Chapter 21c The Digestive System

Chemical and mechanical digestion begins in the mouth The Cephalic Phase Chemical and mechanical digestion begins in the mouth Salivary secretion is under autonomic control Softens and lubricates food Chemical digestion: Salivary amylase and some lipase Saliva also has a protective function Chewing: mastication

Swallowing Reflex Figure 21-24, step 1 1 Tongue pushes bolus against soft palate and back of mouth, triggering swallowing reflex. Soft palate Hard palate Tongue Bolus Epiglottis Glottis Larynx Tonically contracted upper esophageal sphincter Figure 21-24, step 1

Swallowing Reflex Figure 21-24, step 2 2 Upper esophageal sphincter relaxes while epiglottis closes to keep swallowed material out of the airways. Epiglottis Figure 21-24, step 2

Swallowing Reflex Figure 21-24, step 3 3 Food moves downward into the esophagus, propelled by peristaltic waves and aided by gravity. Figure 21-24, step 3

Storage Digestion Protection The Gastric Phase Stomach Acid, enzymes, and signal molecules Protection

Enterochromaffin- like cell The Gastric Phase Activity of secretory cells of the gastric mucosa Gastric mucosa Cell Types Substance Secreted Stimulus for Release Function of Secretion Mucus Tonic secretion; with irritation of mucosa Physical barrier between lumen and epithelium Opening of gastric gland Mucous neck cell Bicarbonate Secreted with mucus Buffers gastric acid to prevent damage to epithelium Gastric acid (HCl) Activates pepsin; kills bacteria Parietal cells Acetylcholine, gastrin, histamine Intrinsic factor Complexes with vitamin B12 to permit absorption Enterochromaffin- like cell Acetylcholine, gastrin Stimulates gastric acid secretion Histamine Pepsin(ogen) Acetylcholine; acid, secretin Digests proteins Chief cells Gastric lipase Digests fats Somatostatin Acid in the stomach Inhibits gastric acid secretion D cells Acetylcholine, peptides and amino acids G cells Gastrin Stimulates gastric acid secretion Figure 21-25

Integration of cephalic and gastric phase secretion in the stomach The Gastric Phase Integration of cephalic and gastric phase secretion in the stomach 1 Food 1 Input via vagus nerve Lumen of stomach Gastric mucosa 1 Food or cephalic reflexes initiate gastric secretion. Enteric sensory neuron Amino acids or peptides 2 Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine. G cell Gastrin 2 3 Acid stimulates short reflex secretion of pepsinogen. D cell Somatostatin + 4 Negative feedback pathway 4 Somatostatin release by H+ is the negative feedback signal that modulates acid and pepsin release. ECL cell Histamine H+ Parietal cell 3 Enteric plexus Enteric sensory neuron Pepsin Pepsinogen Chief cell Figure 21-26

Enteric sensory neuron Amino acids or peptides The Gastric Phase 1 1 Input via vagus nerve Food Lumen of stomach Gastric mucosa 1 Food or cephalic reflexes initiate gastric secretion. Enteric sensory neuron Amino acids or peptides G cell Gastrin Enteric plexus Figure 21-26, step 1

Enteric sensory neuron Amino acids or peptides The Gastric Phase 1 1 Input via vagus nerve Food Lumen of stomach Gastric mucosa 1 Food or cephalic reflexes initiate gastric secretion. Enteric sensory neuron Amino acids or peptides 2 Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine. G cell Gastrin 2 ECL cell Histamine H+ Parietal cell Enteric plexus Figure 21-26, steps 1–2

Enteric sensory neuron Amino acids or peptides Enteric sensory neuron The Gastric Phase 1 1 Input via vagus nerve Food Lumen of stomach Gastric mucosa 1 Food or cephalic reflexes initiate gastric secretion. Enteric sensory neuron Amino acids or peptides 2 Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine. G cell Gastrin 2 3 Acid stimulates short reflex secretion of pepsinogen. ECL cell Histamine H+ Parietal cell Enteric plexus 3 Enteric sensory neuron Pepsin Pepsinogen Chief cell Figure 21-26, steps 1–3

The Gastric Phase Figure 21-26, steps 1–4 H+ 1 1 Input via vagus nerve Food Lumen of stomach Gastric mucosa 1 Food or cephalic reflexes initiate gastric secretion. Enteric sensory neuron Amino acids or peptides 2 Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine. G cell Gastrin 2 3 Acid stimulates short reflex secretion of pepsinogen. D cell Somatostatin + 4 Negative feedback pathway 4 Somatostatin release by H+ is the negative feedback signal that modulates acid and pepsin release. ECL cell Histamine H+ Parietal cell Enteric plexus 3 Enteric sensory neuron Pepsin Pepsinogen Chief cell Figure 21-26, steps 1–4

