GI physiology

Question 1

what enzyme digests starch in the mouth?

Answer 1

ptyalin (a type of amylase)

Question 2

what is responsible for starch digestion in the small intestine?

Answer 2

pancreatic amylase

Question 3

how are the breakdown products of starch absorbed?

Answer 3

• as monosaccharides via a sodium cotransporter mechanism- on the membrane of enterocytes
• fructose absorbed by facilitated transport

Question 4

how are proteins digested in the stomach?

Answer 4

pepsin is secreted by chief cells as its zymogen pepsinogen

Question 5

what is responsible for protein digestion in the small intestine?

Answer 5

• pancreatic enzymes (trypsin and chymotrypsin)

- peptidases lining the small intestinal villi

Question 6

how are the breakdown products of protein digestion absorbed?

Answer 6

• via a sodium cotransporter mechanism on the luminal membranes of intestinal epithelial cells
• absorbed as di/ tri peptides

Question 7

how are lipids first broken down in the stomach?

Answer 7

• via lingual lipase

- swallowed w/ saliva along with food

Question 8

why does emulsification of fats need to occur?

Answer 8

• not water soluble

- and the enzymes that digest them are present in the water

Question 9

how are triglycerides digested?

Answer 9

via pancreatic lipase

Question 10

how do micelles form and what is there function?

Answer 10

• bile salts combine with fats

- allows monoglycerides and fatty acids to be transported to the brush border of intestinal epithelial cells

Question 11

how are the breakdown products of lipids absorbed?

Answer 11

• diffuse into intestinal epithelial cells
• taken up by SER
• released by lymphatic system in the form of chylomicrons

Question 12

where are bile salts reabsorbed?

Answer 12

terminal ileum

Question 13

what are the 2 stages in the absorption of glucose in the intestine?

Answer 13

• 2 stage transcellular process
• stage 1- glucose and 2 Na+ in via a Na+/ glucose symport protein
• stage 2- Na+/K+ ATPase transporter (moves Na+ out and K+ in via ATP), GLUT 2 transporter- glucose diffuses out

Question 14

where do net absorption and secretion occur in small intestine?

Answer 14

absorption- villi

secretion- crypts

Question 15

by what mechanism is potassium reabsorbed?

Answer 15

passive diffusion

Question 16

where are Cl- and HCO3- reabsorbed?

Answer 16

ileum and colon

Question 17

what vitamins are absorbed in the duodenum?

Answer 17

vitamins A and D

Question 18

what vitamins are absorbed in the jejunum?

Answer 18

vitamins A, D and folic acid

Question 19

what vitamins are absorbed in the ileum?

Answer 19

folic acid and vitamin B-12

Question 20

where are parietal cells found and what is their function?

Answer 20

• fundus of the stomach

- secrete HCl and intrinsic factor

Question 21

where are mucosal cells found and what is their function?

Answer 21

• cardia of stomach

- secrete mucous

Question 22

where are chief cells found and what is their function?

Answer 22

• found in fundus of stomach

- secrete pepsinogen

Question 23

what is the function of D cells?

Answer 23

secrete somatostatin

Question 24

what is the function of enterochromaffin-like cells?

Answer 24

release histamine

Question 25

what is the function of G cells?

Answer 25

release gastrin

Question 26

what is the mechanism by which gastric acid is secreted?

Answer 26

• carbonic acid forms
• dissociates into H+ and HCO3-
• chloride shift occurs- chloride into the cell and HCO3- out
• hydrogen ions pumped into lumen via H+/K+ pump
• potassium and chloride ions diffuse into stomach lumen down their concentration gradient

Question 27

what happens in the cephalic phase?

Answer 27

• via PNS
• sight/ smell/ taste of food and chewing
• ACh released, triggers gastrin and histamine release
• net effect= increased acid production

Question 28

what happens in the gastric on phase?

Answer 28

• triggered by gastric distention, presences of peptides and amino acids
• gastrin release, acts on parietal and enterocromaffin like cells
• increased acid production

Question 29

what happens in the gastric off phase?

Answer 29

• low luminal pH inhibits gastrin secretion, indirectly inhibiting histamine release
• stimulates somatostatin release- inhibits parietal cell activity

Question 30

what happens in the intestinal phase?

Answer 30

• triggered by duodenal distension, low luminal pH, hypertonic luminal contents, presence of amino acids and fatty acids
• triggers release of secretin (inhibits gastrin release, promotes somatostatin release) and cholecystokinin (CCK)- reduces ACh release= secretion of bile by promoting contraction of gall bladder and relaxation of sphincter of Oddi

Question 31

how are proteases secreted?

Answer 31

• chief cells synthesise pepsinogen
• activated in lumen of stomach due to low luminal pH
• converted from zymogen (pepsinogen) into active form (pepsin)

Question 32

what is the empty volume of the stomach?

Answer 32

50ml

Question 33

what is receptive relaxation?

Answer 33

• stomach volume changes without luminal pressure change

- parasympathetic nervous system- vagus (X) causes secretion of NO and serotonin

Question 34

what is peristalsis?

Answer 34

• wave begins in gastric body
• most powerful in pyloric antrum
• increased by gastrin, gastric distention and PNS
• inhibited by duodenal distention, low duodenal pH, SNS
• pylorus closes when chyme reaches it- antral contests forced back to body for more mixing

Question 35

what determines the frequency of peristaltic waves?

Answer 35

interstitial cells of Cajal found in muscularis propria

Question 36

what are the 4 main defences against acid secretion?

Answer 36

• alkaline mucus
• occluding junctions between epithelial cells
• replacement of damaged cells
• feedback loops