The Digestive System By Khaled Na3im

The Digestive System

The Pharynx The 3 parts of pharynx

Movement of food through the digestive system chewing and swallowing peristalsis Secretions- exocrine (digestive enzymes and other components) Digestion- catabolism (hydrolysis of macromolecules) Absorption- into blood Elimination –of indigestible substances

Accessory digestive glands

Mucosa- absorption and secretion Submucosa- supports mucosa highly vascularized (absorption) Muscularis-inner circular and outer longitudinal layer; innervation stimulated by parasympathetic nerves also intrinsic system Serosa- outer membrane

Chewing and first stage of swallowing are voluntary; the rest of the process is not Peristalsis pushes food through the esophagus to the stomach Esophageal sphincter ( cardiac sphincter ) keeps food from regur- gitating to the esophagus

Stomach ; peptic ulcer

Secretions of the stomach : Mucus- goblet cells HCl- parietal cells Intrinsic factor- parietal cells (necessary for absorption of vitamin B12) Pepsinogen (pepsin)- chief cells Gastrin- G cells Entrochromaffin-like- histamine, serotonin Somatostatin- D cells These all contribute to gastric juice

Why is stomach so acidic? To digest proteins (pepsin works optimally at low pH) Very little is absorbed through the stomach Peptic ulcers: erosions of the mucous membranes most are caused by bacteria (H. pylori) Treatment of gastritis bicarbonate histamine blockers avoidance of acid-stimulating foods

Small Intestine

Small intestine Duodenum 25cm, jejunum 240cm, ileum360cm Most food molecules are absorbed in duodenum and jejunum Bile salts, water, electrolytes, lipids in ileum Villi and microvilli facilitate absorption

Intestinal enzymes involved in digestion Various enzynes convert disaccharides to glucose Peptidases produce amino acids Phosphatases facilitate absorption of Calcium remove phosphate groups Peristalsis facilitates mixing

Large Intestine

Large intestine Consolidation of waste Absorbs water, electrolytes and vitamins Bacterial flora in intestine help ferment molecules otherwise indigestible Diarrhea transport of salt into intestine, followed by water inflammation lactose intolerance

Secretory role of liver, gall bladder, pancreas Liver secretes bile (via gall bladder) Which facilitates digestion of fat Pancreatic juice helps with digestion in small intestine Liver performs a number of essential functions

Detoxification of blood Kupffer cells conversion of toxic molecules to more innocuous ones Blood glucose to glycogen and back Lipid metabolism- production and excretion of cholesterol; ketone bodies Synthesis of plasma proteins Synthesis and secretion of bile

Gall bladder Stores and concentrates bile Gall stones cholesterol has low solubility; clusters with lecithin and bile salts in micelles when these calcify they form gall stones

Pancreas Exocrine and endocrine gland Acini and islets of Langerhans, respectively Pancreatic juice- water, bicarbonate, amylase, lipase, trypsin- among others Many are synthesized in inactive form

Regulation of the digestive system Gastric secretion Cephalic phase- higher brain centers stimulate gastric juice secretion via vagus nerve (“primes” the stomach) Gastric phase- gastrin is secreted by gastric mucosa. Controls secretion of HCl and pepsin Intestinal phase- neural reflexes and hormonal secretion from duodenum inhibits stomach activity

Enteric nervous system controls intestinal function- smooth muscle contraction Secretion of pancreatic juice and bile controlled by secretin and cholecystokinin (CCK) Other hormones maintain the digestive tract

How and where does digestion happen? Mouth- salivary amylase starts to digest starch completed in small intestine (brush border) Protein digestion- begins in stomach, completed with help of pancreatic enzymes Lipids- emulsified by bile salts, digested in small intestine eventually rebuilt into triglycerides transported to blood via lymph