Lecture 30: Absorption

Question 1

What is process of absorption?

What are the key concepts about absorption

Answer 1

= Passage of substances form the lumen across the lining of the intestine not the interstitial fluid and then into the blood or lymph —-> unidirectional

• the intestine has excess capacity for absorption
• the intestinal epithelium serves as a selective barrier for absorption
• water absorption is dependent on nutrient and Na+ absorption

Question 2

What are the sites of absorption?

- Describe each site

Answer 2

1. Mouth,oesophagus and stomach = minimal absorption
2. Small intestine = main site of absorption due to massive SA
   - 90% water and sodium
   - All nutrients
3. Large intestine
   - 9% of water and sodium

Question 3

What factors affect absorption?

Answer 3

1. Motility
2. Available surface area
   - rate of absorption is proportional to SA
3. Passage of molecules across available surface area
   - Paracellular: Btw cells
   - Cellular: Across cell membranes
4. Removal of substance from the interstitial space
   - maintain gradient by removing substances from space to ensure unidirectional movement

Question 4

There are two pathways in which molecules can cross the intestinal epithelium and arrive in the interstitial space.
What is the paracellular pathway?

Answer 4

• Does not require solutes to cross membrane, rather they pass between cells
• Small molecules
• Cross through tight junctions
• Relatively non selective: if you are small enough you can cross through
• Is PASSIVE requires a gradient

Question 5

What is the cellular pathway?

Answer 5

• Solutes must cross two cell membranes
• Cell membranes are lipid bilayers = substances which are not lipid soluble require transport proteins
• tranport proteins = ACTIVE selective transport

Question 6

How is water absorbed across the intestinal epithelium ?

Answer 6

OSMOSIS
- passive moment of water form the lumen into the blood
- osmotic gradient set up by the absorption of salts and nutrients
= nutrient coupled sodium absorption

Question 7

How is sodium absorbed?

What are the 3 mechanisms for active absorption of Na+?

Answer 7

Passive = via paracellular pathway
Active = requires transporters to cross the cell membrane ---> there are 3 transport mechanisms
1. Na+ transport alone
2. Na+ absorption coupled to glucose
3. Na+ absorption coupled to amino acids

Question 8

How does Na+ absorption coupled to glucose enable absorption of carbohydrates?

Answer 8

Active transport = co-transport with Na+ = enters epithelium
Passive transport = down conc gradient = leaves epithelium
Monosaccharides: glucose, galactose, fructose

Question 9

How does Na+ absorption coupled to amino acids enable absorption of proteins?

Answer 9

Active = Co-transport with Na+
Passive = diffused down conc gradient via paracellular pathway

Question 10

How are products of fat digestion absorbed?

Answer 10

Products of fat are lipid soluble can diffuse through the membrane —> micelles deliver lipids to brush border for diffusion
After diffusion monoglycerides are re-synthesised back into TACs. They are then packaged into chylomicrons which exist the cells via exocytosis and enter bloodstream through lymph channel