that can digest fats
• it hydrolyzes triglycerides into monoglycerides and free fatty acids, both of which can be absorbed

Question 6

control of pancreatic exocrine secretion

Answer 6

• during cephalic phase of digestion, there is a small amount of parasympathetically mediated paracrine secretion and during the gastric phase of digestion, there is a further small increased secretion due to gastrin
• the bulk of pancreatic secretion is under hormonal control
• the presence f chyme in the duodenum causes the release of secretin and CCK

Question 7

secretin

Answer 7

• the presence of acid cauaes duodenal and jejunal mucosa to release secretin into the blood
• the secretin then travels to the pancreas where it stimulates the duct cells to increase their secretions of the NaHCO3- rich fluid
• the amount of secretin released is proprotional to the amount of acid that enters the duodenum

Question 8

CCK

Answer 8

• primarily in response to the presence of fats in the duodenum, and a lesser extent protein, the duodenal mucosa releases CCK into the blood where it travels to the pancreas and stimulates the pancreatic acinar cells to increase digestive enzyme secretion
• the presence of carbs in the duodenum does not stimulate the release of CCK
• since all 3 classes of pancreatic enzymes are stored together, a high protein meal does not preferentiall release proteolytic enzynes

Question 9

most important role of the liver

Answer 9

the production of bile salts which are important of fat digestion and absorption

Question 10

other functions of the liver

Answer 10

• metabolic processing of fats, cabrs, and proteins after they have been absorbed
• detoxification and or degradation of body wastes, hormones, drugs, and other foreign compounds
• synthesizing plasma proteins
• storage of glycogen, fatsm iron, copper, and vitamins
• involved in the activation of vitamin D
• removal of old red blood cells and bacteria
• excreting cholesterol and bilirubin

Question 11

liver blood flow

Answer 11

• each liver cell, or heaptocyte, performs essentially the same tasks
• in order to perform these tasks, each hapatocyte has to be in contact with both arterial blood and venous blood coming from the digestive tract
• each minute, about 1L of venous blood flows from the digestive system, through the hepatic portal vein, into the liver to deliver anything that was absorbed
• the hepatic artery supplies about another 350ml of arterial blood to deliver oxygen and blood-borne metabolites

Question 12

lobules

Answer 12

• the functionla unit of the liver are called lobules
• these are hexagonal arrangements of tissue surrounding a central vein
• at each of the 6 outer corners, a branch of the hepatic artery, a branch of the hepatic portal vein, and a bile duct are located
• the bile ducts converge to form the common bile duct, which transports the bile from the liver to the duodenum

Question 13

wedge of the lobule

Answer 13

• blood from the hepatic artery and portal vein flow from the periphery of the lobule through capillary-like structures called sinusoids
• the sinusoids run between rows of liver cells towards the central vein
• lining the siusoids are Kupfer cells, which are macrophages, that remove old blood cells and bacteria
• the central vein of all liver lobules converge to form the hepatic vein, which carries the blood away from the liver
• also located between the hepatic cells are the thin bile-carrying channels called bile canaliculus
• hepatocytes are constantly secreting bile into the canaliculus which carries the bile from the centre of the lobule to the periphery and into the bile duct
• each hepatocyte is in contact with a sinusoid on one side and a bile canaliculus on the other side

Question 14

bile

Answer 14

• bile is constantly produced, but it does not enter the duodenum unless active digestion is occurring
• the opening of the bile duct into the duodenum is guarded by the sphincter of Oddi that is closed until bile is needed
• when closed, secreted bile is diverted back up the common bile duct and into the gallbladder for storage
• the gallbladder is capable of storing about 50ml of bile and about 250ml of bile are produced each day

Question 15

composition of bile

Answer 15

• bile is composed mainly of bile salts, cholesterol, lecithin, and bilirubin, in an aqueous fluid similar to the pancreatic NaHCO3 secretion
• bile salts are derivatives of cholesterol and are important for fat digestion and absorption

