Lecture 37: GIT 5

Question 0

What is the importance of the small intestine surface area?

Answer 0

The more surface area the more absorption that can occur.

Loss of surface areas impairs digestion and absorption

Question 1

Give an overview of the structure and function of the small intestine

Answer 1

• Enzymatic degradation of carbohydrates, proteins and fats
• Absorption of degradation products
• Absorption of vitamins, bile salts, electrolytes, water
• Secretion of electrolytes and water
• Mixing and aboral movement of digesta at appropriate rate

Question 2

What are the two types of cells on the villi of the small intestine

Answer 2

• Crypt cells =secretory

* Villous cells =absorptive

Question 3

How is carbohydrates digested and absorbed?

Answer 3

• Carbohydrates must be broken down to monosaccharides before they can be absorbed in small intestine
• Initial degradation by salivary amylase
• In small intestine, pancreatic amylase degrades starch and glycogen into saccharides (sugars) containing 2-9 glucose units (maltose most common)
• Brush border enzymes then degrade maltose and other simple carbohydrates to monosaccharides

1. Dietary polysaccharides starch and glycogen converted to maltose by salivary and pancreatic amylase
2. Maltose, lactose and sucrose converted to monosaccharides by brush border enzymes
3. Glucose and galactose absorbed across apical membrane by secondary active transport with Na+ (SGLT1)
4. Fructose absorbed by facilitated diffusion (GLUT5)

Question 4

How is protein digested and absorbed?

Answer 4

• Exogenous (dietary) proteins from plants and animal tissue
• Endogenous protein from sloughed mucosal epithelial cells
• Gastric pepsin degrades some dietary protein to peptides
• Majority of degradation takes place in small intestine, by action of pancreatic proteases
• Degradation products mainly short-chained peptides
• As with carbohydrates, final degradation step is by brush border enzymes
• Produces amino acids and small peptides with 2-3 amino acids

1. Proteins hydrolysed by gastric pepsin and pancreatic proteolytic enzymes
2. Amino acids absorbed across apical membrane by cotransport with Na+ – carriers specific to various types of amino acids
3. Small peptides absorbed by different carrier – broken down to amino acids by brush border aminopeptidases or intracellular peptidases

Question 5

How are lipids digested and absorbed?

Answer 5

• Dietary lipids primarily triglycerides, with some phospholipids and cholesterol
• Other lipids enter intestine in secretions such as bile
• Pattern of lipid digestion similar to carbohydrates and proteins – large molecules must undergo enzymatic breakdown to smaller molecules before absorption can occur
• One major difference – lipids are insoluble in water

Sites and steps in lipid digestion:

1. Fat in stomach: warming and mixing action results in the formation of liquid fat globules
2. Duodenum: exposure to bile leads to the production of emulsified fat droplets
3. Jejunum: combined action of lipase, and bile components leads to micelle formation
4. Jejenum: diffusion of micelles through the unstirred water allows direct transport of most micelle components, except bile acids, into the enterocytes
5. Ileum: specialised Na co-transport proteins in the ileum are responsible for the bile acid absorption
6. Large fat globules emulsified by bile salts
7. Pancreatic lipase hydrolyses triglycerides into monoglycerides and free fatty acids
8. By action of bile salts, these water-insoluble products transported to epithelial cells as micelles
9. At brush border, monoglycerides and free fatty acids leave micelles and passively diffuse through cell membrane
10. Once inside cells, monoglycerides and free fatty acids resynthesised into triglycerides
11. Triglycerides aggregate and are covered with layer of lipoprotein to form chylomicrons, which leave basal membrane by exocytosis
12. Chylomicrons enter lymphatic vessels

Question 6

Sodium absorption

Answer 6

• In small intestine, nutrient-coupled Na+ transport occurs via SGLT1 (secondary active) transporter on luminal surface of cell and GLUT-2 (facilitated diffusion) transporter on basolateral surface
• SGLT1 transporter driven by downhill electrochemical Na+ gradient generated by basolateral Na+/K+-ATPase pump
• Cl- absorbed passively via paracellular pathway, in response to lumen negative potential difference generated by movement of other ions
• In small intestine and colon, extracellular Na+ exchanged for intracellular H+ via NHE3 exchanger
• Driven by electrochemical Na+ gradient generated by basolateral Na+/K+-ATPase pump and pH gradient resulting from moderately acidic intracellular environment
• NHE3 inhibited by both cAMP and cGMP
• In ileum and proximal colon, Na+ absorption is coupled to the movement of Cl- through a Cl–HCO3- anion exchanger
• HCO3- produced by intracellular metabolism and from intracellular CO2
• Tight coupling with NHE3 exchanger in results in electroneutral Na+ and Cl- absorption, maintenance of cell pH and luminal release of H+ and HCO3- (water + CO2)
• In distal colon, electrogenic Na+ absorption occurs via Na+-specific, aldosterone-sensitive ion channel
• Driven by downhill electrochemical Na+ gradient generated by basolateral Na+/K+-ATPase pump and K+ channel
• Cl- absorbed passively via paracellular pathway, in response to lumen negative potential difference generated by movement of other ions

Question 7

Potassium absorption

Answer 7

• Potassium usually plentiful in diet
• Also present in gland secretions
• Therefore present in high luminal concentrations
• Further concentrated by reabsorption of water
• Action of basolateral Na+-K+-ATPase pump produces low K+ concentration in intracellular spaces
• Potassium passively reabsorbed through paracellular spaces, moving down concentration gradient

Question 8

Explain the general mechanism and importance of water reabsorption in the small intestine and large intestine

Answer 8

• As always, passive – in response to osmotic gradients
• Mucosa of upper small intestine freely permeable to water
• In small intestine, much water absorption paracellular
• Total volume absorbed daily is LARGE (>10% bw in healthy individual)

Question 9

Intestinal secretion

Answer 9

• Thus far, have been concentrating on water and electrolytes secreted from liver, pancreas, and salivary glands
• Significant amount of secretion also occurs from intestinal mucosa
• Water moves into lumen in response to passively or actively generated osmotic gradients
• Osmotic gradients passively generated by normal digestion processes (eg breakdown of carbohydrates)
• Osmotic gradients also generated by active secretion of chloride by immature crypt epithelial cells
• In both small and large intestine, Cl- (and HCO3-) secreted from crypt cells
• Provides main driving force for fluid secretion from serosal to luminal compartment (passive movement of Na+ and H2O in response to electrical and osmotic gradients generated)
• Also main source for luminal Cl- for Cl–HCO3- anion exchanger
• Regulated by second messengers such as cAMP, cGMP and Ca2+
• Disruption leads to secretory diarrhoea