Intestinal Absorption (The Digestive System) PDF

Anatomy and Physiology I. Pharmacy The Digestive System: Absorption in the Small Intestine Dr. Teresa Olivar Department of Biomedical Sciences 2 All the chemical and mechanical phases of digestion from the mouth through the small intestine are dir...

Anatomy and Physiology I. Pharmacy The Digestive System: Absorption in the Small Intestine Dr. Teresa Olivar Department of Biomedical Sciences 2 All the chemical and mechanical phases of digestion from the mouth through the small intestine are directed toward changing food into forms that can pass through the absorptive epithelial cells lining the mucosa and into the underlying blood and lymphatic vessels. Carbohydrates: monosaccharides (glucose, fructose and galactose) Proteins: Single amino acids, dipeptides and tripeptides Lipids: Glycerol, fatty acids, monoglycerides Dr Teresa Olivar 3 Absorption of materials occurs via: Diffusion Facilitated diffusion Osmosis Active transport 90% of all absorption of nutrients occurs in the small intestine 10% occurs in the stomach (alcohol and aspirin) and large intestine (water, some vitamins and electrolytes) Dr Teresa Olivar 4 Absorption of monosaccharides All dietary carbohydrates that are digested normally are absorbed, leaving only indigestible cellulose and fibers in the feces. They pass from the lumen through the apical membrane via facilitated diffusion or active transport. (epithelial cell) Fructose: via facilitated diffusion Glucose and galactose: via secondary active transport coupled to the active transport of Na+ Monosaccharides move out of the absorptive cell through their basolateral surface via facilitated diffusion and enter the capillaries of the villi. Dr Teresa Olivar 5 6 Absorption of amino acids, dipeptides and tripeptides Most proteins are absorbed as amino acids via active transport. About half of the absorbed amino acids are present in food. The other half come from the body itself as proteins in digestive juices and dead cells that slough off the mucosal surface. 98% of the amino acids are absorbed. Different transporters carry different types of amino acids. Dr Teresa Olivar 7 Absorption of amino acids, dipeptides and tripeptides Amino acids: via Na+-dependent secondary active transport or active transport Dipeptides and tripeptides: secondary active transport with H+. Peptides are hydrolysed to single amino acids inside the absorptive cells Amino acids move out of the absorptive cells via diffusion and enter capillaries of the villus. Dr Teresa Olivar 8 9 Monaccharides and amino acids  blood Both monosaccharides and amino acids are transported in the blood to the liver by way of the hepatic portal system. If not remove by hepatocytes, they enter the general circulation. Dr Teresa Olivar 10 Absorption of lipids Dietary lipids are absorbed via simple diffusion. Adults absorb about 95% of the lipids present in the chyme. Newborn infants absorb only about 85%, due to lower production of bile. Tryglicerides are broken down into monoglycerides and fatty acids. These fatty acids can be: Short-chain fatty acids Long-chain fatty acids Dr Teresa Olivar 11  Short-chain fatty acids: Although they are hydrophobic, they are very small in size. Because of their size: They can be dissolved in the watery intestinal chyme. They pass through the absorptive cell via simple diffusion. They follow the same route taken by monosaccharides and amino acids into blood capillary of a villus. Dr Teresa Olivar 12  Long-chain fatty acids and monoglycerides: They are large and hydrophobic. They can not be dissolved in the watery intestinal chyme Emulsification: bile salts help to make them more soluble. The bile salts surround the long-chain fatty acids and monoglycerides, forming tiny spheres called micelles. Dr Teresa Olivar 13 Micelles They are formed due to the amphipathic nature of bile salts. The hydrophobic regions of bile salts interact with the long- chain fatty acids and monoglycerides. The hydrophilic regions of the bile salts interact with the watery intestinal chyme. Once formed, they move from the interior of the small intestinal lumen to the brush border of the absorptive cell. The long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cell, leaving the micelles behind in the chyme. Dr Teresa Olivar 14 Micelles The micelles continually repeat this ferrying function. They move from the brush border back through the chyme to the interior of the small intestinal lumen to pick up more long-chain fatty acids and monoglycerides. Micelles solubilize: Long-chain fatty acids and monoglycerides Fat-soluble vitamins (A, D, E and K) Cholesterol Dr Teresa Olivar 15 Once inside the absorptive cell long- chain…….. Chylomicrons! Fatty acids and monoglycerides are recombined to form triglycerides. They aggregate into globules along with phospholipids and cholesterol and become coated with proteins. These large spherical masses (about 80 nm in diameter) are called chylomicrons. Chylomicrons leave the absorptive cell via exocytosis. Because they are so large and bulky, chylomicrons can not enter blood capillaries (the pores in the walls of blood capillaries are too small). Chylomicrons are transported by way of lymphatic vessels to the thoracic duct and enter the blood at the left subclavian vein. Dr Teresa Olivar 16 17 Dr Teresa Olivar 18 Dr Teresa Olivar 19  The hydrophilic protein coat keeps the chylomicrons suspended in blood and prevents them from sticking to each other. Within 10 minutes after absorption, about half of them have already been removed from the blood as they pass through blood capillaries in the liver and adipose tissue. This removal is accomplished by an enzyme attached to the apical surface of capillary endothelial cells (lipoprotein lipase). This enzyme breaks down triglycerides in chylomicrons and other lipoproteins into fatty acids and glycerol. The fatty acids diffuse into hepatocites and adipose cells and combine with glycerol during resynthesis of triglycerides. Two or three hours after a meal, few chylomicrons remain in the blood. Dr Teresa Olivar 20 Bile salts recycling 90-95% of the bile salts are reabsorbed by active transport in the ileum. They are returned by the blood to the liver through the hepatic portal system for recycling. Recall! Bile salts present enterohepatic circulation Bile salt secretion by hepatocytes into bile Reabsorption by the ileum Resecretion into bile Dr Teresa Olivar 21 Absorption of vitamins Fat-soluble vitamins A, D, E and K are included with ingested dietary lipids in micelles and are absorbed via simple diffusion. Most water-soluble vitamins, such as mot B vitamins and vitamin C, also are absorbed via simple diffusion. Vitamin B12, combines with intrinsic factor produced by the stomach, and the combination is absorbed in the ileum via an active transport mechanism, Dr Teresa Olivar 22 Absorption of water All water absorption occurs via osmosis. Because water can move across the intestinal small intestine depends on the absorption of electrolytes and nutrients to maintain an osmotic balance with the blood. The absorbed electrolytes, monosaccharides and amino acids establish a concentration gradient for water. Dr Teresa Olivar 23