Digestive System

1. CHAPTER 18 Digestive System • Functions of the digestive system. • Structure and innervation of the digestive system. • Swallowing, peristalsis, Esophagus and LES. • Structure and secretions of the stomach. • Mechanism of HCl secretion, GI protection, & Ulcers. • Intestine, liver, gallbladder, and pancreas. • Regulation of digestive system. • Digestion and absorption of major food groups.

2. Gastrointestinal Tract and its Accessory Organs

3. Functions of the Digestive System • The major functions of digestive system are to digest food and absorb the final products into blood. • Question) How the digestive system achieves the above goal? • Answer) by the following activities: 1- Motility 2- Secretion 3- Digestion & absorption • The digestive system also must get ride of the undigested waste materials this is achieved through the process of defecation.

4. Layers of the G. I. T. 1- Mucosa. - Direct contact with food, - directly involved in secretion and absorption, - contains lymph nodes, goblet cells, and other secretory cells, - muscularis mucosa, thin muscular layer changes the surface area of mucosa. 2- Submucosa. - Tissue below mucosa, which is supportive to - mucosa, contains blood vessels, glands, nerve plexus. 3- Mascularis. - Responsible for the motility of the GIT, - has inner circular and outer longitudinal smooth muscle, - also contains nerve plexuses. 4- Serosa. - Outermost layer serves supportive and protective functions.

5. Innervation of the G. I. T. 1- Autonomic nerve system (ANS) supply. a) Sympathetic b) Parasympathetic 2- Enteric nervous system (ENS). “intrinsic nervous system of the gut” a) Myenteric plexus. Its stimulation causes increase in motor activity (motility) of the gut. b) Submucosal plexus. Mainly controls secretion and blood flow of the GIT, also serves many sensory functions.

6. Interaction of ANS with ENS

7. Upper Esophageal Sphincter Swallowing • Swallowing reflex is initiated when pressure receptors in walls of pharynx are • stimulated by food or drink. • - Swallowing is co-ordinated by the swallowing center in brain stem (medulla oblongata).

8. Swallowing The food passes down the esophagus by peristalsis and enters into the stomach via the Lower Esophageal Sphincter (L.E.S.) LES

10. Peristalsis • Aboral propulsion of contractions at a velocity of 0.5 to 2 cm per sec. (much faster • in the proximal compared to distal part of intestine). • Peristalsis waves are weak and die out after about 10 cm therefore giving a slow • rate of movement of chyme (1 cm/min). Direction of propulsion Bolus Circular contract Longitudinal relax Circular relax Longitudinal contract Presence of bolus causes local distention small intestine, and this activates myenteric plexus between circular and longitudinal muscle layers of intestine.

11. Storage Mixing & Emptying Retropulsion Structure and Function of the Stomach Structure: divided into: 1- Fundus 2- Body 3- Antrum Fuction: 1- Storage of food 2- Mixing of food 3- Emptying of food into small intestine

12. Control of Intragastric Pressure Empty Stomach Distension Food Entering Stomach R R Laplace law: Pressure = 2 x Surface Tension Radius - To keep the intragastric pressure constant, after food enters the stomach the peristalsis is inhibited for about 1 hour.

13. EXOCRINE ENDOCRINE PARACRINE Structure and Function of the Stomach Gastric glands have several types of cells that secrete different materials which collectively are called gastric juice: 1- Gobblet cells- secret mucus. 3- Chief cells – sceret enzyme (pepsinogen) 2- Parietal (oxyntic) cells – sceret HCl 4- G cells – secret hormone gastrin 5- Enterochromaffin-like (ECL) cells –secret histamine. 6- D Cells – secret somatostatin

14. Acid (HCl) and enzyme (pepsin) help in digestion of food substances. By this action the food is broken into smaller molecules which can be absorbed into blood. How the Mucosa of GI is Protected against Digestion The stomach protects itself from digestion by: 1- The chief cells of the gastric mucosa are extremely impermeable to HCl. 2- A layer of alkaline (HCO3-) mucus covers the mucosa. 3- The cells of mucosa are tightly fused to each other so that the HCl does not leak to the submucosa. 4- Rapid rate of cell division in the mucosa (entire epithelium is replaced every 3 days). 5- The enzyme pepsin is secreted in inactive form (pepsinogen).

15. Activation of Pepsin

16. Small Intestine

17. A Villus Intestine folds pilacae circularis and on these folds there are villi which have a layer of epithelial cells which their membrane is also has folds called microvilli. These arrangement increase the Surface area of the small intestine.

18. Intestinal Motility Types of contractions in small intestine: 1- Segmentation: (mixing movement)

19. 2- Peristalsis: (propulsive movement) Direction of propulsion Bolus Circular contract Longitudinal relax Circular relax Longitudinal contract Intestinal Motility

20. Structure and Motility of the Colon 1- Mixing movements: Haustrations= occur in cecum, entire colon, and rectum. Are characterized by contractions of inner circular muscles. 2- Propulsive movement: Mass peristalsis = strong peristalsis which directed aborally, occur several times a day in cecum and entire colon. Reverse peristalsis = orally directed which gives more time to the fecal matter to be expose of water absorption.

21. Regulation of Intestinal Function The GI has its own nervous system (enteric nervous system), it can sense and form reflex actions due to the presence of food. In the intestine the enteric plexus is very extensive and this system helped by the autonomic nervous system are involved in several reflexes: 1- Gastroilial reflex = increase gastric activity causes increased motility of ileum and movement of chyme (food mixed with digestive juice) through the ileocecal valve. 2- Ileogastric reflex = when ileum is distended this decreases the motility and emptying of the stomach. 3- Intestino-intestinal reflex = over-distension of one part of the intestine causes reflex relaxation of the rest of the intestine.

22. Is the largest internal organ in the body. • Has many functions: • 1- Detoxification of blood • 2- Regulating metabolism • 3- Protein synthesis • 4- Storage site for vitamins and iron • 5- Secretion of bile. • * Bile secretion is the principal digestive function of the liver. • Bile contains: • Bile salts • Urobilinogen • Cholestrol • Lecithin • Bilirubin • Electrolytes • Bicarbonate LIVER 5-Secretion of Bile LIVER Small intestine

23. Enterohepatic Circulation Entero = intestine Hepatic = liver LIVER

24. Gallbladder The bile which is secreted by the liver is stored and concentrated in gallbladder. When bile is needed for digestion, the walls of gallbladder contract and this ejects the bile through the bile duct into duodenum.

25. Pancreas Weighs about 100g, and produces about 1L of juice/day. Secretions are both endocrine and exocrine. Digestive function Metabolic function Islets of Langerhans Secret insulin and glucagon Acini: secret pancreatic juice

26. Sodium Bicarbonate For neutralization of acid Enzymes for digestion of food Pacreatic Juice • Pancreatic juice contains: • Water and electrolytes (Na+, K+, Cl+, Ca++) • Bicarbonate (HCO3-) • Digestive enzymes: • Amylase • Trypsin • Lipase • Ribonuclease • Deoxyribonuclease • Etc.

27. From stomach From small intestine Regulation of the Digestive System • The activities of the G.I. Tract is regulated by: • Neural mechanisms: • Automomic nervous system • Sympathetic • Parasympathetic • Enteric nervous system • myenteric • Submucosal • Action of hormones (endocrine): • Hormones secreted from the glands located in the G.I. Tract act on effector cells of the G.I. Itself. Examples of such hormones are: secretin, gastrin, CCK (cholecystokinin)