Physiology of Digestion and Absorption

Question 1

What are the respective sizes of the duodenum, jejunum and ileum?

Answer 1

Duodenum - 30cm. Jejunum - 3.5m.

Ileum - 2.5m.

Question 2

What substances does the small intestine receive and where do they come from?

Answer 2

Chyme from stomach, pancreatic juice from pancreas and bile from gall bladder.

Question 3

What does the small intestine secrete?

Answer 3

Intestinal juice (succus entericus).

Question 4

What hormone causes the ileocaecal valve to open?

Answer 4

Gastrin.

Question 5

What are the 3 structures of the small intestine that make it well adapted for absorption?

Answer 5

Circular folds (of Kerckring), villi and microvilli (brush border).

Question 6

What cells secrete gastrin, cholescystokinin (CKK), secretin and motilin and where are they located?

Answer 6

Gastrin - G cells of antrum and duodenum.
CCK - I cells of duodenum and jejunum.
Secretin - S cells of duodenum.
Motilin - M cells of duodenum and jejunum.

Question 7

What are the 2 incretins?

Answer 7

Glucagon-like insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1).

Question 8

What is another name for GIP?

Answer 8

Gastric inhibitory peptide.

Question 9

Where are GIP and GLP-1 secreted from?

Answer 9

GIP - K cells of duodenum and jejunum.

GLP-1 - L cells of gut.

Question 10

What are the functions of incretins?

Answer 10

Act upon B-cells of pancreas in feed-forward manner to stimulate the release of insulin.

Question 11

What type of receptor do hormones secreted by the gut act on?

Answer 11

GPCRs.

Question 12

How much succus entericus is secreted per day by the small intestine?

Answer 12

2 litres.

Question 13

How does the composition of the succus entericus vary?

Answer 13

At different parts of the small intestine.

Question 14

What are the control mechanisms of the small intestine secretions?

Answer 14

Distension/irritation, gastrin, CCK, secretin, parasympathetic stimulation (increases), sympathetic stimulation (decreases).

Question 15

What are the 2 components of the succus entericus and their functions?

Answer 15

Mucus (protection/lubrication from goblet cells), aqueous salt (enzymatic digestions from crypts of Lieberkuhn).

Question 16

Does the succus entericus have digestive enzymes?

Answer 16

No.

Question 17

What transporter brings chloride into the enterocyte from the circulation?

Answer 17

Na+/K+/2Cl- co-transporter.

Question 18

What channel does chloride enter the gut lumen through?

Answer 18

CFTR?

Question 19

How does sodium and water enter the lumen after chloride?

Answer 19

Because of the osmotic gradient (goes around the cell rather than through it).

Question 20

When does segmentation occur very vigorously?

Answer 20

After a meal.

Question 21

Why is segmentation in the duodenum activated after a meal?

Answer 21

Due to distension by entering chyme.

Question 22

What causes segmentation in the empty ileum?

Answer 22

Gastrin from the stomach (gastroileal reflex, feedforward mechanism).

Question 23

What is the net movement in segmentation and why?

Answer 23

Aboral. Duodenal has 12 contractions per min, ileum has 9 per min.

Question 24

When does the migrating motor complex (MMC) occur?

Answer 24

Between meals every 90-120 mins.

Question 25

What is the MMC?

Answer 25

A strong peristaltic contraction passing the length of the small intestine (stomach to ileocaecal valve).

Question 26

What is the purpose of the MMC?

Answer 26

Clears small intestine of debris, mucus and sloughed epithelial cells between meals.

Question 27

What inhibits and triggers the MMC?

Answer 27

Inhibits: feeding and vagal activity, gastrin and CKK. Triggers: motilin.

Question 28

Why may macrolide antibiotics cause GI disturbances?

Answer 28

They may mimic the effect of motilin.

Question 29

What does the exocrine part of the pancreas produce?

Answer 29

Digestive enzymes and aqueous sodium bicarbonate.

Question 30

What cells produce digestive enzymes and bicarbonate in the pancreas?

Answer 30

Digestive enzymes - acinar cells. Bicarbonate - duct cells.

Question 31

Describe the path of the pancreas from acni cells into the duodenum.

Answer 31

Acni cells to ducts which then combine into bigger ducts which flow into the main pancreatic duct. This then flows into the duodenum.

Question 32

How much alkaline fluid does the pancreas secrete per day?

Answer 32

1-2 litres.

Question 33

Why is the alkaline fluid important?

Answer 33

It neutralises acidic chyme which provides optimum pH for pancreatic enzyme function and protects the mucosa from erosion by acid.

Question 34

When are enzymes stored in zymogen granules released in acinar cells?

Answer 34

When intracellular calcium concentration is elevated.

Question 35

What causes an increase in calcium concentration in acinar cells?

Answer 35

Parasympathetic or hormonal stimulation.

Question 36

Why are the proteases produced by the pancreas inactive?

Answer 36

They would autodigest the pancreas if they were active.

Question 37

What enzyme in the mucosal cells of the duodenum activates trypsinogen (produced trypsin)?

Answer 37

Enterokinase.

Question 38

What are the enzymes that the pancreas secretes?

Answer 38

Trypsinogen, chymotrypsinogen, procarboxypeptidase A and B, pancreatic amylase and pancreatic lipase.

Question 39

What enzymes can trypsin activate?

Answer 39

Trypsinogen (autocatalysis), chymotripsinogen (produced chymotrypsin) and procarboxypeptidase A and B (produces carboxypeptidase A and B).

Question 40

What are the 3 phases of the control of pancreatic secretion?

Answer 40

Cephalic (mediated by vagal stimulation of mainly the acinar cells [20% secretion]), gastric (distension evokes vagovagal reflex causing parasympathetic stimulation of acinar and duct cells [5-10% total secretion]), and intestinal.

Question 41

In the intestinal phase, what does acid in the duodenal lumen cause?

Answer 41

Increase of secretin release from S cells, carried by blood to pancreatic duct cells, increased secretion of bicarbonate into duodenal lumen.

Question 42

In the intestinal phase, what does fat and protein in the duodenal lumen cause?

Answer 42

Increased CCK release from I cells, carried by blood to pancreatic acinar cells, increases secretion of digestive enzymes into the duodenal lumen.

Question 43

What are the main constituents of carbohydrates?

Answer 43

Starch, cellulose, glycogen, disaccharides.

Question 44

What are the 2 types of digestion that occurs in the small intestine?

Answer 44

Luminal digestion - mediated by pancreatic enzymes. | Membrane digestion - mediated by enzymes on brush border of epithelial cells.

Question 45

What membranes do things have to travel across to be absorbed?

Answer 45

The apical and basolateral.

Question 46

What is assimilation?

Answer 46

The overall process of digestion and absorption.

Question 47

Can di, tri or tetrapeptides be absorbed by the gut epithelium and where are they broken down?

Answer 47

Yes, in the enterocyte.

Question 48

What types of sugars can be absorbed?

Answer 48

Only monosaccharides.