Lecture 7 Dr. Zahoor Ali Shaikh SMALL INTESTINE Lecture 7 Dr. Zahoor Ali Shaikh

SMALL INTESTINE In small intestine most digestion and absorption takes place. Small intestine is divided into duodenum, jejunum and ileum. We will study in Small Intestine: 1. Motility 2. Secretion 3. Digestion 4. Absorption

SMALL INTESTINE ‘MOTILITY’ Motility in small intestine includes: 1. Segmentation 2. Migrating Motility Complex [MMC] It causes mixing and slow propelling of food in small intestine, How ?

SMALL INTESTINE ‘MOTILITY’ Segmentation Because segmentation consist of oscillating, ring like contractions of smooth muscle. Between the contracted segments are relaxed area containing small amount of Chyme. Contractile ring occur every few centimeter, dividing the small intestine into segments. These segmentation contraction cause mixing of Chyme and move the food.

‘SEGMENTATION’

‘MOTILITY’ Segmentation – Initiation and Control Contraction are initiated by the small intestine pace-maker cells, which produce Basic Electrical Rhythm [BER] or slow wave. When BER reaches the threshold level in the circular smooth muscle segmentation are induced. Both duodenum and ileum start to segment as meal enter the small intestine.

‘MOTILITY’ Segmentation – Initiation and Control Increased segmentation is caused by local distension, gastrin secreted in stomach [gastro ileal reflex], parasympathetic stimulation. Sympathetic stimulation decreases segmental activity. Functions of Segmentation - Mixing of food with digestive juices. - Exposing all Chyme to small intestine mucosa for absorption.

‘MOTILITY’ Functions of Segmentation - Propelling [moving the food]. - Segmentation occur about 12/min in duodenum and 9/min in terminal ileum. - Contents take 3 to 5 hours to move through small intestine.

2. Migrating Motility Complex [MMC] MMC sweeps the intestine between meals. When most of the meals are absorbed, segmentation contraction stop and they are replaced by MMC in between the meals. What is MMC ? It consist of weak, repetitive peristaltic wave that move a short distance in the small intestine before dying out.

Migrating Motility Complex [MMC] MMC starts at the stomach and migrate down the intestine. Function of MMC is to move [sweep] the remaining contents of previous meal to the colon, therefore, MMC is called Intestinal House Keeper. MMC is regulated by hormone motilin. Motilin hormone is secreted by endocrine cells of small intestinal mucosa.

Ileocecal Sphincter Ileum opens into the cecum. Ileocecal Sphincter prevents contamination of small intestine from colonic bacteria. Ileal contents are pushed forward in cecum through Ileocecal valve. Ileocecal Sphincter [smooth muscle] is under neural [local myentric] and hormonal control. Gastrin causes relaxation.

SMALL INTESTINE ‘SECRETION’ – Succus Entricus Small intestine secretion is called succus entricus. Succus entricus does not contain any digestive enzymes. But enzymes are present in brush border [micro villi] in small intestine.

Succus Entricus Succus Entricus is secreted into the lumen by exocrine glands present in small intestine mucosa. It consist of: - Water - Mucus - Mineral Salts - pH 7.8 Secreted 1.5 liters per day. Secretion increases after meal due to local stimulation of small intestine mucosa by presence of Chyme.

SMALL INTESTINE ‘DIGESTION’ In small intestine, enzyme present in the brush border membrane complete the digestion. In the lumen of small intestine, pancreatic enzyme are acting for fat digestion, being helped by bile, therefore, fats are completely reduced to absorble units of monogylecrides and free fatty acids.

SMALL INTESTINE ‘DIGESTION’ Protein are broken down into peptide fragments and amino acids by trypsin and chymotrypsin. Carbohydrate are reduced to diasaccharides and some monosaccharides. Therefore fat digestion is completed in small intestine lumen but carbohydrate and protein digestion is not complete yet.

SMALL INTESTINE ‘DIGESTION’ In the lumen surface of small intestine on the epithelial cells there are MicroVilli – hair like projection forming the brush border. What is brush border? It is plasma membrane and contains three types of enzyme: 1. Enterokinase – which activates pancreatic Proteolytic enzyme Trypsinogen.

Brush Border 2. Diasaccharidases – maltase, sucrase and lactase, which complete digestion of maltose, sucrose, and lactose into monosaccharides. 3. Amino pepti dases – which hydrolyze small peptides into amino acids, thereby completing protein digestion. Therefore, carbohydrate and protein digestion are completed in brush border.

Applied Lactose Intolerance Occurs due to deficiency of lactase enzyme, therefore, digestion of lactose [milk and milk products] are not digested properly and lead to action of bacteria in the large intestine producing large quantity of CO2 and methane gas, distension of intestine, cramps and diarrhea.

Small Intestine ‘ABSORPTION’ In small intestine, carbohydrate, protein, fat, electrolytes, vitamins and water are absorbed indiscriminately. 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, although ileum has the power to absorb].

Small Intestine ‘ABSORPTION’ Small intestine has reserve capacity, 50% of small intestine can be removed with little interference with reabsorption. Remember – Vitamin B12 and bile salts are absorbed in terminal ileum [if removed, their absorption will be effected]. We will continue with the Absorption in the Small Intestine.

What Do You Know From This Lecture Small Intestine Motility Secretion Digestion of Carbohydrate, Fat and Protein Brush Border Lactose Intolerance

Thank you