L1 - Introduction to GIT physiology part (1).pdf

General Functions of digestive system By the end of this lecture , you should be able to: 1. Outline the functional structure of the digestive system. 2. Discuss the four basic digestive functions The digestive system consists of : 1-Alimentary canal (starts at the mouth and ends at the anus)...

General Functions of digestive system By the end of this lecture , you should be able to: 1. Outline the functional structure of the digestive system. 2. Discuss the four basic digestive functions The digestive system consists of : 1-Alimentary canal (starts at the mouth and ends at the anus).  It is 5 meters long in living persons and 9 meters outside the body. Why?  It is a muscular tube and its lumen is continuous from the mouth to the anus, thus its lumen is continuous with the external environment. 2-Digestive glands 1. salivary glands, liver 2. gall bladder 3. exocrine pancreas General functions of the Gut: 1. Food digestion. Digestion is the breakdown of complex molecules into simple ones to facilitate their absorption.i.e. CHO a monosaccharides, Proteins a amino acids, Fat a fatty acids). The process of digestion needs:- – Secretion of digestive juices. – Wall motility 2. Food absorption 3. Excretion of waste products (undigested & unabsorbed food) 4. Immune function. Defensive molecules in saliva, gastric HCl., rapid turnover of cells, lymphoid follicles. Functional Structure of the gut: 1 Basic Digestive Processes ( mechanisms): Functions of digestive system is performed by 4 basic processes:- 1. Secretion 2. Motility 3. Digestion 4. Absorption 1-Gut Motility: All GIT muscles are smooth muscles except: 1-Upper 1/3 of esophagus (skeletal muscle). 2-Middle 1/3 of esophagus (skeletal and smooth). 3-External anal sphincter (skeletal muscle). Movement of materials through the digestive tract is accomplished by smooth muscle contraction ( involuntary control), except at the oral & the anal ends of the tract which contain skeletal muscles (.i.e. chewing, swallowing and defecation have voluntary control). 2  Each muscle layer of the gut is made of single unit smooth muscle cells and functions as a functional syncytium. When an action potential is elicited anywhere within the muscle mass, it travels in all directions in the muscle layer because of the presence of gap junctions between cells. The distance that it travels depends on the excitability of the muscle; sometimes it stops after only a few millimeters and at other times it travels many centimeters or even the entire length and breadth of the intestinal tract. Functional syncytium of gut smooth muscles Types of Gut motility: 1. Tonic contractions. 2. Propulsive (peristaltic) movement. 3. Mixing (segmenting) movement. 1-Tonic contractions: It is maintained mild contraction of smooth muscles of GIT wall. Cause: spike potentials or Ca⁺⁺ entry (by unknown mechanism). Function: maintenance of constant intraluminal pressure. 2- Propulsive movement. Seen in the esophagus & the intestine: The basic propulsive movement of the gastrointestinal tract is peristalsis 3  Stimulus: Stretch at one point of the gut or irritation of the mucosa (.e.g. 2ry peristalsis in the esophagus is initiated by reflux of gastric contents in the lower esophagus). Response: Contraction behind (Ach) and relaxation in front (VIP & NO) of the stretched gut segment. This moves the contents in an oral to caudal direction at a rate of 2-25 cm/sec. Control: Auerbach's plexus of ENS, but it can increase or decrease by autonomic nerves. Function: forward movement of food at an appropriate rate for digestion and absorption ( in the stomach antrum, peristalsis cause food mixing). It is fast in the esophagus and slower in the small intestine. Why? 3-Mixing movement: In the stomach antrum , peristaltic contraction cause food mixing in addition to food propulsion. Segmentation contractions are the mixing movement in the small intestine. In the large intestine , mixing movements are called Haustrations. Description: several constrictions appear in the GIT and divide it into equal segments. After few seconds, new constrictions appear in the middle of each segment while the original constrictions relax and so on. Function: help digestion and absorption of food. How? Segmentation contractions 4 2-Gut Secretion There are exocrine glands located along the wall of the gut. Each digestive juice consists of :- 1. Water & electrolytes. 2. Organic constituent.e.g. enzymes & mucus. The gland extract water & raw material from the plasma. The digestive secretion is then reabsorbed back to the blood after participation in the digestive process. Failure of reabsorption of digestive juices may lead to circulatory shock.e.g. vomiting & diarrhea 3-Digestion It is the breakdown of complex molecules into simple absorbable unites. It is accomplished by the enzymes in the digestive juices that hydrolyze certain bonds in the food molecules. 4-Absorption Occurs in the small intestine. Why? When the digested food , water and electrolytes come in contact with the intestinal mucosa , they become absorbed. Mixing movement help absorption. Undigested & unabsorbed materials are excreted to the outside by the process of defecation. 5