Physiology of the Small Intestine

Question 1

what are 5 things that the Exocrine Pancreas and Small Intestine secrete

Answer 1

hormones
alkali
digestive enzymes
water
mucus

Question 2

where is mucus secreted from the small intestine

Answer 2

Brunner’s glands in the duodenal mucosa secrete a viscous, mucus-rich alkaline fluid.

Question 3

How is water secreted by the small intestine

Answer 3

Intestinal epithelium secretes Na+, Cl- and HCO3- into the lumen. Water follows by osmosis.

N.B. overall there is a large net absorption of water from the small intestine.

Question 4

what hormones are secreted by the small intestine

Answer 4

Secretin, cholecystokinin (CCK), motilin, vasoactive intestinal peptide (VIP) and gastric inhibitory peptide (GIP).

Question 5

what is motilin

Answer 5

Stimulates migrating motor complexes (MMCs) via both the enteric and autonomic nervous systems.

Question 6

what are migrating motor complexes

Answer 6

a distinct pattern of electromechanical activity in gastrointestinal smooth muscle inbetween meals. It is thought to serve a “housekeeping” role and sweep residual undigested material through the digestive tube.

Question 7

what is VIP and what does it do

Answer 7

Vasoactive intestinal peptide

increases blood flow to the GI tract

Question 8

what is GIP and what does it do

Answer 8

Gastric inhibitory peptide
glucose-dependent insulinotropic peptide

Inhibits gastric secretion.
Stimulates insulin secretion.

Question 9

what is CCK

Answer 9

Cholecystokinin

inhibiting gastric emptying.

Question 10

what does CCK do

Answer 10

works on the exocrine pancreas and stimulates it to release bicarb ions
Also stimulates the liver to secrete bile

Question 11

what does secretin do

Answer 11

on exocrine panc stimulating release of pancreatic enzymes. Also acts on gallbladder .’ contracts and release its contents into small intestine

Question 12

where are exocrine pancreatic enzymes released from

Answer 12

acinar cells

Question 13

what do acinar cells secrete

Answer 13

exocrine pancreas enzymes

Question 14

what do duct cells of the exocrine pancreas secrete

Answer 14

alkali

Question 15

what is the alkali of the exocrine pancreas

Answer 15

An isotonic solution rich in HCO3-.

Question 16

what is the function of exocrine pancreatic alkali

Answer 16

Neutralizes duodenal contents.

Question 17

that digestive enzymes does the exocrine pancreas secrete

Answer 17

Trypsin (endopeptidase)
Chymotrypsin (endopeptidase)
Carboxypeptidase (ectopeptidase)
Pancreatic amylase 
Lipases
Other enzymes (phospholipase, cholesterol esterase, ribonuclease etc)

Question 18

what do lipases do

Answer 18

break down fats

Question 19

what does pancreatic amylase do

Answer 19

breaks down starches

Question 20

what does trypsinogen released from the pancreas do

Answer 20

trypsinogen -> trypsin which cleaves the zymogens released from the pancreas to produce active enzymes that can work at the intestinal lumen

Question 21

Why are peptidases secreted in their inactive form?

Answer 21

So that they don’t break down the cells that are releasing them

Question 22

when is pancreatic HCO3- secreted

Answer 22

majority of pancreatic secretion arises from intestinal phase stimuli (when chyme reaches the duodenum)

Question 23

how does body know to increase pancreatic HCO3- secretion

Answer 23

majority of pancreatic secretion arises from intestinal phase stimuli (when chyme reaches the duodenum)

secretin and cholecystokinin (CCK) are released by endocrine cells that are located in the duodenal epithelium.

Question 24

how does secretin trigger bicarb release

Answer 24

secretin release is triggered by H+ ions (low pH). Secretin then travels via the circulation to stimulate bicarbonate secretion by duct cells.

Question 25

what is the mechanism of pancreatic HCO3- secretion

Answer 25

In CSF have this complex reaction of 3CO + 3H2O catalysed by carbonic anhydrase to give rise to bicarb ions and H+ ions.

HCO3- are secreted into the duct lumen.

H+ are pumped out of duct cells across basolateral membrane and released into the blood.

Question 26

how is pancreatic enzyme secretion controlled

Answer 26

stimulated by CCK and acetylcholine (Ach; from parasympathetic, postganglionic neurones of vagus

Question 27

how is pancreatic alkali secretion controlled

Answer 27

stimulated by secretin and potentiated by CCK and ACh (vagal tone).

Question 28

how are fats digested in the small intestine

Answer 28

digested by pancreatic lipase to monoglyceride and fatty acids.

Question 29

how is starch digested

Answer 29

by pancreatic amylase.

Disaccharides are broken down into monosaccharides by brush border enzymes.

Question 30

how are proteins digested in the small intestine

Answer 30

broken down to peptide fragments by trypsin and chymotrypsin. Peptide fragments are digested to free amino acids by carboxypeptidase (pancreas) and aminopeptidase (luminal membranes of small intestine epithelial cells).

Question 31

how are fats digested and absorbed

Answer 31

1. broken down by emulsification by bile salts and phospholipids
2. digested by pancreatic lipase
3. products held in micelles, combined with bile salts an phospholipids
4. The micelles diffuse into an “unstirred layer” next to the surface of epithelial cells.

Question 32

what happens to micelles containing fat after they have diffused into the unstirred layer (next to surface of epithelial cells)

Answer 32

fatty acids &monoglycerides diffuse into cell membrane.

• > fatty acids &monoglycerides reassembled into fats.
• > Triglyceride droplets packaged into chylomicrons.
• > Chylomicrons exported across basolateral membrane -> they leave the intestinal villus via its lymph vessel