Bicarbonate is a chemical barrier that neutralizes acid. The Gastric Phase The mucus-bicarbonate barrier of the gastric mucosa Stomach lumen Gastric juice pH ~ 2 The mucus layer is a physical barrier. Mucus layer HCO3– Bicarbonate is a chemical barrier that neutralizes acid. HCO3– pH ~ 7 at cell surface Mucus droplets Gastric mucous cell Capillary Figure 21-27

Reflexes of Cephalic and Gastric Phases Long and short reflexes of the cephalic and gastric phases of digestion Figure 12-23

Chyme in the small intestine inhibits gastric motility and secretion The Intestinal Phase Chyme in the small intestine inhibits gastric motility and secretion Food into stomach Acid secretion STOMACH Pepsin and lipase secretion Gastric motility Chyme into small intestine Enteric nervous system SMALL INTESTINE Hyper- osmotic solution Fats, proteins Carbohydrates Acid ? Endocrine cell GIP GLP-1 CCK Secretin Insulin secretion Pancreatic enzyme secretion Pancreatic bicarbonate secretion PANCREAS Figure 21-28

Bicarbonate neutralizes gastric acid The Intestinal Phase Bicarbonate neutralizes gastric acid Goblet cells secrete mucus for protection and lubrication Bile Fat digestion Digestive enzymes Enteropeptidase

Activation of pancreatic zymogens The Intestinal Phase Activation of pancreatic zymogens Lumen of small intestine Pancreatic duct Pancreatic secretions (include inactive zymogens) ZYMOGENS Trypsinogen • Chymotrypsinogen • Procarboxypeptidase • Procolipase • Prophospholipase Enteropeptidase in brush border activates trypsin. Trypsin activates ACTIVATED ENZYMES Intestinal mucosa • Chymotrypsin • Carboxypeptidase • Colipase • Phospholipase Figure 21-29

Digestive tract arteries Hepatic Portal System Most fluid is absorbed in the small intestine Aorta Capillaries of liver Hepatic vein Inferior vena cava Liver Hepatic artery Capillaries of digestive tract: stomach, intestines, pancreas, and spleen Hepatic portal vein Digestive tract arteries Figure 21-30

Most digestion occurs in small intestine The Intestinal Phase Most digestion occurs in small intestine Large intestine concentrates waste for excretion Motility in large intestine Mass movement triggers defecation Defecation reflex Digestion and absorption in large intestine Diarrhea can cause dehydration

The Intestinal Phase: Anatomy of the Large Intestine Lymphoid nodule Intestinal glands Hepatic portal vein Aorta Tenia coli Inferior vena cava Transverse colon Muscularis mucosae Submucosa Ascending colon Longitudinal layer (tenia coli) Descending colon Muscularis externa Circular muscle Ileocecal valve Ileum Haustra Cecum Appendix Sigmoid colon Rectum Rectum Internal anal sphincter External anal sphincter Anus Figure 21-31

The Intestinal Phase: Anatomy of the Large Intestine Figure 21-31 (1 of 3)

The Intestinal Phase: Anatomy of the Large Intestine Figure 21-31 (2 of 3)

The Intestinal Phase: Anatomy of the Large Intestine Figure 21-31 (3 of 3)

M cells sample the contents of the gut Immune Function M cells sample the contents of the gut Immune cells secrete cytokines Cytokines trigger inflammatory response Increase in Cl–, fluid, and mucus secretion Vomiting is a protective reflex

Function and processes Summary Function and processes Digestion, absorption, motility, secretion, and GALT Anatomy Motility Tonic and phasic contractions Slow wave potentials, interstitial cells of Cajal, migrating motor complex, peristaltic contractions, and segmental contractions

Secretion Regulation Summary Parietal cells, CFTR chloride channel, mucous cells, goblet cells, saliva, and bile salts Regulation Short reflexes, ENS, long reflexes, intrinsic neurons, gastrin family, secretin family, and other hormones

Digestion and absorption Summary Digestion and absorption Amylase, disaccharidases, endopeptidases, exopeptidases, lipase, colipase, micelles, chylomicrons, and intrinsic factor The cephalic phase The gastric phase Mucous cells, chief cells, D cells, ECL cells, and G cells

The intestinal phase Immune functions Summary Brush border, mass movement, and defecation reflex Immune functions M cells and vomiting