It is the passage of the nutrients, the water, the electrolytes from the light of the digestive tract into the blood or the lymph.
Sites of absorption
- there is little absorption at the level of the stomach : part of alcohol, the aspirin.
- at the level of the colon : water, electrolytes, biliary salts (minor action).
The elective site of absorption is the small intestine, with only one enterocytes base and villosities.
All is made to increase the surface of absorption which is especially made on the level proximal intestine (whereas the distal one contains the faeces)
It is a tube:
- 3 m long,
- 4 cm diameter,
- 0,33m2 of surface.
The surface is multiplied by:
- circular folds ; at the level of the duodenum, they have as a role to slow down the arrival of the chyle. Their, they increase the surface of a factor 3,
- villosities, multiply surface by a factor 10. The surface passes to a factor 30 (10x3),
- microvillosities; multiplication by a factor 20. The surface passes to a factor 600. The total surface of absorption is thus of 200m2.
Ways of absorption
Absorption is done through blood vessels or lymphatic ways.
Villosity is very richly vascularized and is centered by lymphatic: central chyle vessel.
Except pathological situation, circulation is never the factor limiting absorption thanks to the capillary network and the important deviation of blood towards the digestive tract (for this reason it is necessary to avoid eating before an important muscular exercise, makes an attempt approximately 3h).
Absorption of glucids
Sites
The absorption of glucids is done primarily on the level of the duodenum (60%) and first half of the jejunum. Absorption very proximale.
Almost complete absorption: 0.5 to 1% in the faeces .
This sugar passes by blood way in the Liver.
Mechanism
Absorption relates to 3 simple sugars at the end of digestion: glucose, galactose, fructose.
80% of absorption are done by transmembrane mechanisms through the entérocytes. It is a mechanism of facilitated diffusion; these sugars use a conveyor to cross the membrane.
Glucose and galactose have the same dependent Na+ conveyor. Na+ enters the cell with sugar, it is detached from the conveyor of the fact of the gradient and involves sugar, once in the Na+ cell is expelled and glucose and galactose have two possibilities: glucose passes directly in blood by the portal vein and is brought back to the liver is glucose and galactose undergo glycolysis and give lactate and pyruvate which pass in you blood by the portal vein.
The conveyor of the fructose is not dependent Na+: either the fructose passes just as it is in the portal vein, or it gives glucose which can pass just as it is or become lactate or pyruvate. the fructose is a simple sugar with index glycemic low collected by the liver to become glucose it does not raise much the glycemia (used cherished sporting).
But for the fructose there is limitation of the number of conveyors, whereas for you glucose and galactose it does not have there a limitation on the level of absorption, conveyors in a practically infinite number.
20% of absorption paracellulaire between the tight junctions or by the holes which exfoliation leaves.
Mechanism of attraction by solvents (solvent drag); glucose attracts water and the electrolytes when it passes by way paracellulaire.
Absorption of proteins
Site
The absorption of proteins is done primarily in the third part of the duodenum and with the level of the jejunum.
Absorption is more proximale for the oltgopeptides that for the amino acids.
Absorption less complete than that of glucids: one finds 10% in the saddles of them.
Absorption is done by blood way in the portal vein then in the liver as for glucids.
Mechanisms
- amino acids
Absorption by diffusion facilitated one knows four types of conveyors:
- conveying for acidesaminés neutral, dependent Na+, it is most important,
- conveying for basic amino acids, dependent Na+, 1/10th of the activity of the precedent,
- conveying for the proline, the hydroxyproline, the glycine, Na+ independent,
- conveying for the amino acids dicarboxylic, dependent Na+, while arriving in the cell it directly engages the amino acids in the reaction of transamination.
- oligopeptides
Active transport: consume energy, it is a direct and fast mechanism of absorption. In the blood of the portal vein one does not find a di and tri peptides bus inside the entérocyte there are enzymes which finish degradation.
Absorption of the lipids
Collecting by the entérocytes
The first stage of the absorption of the lipids is micellar disintegration: one finds in contact with the edge out of brush: fatty acids, mono glycérides, cholesterol and lysolécithines.
They pass in the entérocyte by simple phenomenon of diffusion in lipidic phase, according to the gradient of concentration.
Site
All the length of the small intestine starting from the second duodenum, all the jejunum and the major part of the iléon. Great surface makes it possible biliary salts to play their part of micelle formation.
Absorption is not complete: one finds 4% in the faeces of them
Transformations intra-entérocytaires
The lipids return in molecular mono form, and leave the entérocyte in particulate form: will chylomicrons and lipoproteins, they thus undergo physical modifications. They also undergo chemical modifications like the esterification and the integration of non lipidic components (protidic).
Resynthèse of triglycerides:
The fatty acids are not free in the cytoplasm, they are dealt with by Fatty Acid Binding Protein FABP which has two roles: while taking charges the fatty acids of them, it takes part in their reesterification and prevents them from having a cytotoxic action on the cytoplasm.
The resynthèse of triglycerides is carried out:
- to 70% by the way of the mono glycérides: two fatty acids activated in the form of acylcoA and of a mono glycéride give a triglyceride
- to 30% by the way of L3-glycerophosphate or sees acids phosphatidic: a fatty glycérophosphate (metabolism of glucids) and three acids form a triglyceride.
Cholesterol and phospholipides
Cholesterol for its greater part will be esterified.
The phospholipides are partially esterify, the greatest part will be used on the spot
Transfer out of the cell
This fact in the form of particles: will chylomicrons and lipoproteins.
Will chylomicrons are of low density, they consist of 98% of lipids and protein 2%; they have a heart of triglycerides surrounded by cholesterol and surrounded by phospholipides and proteins.
For the various particles plus the density increases, plus triglycerides decrease, phospholipides and the proteins increase. It is on the level of the LDL that there is the most cholesterol.
Chylomicrons and lipoproteins are expelled of the cell by exocytose towards the lymph.
The lipids all are absorbed by lymphatic way, except the fatty acids lower than five carbons which can be absorbed by blood way.
This exocytose is supported by the presence of the apoproteins, which either are brought to the entérocyte (80%), or synthesized by the entérocyte (20%). These apoproteins will be fixed on the particles and will allow:
- the exocytose
- the later transport of the chylomicron or the lipoproteins towards the site of the metabolism.
"L'absorption digestive" french version of Digestive absorption.
