1. Gastrointestinal absorption and secretion

Question 1

Outline the regional variations in water and electrolyte uptake in the large intestine.
The anatomy behind this?

Answer 1

Structural difference:
– Tight junctional resistance increases towards the colon
– Tight junctions in the small intestine are ‘leaky’
(permeable via the paracellular route)
– Tight junctions in the colon are ‘tight’ (impermeable via the paracellular route

Regional variations in uptake mechanisms:

• H2O (majority in the small intestine 6.5 litres)
• Na+ in the jejunum, ileum and colon

Electrolyte uptake: Na+, Cl-, HCO3- and K+ are absorbed isosmotically with water by the villi (as in the renal proximal tubule)

Question 2

Describe the mechanism of intestinal secretion from the crypts of Lieberkuhn

Answer 2

Zone of cell proliferation in the base of the crypts replaces the villous cells every 48 – 96 hours

Mechanism:
– Cl- enters the cell via the Na++K++2Cl- basal transporter
– Cl- diffuses across the apical membrane through apical Cl- channels (regulated by secretagogues)
– Na+ follows Cl- secretion passively via the paracellular pathway
– H2O follows NaCl secretion

Question 3

e) List common secretagogues and briefly explain their mode of action

Answer 3

2 groups:

1. Hormones/NTs: VIP, ACh, Bradykinin. Lead to release of second messenger e.g. cAMP, Ca2+
2. Bacterial enterotoxins: Cholera, E coli leads to release of second messengers e.g. cAMP, Ca2+

Second messages increase anion secretion and inhibit NaCl absorption.

I.e. Secretagogues –> Bowel movement

Question 4

Digestion and absorption of lipids

Answer 4

1. Dietary lipids are hydrophobic (insoluble in water)
2. They must be solubilised before digestion and absorption can occur
3. Digestion begins the stomach with the action of lingual and gastric lipases
4. Digestion is completed in the small intestine with the action of the pancreatic enzymes

Question 5

Describe the digestion of lipids in the stomach

Answer 5

Stomach churns and mixes lipids to initiate enzymatic digestion
Lingual and gastric lipases hydrolyse 10% of ingested triglycerides to glycerol and free fatty acids

Question 6

Key role of gastric lipase in lipid digestion in the stomach?

Answer 6

Slows the rate of gastric emptying so that pancreatic enzymes are able to digest lipid

Question 7

What is secreted in the duodenal and jejunal mucosa in response to the presence of monoglycerides and fatty acids and small peptides and amino acids?

Answer 7

Cholecystokinin secreted from the I cells of the duodenal and jejunal mucosa

Question 8

How are lipids digested in the SI?

Answer 8

1. Bile salts, lysolecithin and products of lipid digestion emulsify dietary lipids.
2. Emulsification produces small droplets of lipids dispersed in an aqueous solution creating a large surface area for pancreatic enzyme digestion
3. Pancreatic enzymes (pancreatic lipase, cholesterol ester hydrolase and phospholipase A2) and the protein, colipase are secreted to complete digestion

Question 9

Stages of digestion and absorption of lipids

Answer 9

1. Bile salts from liver coat fat droplets
2. Pancreatic lipase and colipase break down fats into monoglycerides and fatty acids stored in micelles
3. Monoglycerides and fatty acids move out of micelles and enter cells via diffusion
4. Cholesterol is transported into cells by a membrane transporter
5. Absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons
6. Chylomicrons are released into the lymphatic system

Question 10

Chylomicrons:
Structure?
Location?
Transport?

Answer 10

Structure: Chylomicrons (100 nm diameter) have a core of triglycerides and cholesterol ester - phospholipids and apoproteins on the outside (80%/20%)

Location: Chylomicrons are packaged into secretory vesicles on the Golgi membrane and are exocytosed across the basolateral membrane

Transport: The lymphatic circulation carries the chylomicrons to the thoracic duct which empties into the blood stream

Question 11

What structures in the Si and LI are responsible for absorption and secretion?

Answer 11

Small intestine:
Villi – absorption
Crypts of Lieberkuhn – secretion

Large intestine:
Surface epithelial cells - absorption
Colonic glands – secretion

Question 12

Effect of cholera toxin on intestinal secretion

Answer 12

Activates GCP in epithelial cells of the crypts. Leads to Cl- released into the lumen –> Diarrhoea

Question 13

In the clinical examination of abdomen/ GU:

What is looked at during close inspection of the hands/arms?

Answer 13

Finger clubbing: Due to Malabsorption, Inflammatory Bowen disease,lymphoma, cirrhosis
Asterixis (coarse flapping tremor): Due to hepatic encaphalopathy
Leuconychia: White deposits on nails
Koilongchia: Spoon shaped nails
Palmar erythema
Dupuytrens contracture
Spinal naevus
Purpura