SMALL INTESTINE II ABSORPTION DR. AMEL EASSAWI DR. Sharique Ahmed Quadri

Small Intestine ‘ABSORPTION’ In small intestine, carbohydrate, protein, fat, electrolytes, vitamins and water are absorbed. Absorption of calcium and iron is adjusted according to the body needs. Most absorption occurs in duodenum and jejunum and very little in the ileum [as most absorption has already taken place]. About 50% of small intestine can be removed with little interference with absorption. The vitamin B12 and bile salts are absorbed in terminal ileum [if the terminal ileum removed, their absorption will be effected].

Small Intestine ‘ABSORPTION’ The mucous lining of the small intestine is well adapted to the absorptive function of the small intestine for two reasons: Has a very large service area. The epithelial cells in the mucous lining have a variety of specialized transport mechanism. During absorption a substance must pass through the epithelial cell, diffuse through the connective tissue and then cross the wall of the capillary or lymph vessel. Intestinal absorption could be passive or active.

Small Intestine ‘ABSORPTION’ Structure of A Villus: Each villus has: Epithelial cells: joined by tight junction at the lateral border which limit the passage of the luminal content between the cells. At the luminal border the epithelial cells have carriers for absorption of nutrients and electrolyte. Connective tissue core: formed by the lamina propria. A capillary network: Each villus supplied by arteriole that break up into a capillary network within the villus core. The capillaries rejoin to form a venule that drains away the villus. A terminal lymphatic vessel: Each villus supplied by a single blind ended lymphatic vessel known as the central lacteal, which occupies the center of the villus core.

Small Intestine ‘ABSORPTION’ Dipping down into the mucosal surface between the villi are shallow invaginations known as crypts of Lieberkuhn. Crypts of Lieberkuhn secrete water and electrolyte along with the mucus secreted by the cells on the villus surface constitute succus entericus. The crypts function as a nurseries. New cells are continuously produced in the crypts migrate to up the villi. Usually the epithelial lining is replaced every three days.

Small Intestine ‘ABSORPTION’ Sodium and Water Absorption: Sodium absorbed by both passive and active transport. Sodium enter the epithelial cells across the luminal border passively through the sodium channels or by secondary active transport by Na+ Cl- symporter, Na+ H+ antiport or Na+ glucose symporter. Sodium pumped out of the cell by Na+ K+ pump. Water absorption followed the Na+. Sodium absorption result in movement of the water by osmosis.

Small Intestine ‘ABSORPTION’ Carbohydrate Absorption: Glucose and galactose are both absorbed by secondary active transport , in which the symport carriers such as sodium and glucose cotransporter (SGLT) on the luminal membrane. The glucose or galactose leave the cells down their concentration gradient via glucose transporter GLUT-2 to enter the blood. Fructose is absorbed by facilitated diffusion. It enters the epithelial cells via GLUT-5 and enter the blood via GLUT-2

Small Intestine ‘ABSORPTION’ Proteins Absorption: The end product of protein digestion is amino acid and small peptide. Amino acids are absorbed in a similar way to glucose and galactose via the amino acid symporters. The amino acid symporter are selective for different amino acids. Small peptide are absorbed with tertiary active transport. The glucose, galactose, amino acid all get a free ride on the energy expended for Na+ transport. Small peptide break down into amino acid by the action of the brush border enzymes. Amino acids enter the capillary net work within the villus.

Small Intestine ‘ABSORPTION’ Fat Absorption: The end product of fat digestion are monoglycerides and free fatty acids. The micelle are water soluble particles carry the monoglycerides and free fatty acids within their lipid soluble interiors. When the micelle approaches the absorptive epithelial surface, the monoglycerides and free fatty acids leave the micelle and passively diffuse through the lipid bilayer of the luminal membranes. The monoglycerides and free fatty acids are resynthesized into triglycerides inside the epithelial cells. These triglycerides aggregates and are coated with a layer of lipoprotein from the endoplasmic reticulum to form water-soluble chylomicrons. Chylmicrons are extruded through the basal membrane of the cells by exocytosis. Chylmicrons enter the lymphatic vessels.

Small Intestine ‘ABSORPTION’ Iron Absorption: Only a portion of ingested iron is in a form that can be absorbed, either heme iron ferrous iron (Fe+2). Iron absorbed by different energy-dependent carriers for heme and Fe+2. Dietary iron needed for red blood cell production is transferred into the blood by ion transporter ferroprtin. Iron absorption is controlled with the hormone hepcidin. In the blood iron is carried bound to plasma protein transferrin. Absorbed dietary iron that is not needed immediately is stored in the epithelial cells as ferrtin. The unused iron is lost in the feces as the ferritin-containing epithelia; cells are sloughed. Dietary ion that was not absorbed is lost in the feces.

Small Intestine ‘ABSORPTION’ Calcium and Vitamins Absorption: Calcium enter the luminal membrane of the small intestine epithelial cells down its electrochemical gradient through a specialized Ca+2 channel. Calcium exit the basolateral membrane by Ca+2 ATPase pump and Na+ Ca+2 antiporter. Vitamin D enhance calcium absorption Water soluble vitamins are absorbed passively with water Fat soluble vitamins are carried within the micelle. All nutrients absorbed through the small intestine enter the liver through the hepatic portal vein. After passing through the liver join the systemic circulation. Fat enter the lymphatic system and bypass the liver and directly join the systemic circulation. Harmful substance that may have been absorbed are detoxified by the liver before entering the systemic circulation.

Volumes Absorbed by the Small and Large intestine per day Volume entering the small intestine per day Ingested Food eaten 1250 g Fluid drunk 1250 ml Sources Secreted Saliva 1500 ml from the Gastric juice 2000 ml plasma Pancreatic juice 1500 ml Bile 500 ml intestinal juice 1500 ml 9500 ml Volume absorbed by the small intestine per day 9000 ml Volume entering the colon from the small 500 ml intestine per day Volume absorbed by the colon per day 350 ml Volume of feces eliminated from colon per day 150 g

Diarrhea Diarrhea: passage of highly fluid fecal matter, often with increase frequency of defecation. Diarrhea can be beneficial in rapid loss of harmful materials. Excessive loss of intestinal contents result in dehydration. Causes: Excessive motility result from local irritation by bacterial or viral infection. Excess osmotically active particles such as in lactase deficiency Toxins of Vibrio cholera.

Celiac Disease [Gluten Entropathy] Malabsorption may be caused by damage or reducation of the surface area of the small intestine. One of the common causes is Celiac Disease [Gluten Entropathy]. A condition in which the small intestine is sensitive to gluten (a protein constituent of wheat, barley, and rye). This condition is a complex immunologic disorder in which exposure to gluten activates T-cell response that damage the intestinal villi causing decrease of the surface area for absorption. Therefore, absorption of all nutrients is impaired.

References Human physiology by Lauralee Sherwood, seventh edition Text book physiology by Guyton &Hall,11th edition Text book of physiology by Linda .s contanzo,third edition