VPB 121: Veterinary Physiology - II (2+1)

LIPID ABSORPTION

• Lipid absorption begins in the dental duoderium and ends in proximal part of the jejunum.
• As the micelles come in close contact with surface of enterocytes, the lipid components diffuse through the glycocalyx to the apical membrane by a special fatty acid binding proteins which aids the transport the fatty acids across the cell membrane.
• Other components in the micelle such as monoglycerides, cholesterol and vitamin A diffuse into the apical membrane.
• Bile salts get detached from  the micelles during fat absorption and remain in a free state. In the ileum, Na-co transport system function as specific bile acid transport which nearly complete the absorption of bile salts.
• After absorption, the bile salts are transported to liver by the portal blood.
• The liver extracts bile acids and maintains the normal concentration of bile acids in systemic blood. This process of recirculation  of  between liver and intestinal lumen is referred as enterohepatic circulation of bile.
• Glycerol is absorbed by passive diffusion into the mesentric venous blood.
• Short chain fatty acids up to C¹⁰ are water-soluble  get into mesentric portal blood.
• Monoglycerides and long chain fatty acids enter the microvilli and pass on to the lacteal by simple diffusion.
• Only free form of cholestrol can be absorbed, whereas cholestrol esters must be hydrolyzed by pancreatic brush border hydrolases..
• Cholesterol is absorbed less efficiently than triglycerides. Presence of hydro cholesterol or plant sterols inhibits cholesterol absorption.
• In the epithelial cells, cholesterol is re-esterified before their transfer to lacteals.
• Before absorption phospholipids are hydrolysed to free fatty acids and lysophospholipids by phospholilpase of pancreas and intestinal epithelium.
• Within the epithelial cells, long chain fatty acids are converted into fatty acyl-CoA involving co-enzyme A and ATP.
• The fatty acyl co-enzyme reacts with monoglycerides to form di and tri glycerides.
• The newly formed triglycerides differ from that of dietary fat. GlycerolPO₄ derived from glucose metabolism provides glycerol residue for the triglyceride synthesis.
• In addition, phospholipids and cholesterol esters are produced in the epithelial cells. Small amounts of proteins are added to the lipid droplet before their transfer from epithelial cells to lymph.
• Chylomicrons are the products containing high amount of triglycerides, low level of phospholipids, cholesterol esters and proteins. They leave the cell by reverse pinocytosis and enter into the lacteals.
• Lipid absorption begins in the distal duodenum and completed in the proximal jejunum.
• The absorbed fat is in the form of an emulsion and imparts a milky appearance to the lymph,called as “chyle”.  This leave the cell by reverse pinocytosis and enter in the lacteal. Though lymphatic channel and thoracic duct it is added into the blood for its distribution to tissues.
• Short chain fatty acids are absorbed by blood from colon and caecum in sheep and horse and ceecum in pigs.