Lecture 27: ABSORPTION Flashcards
What is absorbance?
The (net) passage of substances from the GI lumen across the lining of the intestine into the interstitial fluid and then into the blood or lymph
Where are the sites of absorption?
Mouth, esophagus, stomach, small intestine and large intestine
What absorption happens at the mouth, esophagus and stomach?
Minimal (lipid soluble substances only)
What is the main site of absorption?
Small intestine
What absorption happens at the large intestine?
9% of water and sodium
What factors affect absorption?
Motility, surface area available for absorption, transport across epithelium (reduction in size by chemical digestion and membrane transporters), removal from interstitial fluid
What is required to allow digestion and absorption?
The correct rate of propulsion
What motility affects absorption?
Peristalsis at an appropriate rate and segmentation to expose products of digestion to absorptive surfaces
What is the rate of absorption proportional to?
Surface area - the greater the surface area the faster the rate of absorption
What are the anatomical adaptations to maximise surface area for absorption?
Length of intestine (6m), circular folds, villi and microvilli
What is the problem with transporting molecules across the epithelium?
The lumen of the intestine is continuous with the outside world and the intestinal epithelium is a barrier
What is the paracellular pathway?
The ‘gap’ between cells
What do solutes fo in the paracellular pathway?
Don’t cross the cell membrane, the only barrier is tight junctions
How selective is the paracellular pathway?
Relatively non-selective - if the solute is small enough it can get across
Is the paracellular pathway passive or active?
Passive - requires a gradient (high concentration in the lumen to low concentration in the interstitial)
What do solutes do in the cellular pathway?
Travel across two cell membranes and through the cytoplasm
What is required if the solute isn’t lipid soluble?
A transport protein as membranes are lipid bilayers
What are specific transport proteins required for?
In order to absorb what is required and allow active transport against a gradient
What is required to remove substances from the interstitial fluid?
A large blood flow to the intestine (25% of cardiac output) and the arrangement in the villi of blood vessels and lacteals prevents build up in the interstitial fluid
How much water do we drink?
About 1.5L per day which replaces the water lost in sweat, urine, faeces and as we breathe
What is the total amount of water delivered to the small intestine?
About 9-10 L per day (including secretion)
What happens if we don’t replace the losses and secreted water?
There is a major problem
What is osmosis?
Passive movement of water from lumen (high water concentration) into the blood (low water concentration)
How is the osmotic gradient set up?
By absorption of salts and nutrients
