GI Physiology - digestion and absorption

Question 1

List the five main hormones involved in regulating digestion

Answer 1

Gastrin
Secretin
Cholecystokinin (CCK)
Gastric inhibitory peptide (GIP)
Motilin

Question 2

Describe the function of secretin

Answer 2

Secreted from S-cells in the duodenal mucosa in response to the presence of acid in the duodenum

Stimulates pancreatic secretion of bicarbonate
Also stimulates the liver to produce and secrete bile

Question 3

Describe the function of cholecystokinin (CCK)

Answer 3

Secreted from enteroendocrine cells in the duodenum in response to the presence of fats and amino acids in the duodenum

Stimulates pancreatic secretion of pepsinogen and other zymogens
Causes the sphincter of Oddi to relax and gallbladder smooth muscle to contract, releasing bile into the duodenum
Causes the pyloric sphincter to contract and the stomach wall to relax to slow gastric emptying

Question 4

Describe the function of gastrin

Answer 4

Secreted from G-cells in the duodenum and the pyloric antrum of the stomach in response to the presence of protein in the stomach

Stimulates parietal cells to secrete HCl
Stimulates ECL cells to produce histamine which also stimulates parietal cells

Question 5

Describe the function of GIP

Answer 5

Gastric inhibitory polypeptide
Secreted by K-cells in the duodenal mucosa in response to the presence of fat in the duodenum

Decreases gastrin secretion
Decreases parietal HCl secretion

Question 6

Describe the function of motilin

Answer 6

Secreted into the duodenum

Increases migrating myoelectric complex (MMC) component of GI motility
- promotes peristalsis in the small intestine and clears
out the gut to prepare for the next meal
Stimulates the production of pepsin

Question 7

Which two proteins are involved in iron absorption?

Answer 7

Ferritin and Transferrin

Question 8

Describe the role of ferritin and why it is so important

Answer 8

Forms a protein-iron complex that acts as an non-toxic intracellular store of iron
Unbound iron is toxic; would react with oxygen to form free radicals
Ferritin expression is regulated by the gut itself depending on how much iron is in the body

Question 9

What is the normal range for ferritin levels? What does it mean if ferritin level is outside this range?

Answer 9

Normal ferritin 24-300ug/L
High ferritin: 10% cases caused by iron overload, the other 90% caused by disease (MAIL):
- M: malignancy
- A: alcohol consumption (chronic)
- I: inflammation
- L: liver disease (e.g. NAFLD, viral hepatitis)
Low ferritin: evidence of iron deficiency

Question 10

Which vitamins are fat soluble and which are water soluble?

Answer 10

Fat soluble: Vitamins A, D, E, K

Water soluble: B-group, C, folic acid

Question 11

Which ion is essential for transporting monosaccharides across enterocytes? Briefly describe this mechanism

Answer 11

Sodium (co-transportation)
Low intracellular sodium concentration is maintained by Na/K ATPases (pumps) in the basolateral membrane. This establishes an electrochemical gradient of sodium across the epithelial cell boundary of the intestinal lumen (low [Na+] inside lumen, high on other side of enterocytes)
Sodium binds to SGLUT-1, causing the glucose-binding site to be exposed. This allows glucose to bind and then be transported into the cell

Question 12

What are enterocytes? Describe their structure and function

Answer 12

Intestinal absorptive epithelial cells
Simple columnar epithelial cells lining the intestines
Have microvilli on apical surface which increase surface area for absorption (this is referred to as the brush border membrane)
Transport nutrients from the intestinal lumen into the blood

Question 13

Describe three features of the intestinal lining which increase absorptive surface area

Answer 13

Mucosa is folded, creating “corkscrew” ridges along the tube lining
Villi - finger-like projections into intestinal lumen
Microvilli on enterocyte apical surfaces create brush border membrane

Question 14

Define transcellular transport

Answer 14

molecules pass through the cell; this requires specific transport proteins in both the apical and basolateral membranes

Question 15

Define paracellular transport

Answer 15

molecules are transported around the cell, through the tight junctions that separate the apical from the basal membrane

Question 16

How is water absorbed from the intestine?

Answer 16

Paracellular transport

Sodium ions create an osmotic imbalance across the enterocytes

Question 17

Which protease enzyme is secreted from the stomach? Which cells secrete it?

Answer 17

Pepsinogen (is then converted to pepsin by stomach acid)

Secreted from chief cells

Question 18

Give three preotolytic enzymes that are secreted by the pancreas

Answer 18

trypsin
chymotrypsin
carboxypeptidase

Question 19

How are amino acids absorbed from the intestine?

Answer 19

sodium-coupled transport, very similar to glucose

Question 20

How are peptides absorbed from the intestine?

Answer 20

co-transport with hydrogen ions (instead of sodium)
- protons are supplied by mucus which is acidic
uses PepT1, a proton-coupled transporter in the apical membrane

Question 21

What is the clinical relevance of being able to absorb peptides?

Answer 21

Allows us to take penicillin orally

Question 22

What is the exception to the rule that proteins aren’t generally absorbed from the small intestine?

Answer 22

Neonates; neonates can absorb intact proteins for the first few days after birth. This allows them to aquire passive immunity by absorbing immunoglobulins in colostral milk

Question 23

What are micelles and what is their function?

Answer 23

small aggregates of mixed lipids and bile acids/salts (smaller than emulsion droplets)
bump into brush border membran when material is mixed by the small intestine, allowing monoglyceride and fatty acids to be taken up into enterocytes

Question 24

How are fatty acids and monoglycerides absorbed?

Answer 24

Once in the enterocytes, they enter smooth ER and are reformed into triglycerides
Are emulsified then transported to the golgi apparatus
- are transported in vesicles due to the aqueous environment inside the cell (fat it hydrophobic!)
Chylomicrons are formed inside the golgi - these are exocytosed through basal membrane and pass into lacteals
Lacteals drain into larger lymph vessels and eventually into the blood via the thoracic duct

Question 25

What are lacteals and why are they necessary?

Answer 25

lymph vessels that penetrate into each villus, between epithelial cells
necessary because chylomicrons are too big to pass through the basement membrane

Question 26

Which lymph nodes drain the small intestines?

Answer 26

Mesenteric lymph nodes

Question 27

Describe the role of intrinsic factor in B12 absorption

Answer 27

Intrinsic factor is secreted from parietal cells in stomach
IF binds of B12 in the stomach, forming a complex
This allows the complex to bind to IF-receptors on the apical surfaces of enterocytes in the distal ileum

Question 28

What is pernicious anaemia?

Answer 28

Failure of red blood cell maturation due to B12 deficiency.
B12 deficiency is caused by loss of intrinsic factor due to autoimmune attack (either the parietal cells, the IF or the IF-receptors are attacked by auto-antibodies).