GIT physiology

Question 1

What are the four layers of the GI tract (starting with the innermost layer)?

Answer 1

Mucosa, submucosa, muscularis externa, serosa

Question 2

What are the two main nerves supplying the GI tract?

Answer 2

Myenteric plexus (in the muscularis externa layer) and submucosal plexus

Question 3

Name four cell types in the intestinal epithelium.

Answer 3

Absorptive cell, Goblet cell, enteroendocrine cell (secrete hormones), paneth cell (large granuals in their cytoplasm have anti-microbial properties)

Question 4

How much fluid do humans ingest per day (avg)?

Answer 4

2L

Question 5

What are the three pairs of salivary glands called?

Answer 5

Sublingual (under the tongue) - largely mucous acini;
Parotid - serous solution containing amylase;
Submantibular - mucous and serous acini

Question 6

What type of cells are involved in saliva secretion?

Answer 6

Acinar cells (serous secretions) and duct cells (ion exchange)

Question 7

What are two functions of the stomach? [2-4 hours]

Answer 7

Secretion of intrinsic factor for absorption of vitamin b12

Pepsin action - pepsinogen is converted to pepsin for protein digestion

Question 8

What stops the stomach from digesting itself?

Answer 8

A layer of alkaline mucus is secreted to neutralise the acid and to stop the tissue from being digested by the stomach acid. The HCO3- ‘alkaline tide’ comes from the parietal cells’ production of HCl. Aspirin reduces its effectiveness (stomach ulcers), local factors promoting prostaglandin release increase this mucous secretion.

Question 9

In which part of the intestine are most electrolytes reclaimed?

Answer 9

Small intestine

Question 10

What is secreted from the large intestine?

Answer 10

K+ and HCO3- ions

Question 11

What is the splanchnic circulation?

Answer 11

Circulation of the intestines, stomach, liver, pancreas, spleen, providing a rich blood supply for the GI tract. [~25% of the cardiac output]

Question 12

What is neurocrine signalling?

Answer 12

Secretion from neurones acting on non-excitable tissue (e.g. ACh release stimulating ion secretion in the GI mucosa)

Question 13

Main endocrine hormones?

Answer 13

Secretin, gastrin, CCK (cholecystokinin) and also GIP, GLP1/2

Question 14

What is PYY?

Answer 14

Peptide YY - regulates food intake by sending satiety signals to the CNS

Question 15

Paracrine hormones?

Answer 15

Somatostatin, histamine

Question 16

How is HCl secretion regulated?

Answer 16

G cells secrete gastrin that can directly influence the parietal cell to secrete HCl, but it also stimulates ECL (entero-chromaffin like cells) to release histamine that affects the parietal cells:
G cells -> ECL cells -> parietal cell

Somatostatin…

Question 17

What inhibits HCl secretion?

Answer 17

Somatostatin. When [HCl] is being secreted, it inhibits the G cells, the parietal cells and the ECL cells.

Question 18

What cells are in the:
stomach
pancreas
duodenum
ileum/colon?

Answer 18

G cells (gastrin)
Alpha and beta cells (glucagon and insulin respectively)
S cells (secretin)
L cells (GLP1/2)

Question 19

How is saliva secretion controlled?

Answer 19

An NKCC co-transporter with the acinar cells: the Na enters cells to partake in the Na/K ATPase; HCO3- and H2O enter cells.

In duct cells, K+/ HCO3- in, Na+ and Cl- out

Question 20

What is the name of the spincter in the stomach?

Answer 20

Pyloric sphincter

Question 21

What are the endocrine and exocrine regions of the stomach?

Answer 21

Endocrine - pyloric antrum: gastrin is secreted here to stimulate HCl production
Exocrine - in the body of the stomach - protein digestion

Question 22

5 main cells in the stomach and their functions

Answer 22

Chief cells - secrete pepsinogen
Parietal cells (HCl secretion)
D cells - somatostatin secretion (inhibits HCl secretion)
ECL cells (entero-chromaffin-like cells) - secrete histamine (involved in HCl secretion inhibition)
Enterochromaffin cells

Question 23

How is HCl produced from the parietal cells?

Answer 23

CO2 enters the cell, combines w water to make H2CO3 - this splits into HCO3- [alkaline tide, leaves cell at basal membrane] and H+ [ACID] -
The HCO3- and Cl- counter-transport each other [Cl- ACID]
Na/K ATPase - K+ leaves at apical surface and re-enters - drives release of H+ and Cl-???

Question 24

How is pepsinogen activated?

Answer 24

Exposure to HCl –> peptin

Question 25

What is intrinsic factor and whyis it important?

Answer 25

It is secreted by parietal cells and aids vitamin B12 absorption in the upper small intestine. B12 is needed for erythrocyte maturation.

Question 26

What controls gastric secretion?

Answer 26

Parasympathetic nervous system, endocrine hormones (gastrin) and paracrine hormones (ECL cells)

Question 27

What are the 3 phases of gastric secretion?

Answer 27

CGI:
Cephalic (as in, at head end)
Gastric (stomach)
and Intestinal

Question 28

4 facts about the cephalic phase

Answer 28

Occurs before food enters the stomach
Stimulated by chewing, taste, smell, sight
Direct vagal stimulation of submucosal plexus (sensory)
Indirect stimulation - G cells secrete gastrin

Question 29

Gastric phase: where, function, stimuli?

Answer 29

Occurs in stomach
Function: to acidify chyme
Stimuli: increased pH, presence of amino acids and peptides in the stomach [peptides stimulate gastrin secretion], DISTENTION (swelling of the stomach)

Question 30

What does distention do in the stomach?

Answer 30

Stimulates vaso-vagal and enteric reflexes

Question 31

Intestinal phase: when/where, stimuli, functions?

Answer 31

When chyme enters the duodenum.
Stimuli: duodenal stretch, presence of lipids
Functions: controls rate of chyme entry to duodenum; gastric secretions [gastrin released in response to presence of chyme and undigested proteins].
Release of CCK and secretin (inhibitory effect on gastric emptying).