Question 16

enterohepatic circulation

Answer 16

• almsot all of the bile salt are reabsorbed in the ileum and transported back through the hepatic portal system to the liver, where they are recycled by secretion back into the bile
• this recycling between the small intestine and liver is called enterohepatic circulation

Question 17

steps of enterohepatic circulation of bile

Answer 17

1. secreted bile salts consist of 95% old, recycled and 5% newly synthesized bile salts
2. 95% of bile salts are reabsorbed by temrinal ileum
3. reabsorbed bile salts are recycled by enterohepatic circulation
4. 5% of bile salts are lost in feces

Question 18

structure of bile salts

Answer 18

• have a detergent-like effect on fats
• they convert large fat globules int he chyme into a lipid emulsion of numerous small fat droplets, each less than 1mm in diameter
• this effectively increases the surface area of the luminal fats for the pancreatic lipase
• bile salts have a lipid-soluble part, which interacts with the surface of the fat droplets, and a negative charged water soluble end
• this allows the bile salts to decrease the surface tension of lipid droplets and break them down into smaller droplets

Question 19

fat digestion and absorption

Answer 19

• the polar ends of the bile salts repel each other and prevent the small lipid droplets from coming back together
• the lipid droplets are dispersed as an emulsification
• lipase alone cannot penetrate the layer of bile salts surrounding the lipid droplet
• along with lipase, the pancreas secretes colipase, which binds both lipase and the bile salts to hold the lipase at its site of action so it can break down the triglycerides

Question 20

micelles

Answer 20

• the structure of lipid-soluble compounds surrounded by water-soluble bile salts
• along with lethicin, which also has lipid-soluble and water-soluble ends, and cholesterol, bile salts form these clusters with an outer lipophobic shell while maintaining a lipophilic core
• the cholesterol is found within this core along with any digested fats and other lipid-soluble compounds
• the micellular formation allows lipid-soluble substaces to be transported within the chyme to their site of absorption

Question 21

bilirubin

Answer 21

• not involved in digestion but instead is a waste product that is secreted in the bile
• it primarily comes from the degradation of haemoglobin in old RBC

Question 22

how does the colour of bilirubin play a role in the bodys appearance

Answer 22

• has a yellowish colour but once it is in the digesitve tract, bacterial enzymes, mainly found in the large intestine, modify the colour to the brown colour associated with feces
• in the absence of bile secretion, the feces would be greyish white
• most bilirubin is excreted in the feces but some is reabsorbed in the small intestines where it is eventually excreted int he urine, whcih is why urine has a yellow colour to it

Question 23

regulation of bile secretion - chemical

Answer 23

• the most potent chemical signal is bile salts themselves
• when bile salts are reabsorbed and return to the liver via the enterohepatic circulation, they stimulate the further release of bile

Question 24

regulation of bile secretion - hormonal

Answer 24

secretin stimulates the secretion of the aqueous alkaline component of bile but does not stimulate the release of bile salts

Question 25

regulation of bile secretion - neural

Answer 25

during the cephalic phase, vagal stimulation of the liver promotes the release of bile before food reaches the stomach or intestine

Question 26

bile storage in the gallbladder

Answer 26

- bile is secreted by the liver, but cannot enter the duodenum because the sphincter of Oddi is closed, backs up into the gallbladder for storage - active transport of salt out of the gallbladder, with water osmotically folllowing it, concentrates the bile 5 to 10 times - druing digestion, when chyme reaches the small intestine, the presence of fat in the duodenum triggers the release of CCK - CCk both causes the galbladder to contract and causes the sphincter of Oddi to relax allowing bile to flow into the duodenum

Question 27

morbiditity of the gallbladder

Answer 27

- because the gallbladder concentrates bile, the bile salts can precipitate ad form gallstones - the gallbladder can be removed with no negative effects on digestion - bile is still produced but instead of being stored in the gallbladder, excess bile is stored in the common bile duct, which dilates