GASTROINTESTINAL PHYSIOLOGY PHYSIOLOGY DEPARTMENT KAMPALA INTERNATIONAL UNIVERSITY DAR ES SALAAM TANZANIA Anatomy of the GI Tract The GI tract is essentially a hollow tube connecting the mouth to the anus. The GI tract has a similar layout through out its length; An inner mucosal layer with an epithelial lining A submucosal layer

A thin layer of muscle , the Muscularis Mucosa is at the junction of the mucosal and sub mucosal layers, outside this are the nerves of the mucosal plexus A muscular layer with an inner circular muscle layer and an outer longitudinal layer Between the muscles are the nerves of the myenteric plexus A serosal layer which is continuous with the mesentry GIT Gastrointestinal tract Long hollow muscular tube consisting of structures from the mouth to the anal canal It is about 5 meters long

GIT consists of; Oral cavity Pharynx Esophagus Stomach Small intestine Large intestine Rectum Anal canal Mouth: Foodstuffs are broken down mechanically by chewing and saliva is added as a lubricant. In some species, saliva contains amylase, an enzyme that digests

starch. Esophagus: A simple conduit between the mouth and stomach - clearly important but only marginally interesting compared to other regions of the tube. Stomach: Where the real action begins - enzymatic digestion of proteins initiated and foodstuffs reduced to liquid form. Small Intestine: The most exciting place to be in the entire digestive system - this is where the final stages of chemical enzymatic digestion occur and where almost almost all nutrients are absorbed. Large Intestine: Major differences among species in extent and importance - in all animals water is absorbed,

bacterial fermentation takes place and feces are formed. Accessory glands of GIT include; Salivary glands Liver: The center of metabolic Liver and gall bladder activity in the body - its major Pancreas role in the digestive process is to provide bile salts to the small intestine, which are critical for digestion and absorption of fats. Pancreas: Important roles as both an

endocrine and exocrine organ - provides a potent mixture of digestive enzymes to the small intestine which are critical for digestion of fats, carbohydrates and protein. OVERVIEW OF Digestive System Four major activities of GI tract; 1. Motility Propel ingested food from mouth toward rectum 2. Secretion of juices e.g. saliva

Aid in digestion and absorption 3. Digestion Food broken down into absorbable molecules 4. Absorption Nutrients, electrolytes, and water are absorbed or transported from lumen of GIT to blood stream GIT MOTILITY 1. Motility in the mouth 2 types; a) Chewing or Mastication:

It is reflex in nature Significance: 1. Breaks the food into small pieces to be easily swallowed 2. Expose food to salivary amylase enzyme, which begins digestion of starch 3. Help digestion of all types of food especially cellulose containing food e.g. vegetables b) Swallowing: Def.

Swallowing is the transport of food from mouth to stomach Steps: It consists of 3 phases or steps; 1) Buccal Phase: food is pushed back into pharynx from mouth Motility of the GIT b) Swallowing:

2) Pharyngeal Phase: food pass through pharynx to esophagus Motility of the GIT b) Swallowing: 3) Oesophageal Phase: food pass through esophagus to stomach by peristaltic movements

Motility of GIT 2. Motility of Esophagus The esophagus is 25 cm ms tube It is guarded by 2 sphincters; 1.Upper esophageal sphincter prevents air from entering the GIT 2.Lower esophageal sphincter prevents gastric contents from re-entering the esophagus from the

stomach Esophageal peristalsis sweeps down the esophagus Motility of GIT 3. Motility of Stomach The stomach consists of fundus, body and pylorus Proximal area (fundus and body) has a thin wall and contracts weakly and infrequently holds large volumes of food (to store food) because of receptive relaxation

Distal area (pylorus) has thick wall with strong and frequent peristaltic contractions that mix and propel food into the duodenum. Also, distal area is responsible for gastric emptying into duodenum MOTILITY OF GIT 3. Motility of stomach Gastric peristalsis MOTILITY OF GIT

4. Motility of Small intestine Types: Two basic motility patterns exist; segmentation and peristalsis. Significance: Motility of the small intestine serves 3 functions: 1. Mixing contents with enzymes and other secretions help digestion 2. Maximizing exposure of the contents to membranes of intestinal cells help absorption and

digestion. 3. Propulsion of contents into the large intestine. MOTILITY OF GIT Segmentation movements 1 ~ 5 cm MOTILITY OF GIT PERISTALSIS Orad

Peristalsis caudad Motility of GIT 5. Motility of Large intestine Types: Include : a) Segmentation in the large intestine causes the contents to be continuously mixed b) Mass movement propels the contents of one segment of the large

intestine into the next downstream segment. c) Defecation involves involuntary reflexes and voluntary reflexes evacuation of colonic content through anal canal Secretory Functions OF GIT/ Secretions of GIT SECRETIONS IN THE MOUTH Salivary secretion= 1500ml/day on average

Salivary Glands Three major glands Parotid Sublingual Submandibular Minor salivary gland such as Ebners gland Functions of saliva 1.Lubricates, cleanes oral cavity 2.Dissolves chemicals

