See also: malabsorption
Micelles
At the level of the small intestine, pancreatic lipases digest fatty acids, giving rise to many very fine aggregates called micelles. Inside these small "transporters", essential for carrying the lipophilic molecules into the cells responsible for their absorption, are contained the products of lipid digestion:
cholesterol, vitamins, bile salts
monoglycerides and fatty acids resulting from the digestion of triglycerides
lysophospholipids and fatty acids deriving from the digestion of phospholipids
The absorption of fats occurs mainly in the intermediate tract of the small intestine, called jejunum.
Absorption of nutrients
Thanks to the small size and the solubilizing action of the bile salts, the micelles are soluble in the aqueous environment.
Arrived in proximity of the microvilli that cover the external surface of the intestinal villi (brush border), the micelles release their contents. The individual components, by virtue of their lipophilicity, are able to cross the plasma membrane of the brush border and penetrate the enterocytes.
At this point these substances, in order to be released into the plasma or lymph, must necessarily come together and form lipoproteins, real agglomerates consisting of a lipid portion and a protein portion.
- In the cytoplasm of the enterocyte, monoglycerides are joined to fatty acids to reform triglycerides (exactly the reverse of what happened in the stomach and especially in the initial sections of the small intestine). Similarly, lysophospholipids are joined to fatty acids, giving rise to phospholipids .
- At this point, a lipoprotein is produced, called chylomicron and made up of a lipid heart (made up of triglycerides, phospholipids, cholesterol and vitamins), surrounded by protein molecules. This sort of mantle, thanks to the water solubility given to it by the proteins, increases the degree of solubility of the chylomicron in the aqueous medium.
After exiting the enterocyte with a mechanism of exocytosis, the chylomicrons pass into the interstitial fluid and from there to the lymphatic vessels inside the villus. The acids and bile salts are instead absorbed in the ileum (terminal part of the small intestine), conveyed into the bloodstream and transported to the liver, where they are recycled and secreted again with the bile.
- We remind you that, unlike lipids, all the other products of intestinal absorption (carbohydrates, amino acids, water, mineral salts and water-soluble vitamins) enter directly into the blood capillaries by simple diffusion, facilitated or active transport. Even short and short fatty acids medium chain, which represent only a small part of the lipids contained in food, reach the blood capillaries directly.
- As for the mineral salts, some, such as sodium, chlorine, potassium and phosphorus, are easily absorbed, while divalent cations, such as calcium, iron and zinc, are absorbed with greater difficulty. This explains why the lack of these minerals is so common and how their integration is often proposed in both health and sports.
- Water-soluble vitamins, with the exception of B12 which requires the intrinsic factor secreted by the gastric glands, are absorbed with relative ease.
After being produced by the intestinal cells, the chylomicrons are conveyed into the lymphatic circulation, which flows into the bloodstream at the level of the subclavian veins. This apparently useless obligatory path is thought to be linked to the permeability of chylomicrons. These large agglomerations, by virtue of their large size, would in fact encounter many difficulties in crossing the blood capillaries inside the villus.
In summary, chylomicrons pass into the blood only after being absorbed in the intestine and transported from the lymph to the blood.
- All the other nutrients absorbed in the intestinal villi are poured into the blood capillaries which, gathering in the venules, flow into a vessel called the mesenteric vein, in turn connected into a larger duct directed to the liver and called the portal vein. Therefore, with the exception of lipids, all the products of intestinal absorption meet as the first organ the liver, an essential structure for their metabolism. It receives blood from two imported vessels: the hepatic artery, which comes from the aorta and the portal vein, coming from the intestine.
Arterial blood directed to the liver is transported by the hepatic artery which, once reached the organ, branches into many small arterioles and capillaries. As mentioned a few lines ago, the liver also receives blood from the portal vein which carries the blood rich in nutrients (with the exception of lipids) flowing from the intestine.
The venous blood leaves the liver through the hepatic vein, flows into the inferior cava and from there it reaches the heart and the systemic cycle.
The chylomicrons
Once in the vicinity of muscle or fat cells, the chylomicrons carried by the blood slow down their march and bind to sites on the capillary wall. Thanks to this bond, the chylomicron transfers part of the triglycerides to the tissues (especially the muscle and adipose tissue), reducing its lipid load.
Subsequently, the chylomicrons low in triglycerides (called remnants) arrive at the liver, penetrating it. The hepatocytes, after having incorporated them, digest the external protein envelope, releasing their lipid content (residual triglycerides, cholesterol, phospholipids and fat-soluble vitamins).
Triglycerides are partly used as a reserve and partly degraded for energy into glycerol plus fatty acids.The latter, after entering the Krebs cycle, will be further degraded to water and carbon dioxide, with the formation of ATP.
Phospholipids can be used for energy or structural purposes, in the latter case they participate in the renewal of plasma membranes.
The fat-soluble vitamins are partly released into the circulation and partly stored in the liver, to cope with any vitamin deficiencies. Just to cite an example, the reserves of vitamin A in a healthy and well-nourished organism are such as to guarantee the proper functioning of the body for a period of one or two years.
Cholesterol, essential for supporting various metabolic functions, is partly used as a constituent of plasma membranes and partly as a precursor of steroid hormones and bile salts. Unlike other nutrients, cholesterol cannot be transformed or broken down for energy purposes. Any excess can only be eliminated through the bile which, when released into the liver, favors its elimination with the faeces.
