Lecture 27: Absorption Flashcards
What is absorption?
The passage of substances from the GI lumen across the lining of the intestine into the interstitial fluid and then into the blood or lymph
What are the sties of absorption?
5
- Mouth
- Oesophagus
- Stomach
- Small intestine
- Large intestine
What does the correct rate of propulsion allow?
2
- Digestion
- Absorption
What does the correct rate of motility ensure?
Exposure of products of digestion to absorptive surface
What is rate of absorption proportional to?
Surface area
What happens when the surface area of absorption is greater?
The rate of absorption increases
What are anatomical adaptations that maximise surface area?
4
- Length of intestine
- Circular folds
- Villi
- Microvilli
What is the lumen of the intestine continuous with?
The outside world
What is a barrier between the underlying components of the intestinal lumen and the environment?
The intestinal epithelium
What are the 2 pathways across the epithelium barrier?
- Paracellular
- Transcellular
What is the paracellular pathway across the epithelium?
A gap between the cells
What is the cellular pathway across the epithelium?
Across the cell membranes through the cytoplasm
What do solutes not do?
Cross cell membranes
What is the only barrier for solutes using paracellular pathways?
Tight junctions binding cells together
Paracellular pathways are relatively what?
Non-selective
If a solute is relatively what? it can get across a paracellular pathway
Small
What does movement through a paracellular pathway require?
A concentration gradient
Using a cellular pathway what must solutes cross?
Two cell membranes
What are cell membranes?
Lipid bilayers
If a solute is not lipid soluble what does it require for the cellular pathway?
A transport protein
To maximise the absorption across available surface area nutrients are reduced to what?
Their smallest possible unit
What do specific transport proteins do?
Absorb what is required
What do specific transport proteins allow?
Active transport against a concentration gradient
What is there a large blood flow to?
The intestine
What do the arrangement of blood vessels and lacteal in villi prevent?
The build up in interstitial fluid
What is the mechanism of water absorption?
Osmosis
What is osmosis?
Passive movement of water from lumen into blood
How is the osmotic gradient set up?
By the absorption of salts and nutrients
How is sodium absorbed?
2
- Passive movement via paracellular pathways
- Active transport via cells
Active transport of sodium across cell membranes can use what mechanisms?
(3)
- sodium transport alone
- sodium transport coupled to monosaccharides
- sodium transport coupled to amino acids
When moving alone what does sodium move down?
Its concentration gradient
When moving alone what does sodium move through?
Transport proteins
When moving out of a cell what does sodium need to move from and to?
From a low concentration to a high concentration
What does sodium use to move out of a cell?
ATP
How are carbohydrates absorbed?
Active absorption/Cotransport with sodium via cellular pathway
What are carbohydrates absorbed as?
Monosaccharides
How are proteins absorbed?
By passive or active absorption
When moving passively what do amino acids move down?
Concentration gradient via paracellular pathways
What are Di- and tri- peptides absorbed via?
via H+ dependant cotransport
What are amino acids absorbed via?
Via sodium dependent cotransport
Are products of fat digestion lipid-soluble?
Yes
What can lipid soluble products diffuse across?
The cell membrane
How are lipid soluble products delivered to the brush border?
Micelles
In the cell what are products of fat resynthesised into?
Triglyerides
What do products of fats in the cell get packaged into?
Chylomicrons
What do chylomicrons exit the cell by?
Exocytosis
What do chylomicrons enter?
The lacteals
what happens to bile salts in micelles?
They are eventually absorbed
Where are bile salts from micelles absorbed?
Ileum
When are bile salts from micelles absorbed?
After fat absorption is complete
How are bile salts absorbed in the colon?
by passive absorption
What are the 2 classes of vitamin?
- Fat soluble
- Water soluble
What are 4 fat soluble vitamins?
vitamin A, D, E, K
What are fat soluble vitamin absorbed with?
Fats
What are water soluble vitamins?
2
- Sodium
- Vitamin C
Where are faeces formed?
Large intestine
