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
How may sodium be absorbed?
Passive movement via the paracellular pathway or active transport via the cells
HOw is active transport of sodium described?
Transcellular
What does active transport of sodium require?
Transporters to cross the cell membrane
What are some mechanisms of sodium transport?
Na+ transport alone, Na+ transport coupled to monosaccharides, Na+ transport coupled to amino acids
How does Na+ move into the cell?
Down its gradient via transport proteins (Na+ channel or Na+/H+ exchanger)
What does sodium need to do to get out of the cell?
Go against the concentration gradient by active transport, using energy (ATP) via Na+/K+ - ATPase
What happens in passive absorption of carbohydrates?
Monosaccharides diffuse down their concentration gradient via the paracellular pathway (minor role in absorption)
What is involved in active absorption of carbohydrates?
Cotransport with Na+ via the cellular pathway
What carbohydrates are absorbed?
Monosaccharides
What is the carbohydrate transporter on the apical membrane?
Na+/glucose cotrasnporter (low concentration to high concentration)
What is the carbohydrate transporter on the basolateral membrane?
Glucose carrier (high concentration to low concentration)
What happens in passive absorption of amino acids?
Amino acids diffuse down their concentration gradient via the paracellular pathway
What pathway is active absorption of carbohydrates via?
The cellular pathway
How are di and tri-peptides absorbed?
Via H+ dependent cotransport
How are amino acids absorbed?
Via Na+ dependent cotransporters
What is the amino acid transporter on the apical membrane?
Na+ amino acid cotransporter
What is the amino acids transporter on the basolateral membrane?
An amino acid carrier
What are the products of fat digestion?
Lipid soluble so can diffuse across the cell membrane freely without a transporter
How are products of fat digestion delivered to the brush border?
By micelles
Is the whole micelle absorbed?
NO, it releases its contents and the bile salts are absorbed later
What products of fat digestion move into the cell?
Fatty acids and monoglycerides
What happens to the fatty acids and monoglycerides in the cell?
They re resynthesized into triglycerides, packaged into chylomicrons which then exit the cell by exocytosis and enter the lacteals
Where do the bile salts which are absorbed come from?
Micelles
When does bile salt absorption occur?
After fat absorption is complete
How is bile salt absorption in the ileum?
An active transport process by apical Na+ dependent bile acid cotransporter
How is bile salt absorption in the colon?
Passive
How are fat soluble vitamins absorbed?
(A, D, E, K) are absorbed with fats
What do water soluble vitamins rely on for absorption?
Na+ dependent absorption (Vitamin C)
Where is vitamin B12 absorbed?
In the ileum
What does vitamin B12 do?
It binds to intrinsic factor in the stomach and a specific transporter found in the ileum allows it to be asorbed
What is vitamin B12 deficiency?
Perinicious anemia
Do we absorb everything?
No