3.Suppresses bacterial growth 4.Digest starch by amylase Regulation of salivary secretion GASTRIC SECRETION Cephalic Phase 1. The taste or smell of food, tactile sensations of food in the mouth, or even thoughts of food stimulate the

medulla oblongata (green arrow). Taste or smell of food Tactile sensation in mouth 2. Parasympathetic action potentials are carried by the vagus nerves to the stomach (pink arrow). 3. Preganglionic parasympathetic Medulla oblongata

1 vagus nerve fibers stimulate postganglionic neurons in the enteric plexus of the stomach. 4. Postganglionic neurons stimulate secretion by parietal and chief cells and stimulate gastrin secretion by endocrine cells. 5 Vagus nerves

2 3 Gastrin 4 Circulation 5. Gastrin is carried through the circulation back to the stomach (purple arrow), where it stimulates secretion by parietal and chief cells.

Secretions stimulated Stomach Vagus nerves Medulla oblongata 1

Gastric Phase Secretions stimulated 1. Distention of the stomach activates a parasympathetic reflex. Action potentials are carried by the vagus nerves to the medulla oblongata (green arrow). 2 2. The medulla oblongata stimulates stomach

Distention 3 secretions (pink arrow). 3. Distention of the stomach also activates local reflexes that increase stomach secretions (purple arrow). Stomach Local reflexes

stimulated by stomach distention Intestinal Phase 1. Chyme in the duodenum with Vagus nerves a pH less than 2 or containing fat digestion products (lipids) inhibits gastric secretions by

three mechanisms. Medulla oblongata 2. Sensory vagal action potentials to the medulla oblongata (green arrow) inhibit motor action potentials from the medulla oblongata (pink arrow). 3. Local reflexes inhibit gastric secretion (orange arrows). 4. Secretin, gastric inhibitory

polypeptide, and cholecystokinin produced by the duodenum (brown arrows) inhibit gastric secretions in the stomach. Decreased gastric secretions Vagus nerves 2

Local reflexes 1 pH<2 or lipids Secretin, gastric inhibitory peptide, cholecystokinin 3

4 Circulation FUNCTIONS OF GASTRIC HCl 1. Activates pepsinogen into pepsins Pepsinogen HCl pepsin 2. Provides optimum pH for action of pepsins

3. Kills bacteria in food 4. Help Fe2+ Ca2+ absorption. 5. Promotes pancreatic, small intestinal and bile secretion Pancreatic secretion Secretion=1500ml/day Pancreas has 2 functions; a) Endocrine functions: secretes insulin and glucagon from islets of Langerhans b) Exocrine function: secretion of pancreatic juice

It has 2 components: aqueous and enzymatic components. Aqueous component (contains HCO3) is important for neutralizing stomach acid in the duodenum so pancreatic enzymes can function properly Enzymatic component is essential for the proper digestion and absorption of carbohydrates, fats, and proteins Pancreatic enzymes include trypsin, chemotrypsin, lipase, and amylase Regulation of pancreatic secretion

Liver and gall bladder Functions of the Liver; Liver secretion=500ml/day 1) Metabolic regulation Store absorbed nutrients, vitamins Release nutrients as needed 2) Hematological regulation Plasma protein production Remove old RBCs 3) Production of bile

Required for fat digestion and absorption Small intestine secretion Secretion from duodenal gland and intestinal gland Secretory volume is 1 3L/day It contains inorganic ion, mucoprotein, IgA, various enzyme, e.g. enterokinase ,etc Function: 1. Protective effect by mucous 2. Digestion by enzymes such as peptidase, sucrase, lipase 3. Dilution Small intestine enzymes

Secretion of large intestine 1. Colonic alkaline secretion to neutralize acids produced by intestinal bacteria 2. Secretion of mucous for protection, lubrication of fecal matter 3. Vitamin B and K absorption made from bacterial flora in colon DIGESTION AND ABSORPTION

Digestion and absorption Digestion is a process essential for the conversion of food into a small and simple form. Mechanical digestion by mastication and swallowing Chemical digestion by enzymes Absorption is the process of transporting small molecules from the lumen of the gut into blood stream or lymphatic vessel.

Digestion and absorption Small intestine is the primary site for digestion and absorption of food. Digestion occurs in the GI lumen by secreted enzymes and on surface of enterocytes by membrane-bound enzymes. Absorption occurs by simple diffusion, facilitated diffusion, active transport, endocytosis, and paracellular transport. Surface area of small intestine is greatly increased by extensive folding

Digestion of carbohydrate (salivary and pancreatic) Absorption of Carbohydrate Enterocytes absorb glucose and galactose through an Na-dependent secondary active transport process, while fructose is absorbed by facilitated transport.

Digestion and absorption of proteins Absorption of proteins The whole proteins by endocytosis Amino acids and di and tripeptides by Nadependent secondary active transport Lumen Endocytosis

K N + a+ Pum p Exocytosis Digestion of fats

Cholesterol esters Cholesterol esterase Phospholipids

Cholesterol Phospholipase A2 Phosphate + Fatty acids

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