The Cephalic Phase Chemical and mechanical digestion begins in the mouth Saliva is an exocrine secretion 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: deglutition
Figure 21.7-1 Deglutition: The swallowing reflex Tongue pushes bolus against soft palate and back of mouth, triggering swallowing reflex. Soft palate elevates, closing off the nasopharynx. Hard palate Tongue Bolus Epiglottis Glottis Larynx moves up and forward. Tonically contracted upper esophageal sphincter
Figure 21.7-2 Deglutition: The swallowing reflex Breathing is inhibited as the bolus passes the closed airway. Epiglottis folds down to help keep swallowed material out of the airways. Upper esophageal sphincter relaxes.
Figure 21.7-3 Deglutition: The swallowing reflex Food moves downward into the esophagus, propelled by peristaltic waves and aided by gravity.
The Gastric Phase Three functions of the stomach Storage Digestion Lipids and proteins Acid, enzymes, paracrine signal molecules, and hormones Defense against swallowed pathogens Digestive activity in the stomach begins with the long vagal reflex of the cephalic phase
The Gastric Phase Gastric secretion Mucus and bicarbonate from mucous cells Pepsinogen from chief cells Somatostatin from D cells Histamine from ECL cells Gastrin from G cells HCl from parietal cells
Figure 21.8 Cephalic and gastric phase reflexes
Control of the Digestive System Interactive Physiology Animation: Digestive System: Control of the Digestive System
Figure 21.9a Gastric Secretions
Figure 21.9b Gastric Secretions
Figure 21.9c Gastric Secretions
Figure 21.10 Integration of cephalic and gastric phase secretion Slide 1 Food Input via vagus nerve Food or cephalic reflexes initiate gastric secretion of gastrin, histamine, and acid. Lumen of stomach Amino acids or peptides Gastric mucosa Enteric sensory neuron KEY Short reflexes Long reflexes Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine. G cell Gastrin Acid stimulates short reflex secretion of pepsinogen. + D cell Somatostatin Somatostatin release by H + is the negative feedback signal that modulates acid and pepsin release. Negative feedback pathway H + Parietal cell Histamine ECL cell FIGURE QUESTIONS 1. Is the autonomic vagal input sympathetic or parasympathetic? 2. What are the neurotransmitter and receptor for this input? Pepsin Pepsinogen Chief cell Enteric sensory neuron Enteric plexus
Dual role of pancreas The Intestinal Phase Endocrine: secretes insulin and glucagon Exocrine: secretes digestive enzymes and bicarbonate Bile Digestive enzymes are secreted as zymogens activated in the small intestine Fat digestion Bicarbonate neutralizes gastric acid
The Intestinal Phase Goblet cells secrete mucus for protection and lubrication Isotonic NaCl solution Liver hepatocytes secrete bile Bile salts, bile pigments, and cholesterol Gallbladder stores and concentrates Hepatic duct and common bile duct
Figure 21.13 Isotonic NaCl secretion Slide 1 Lumen Intestinal cell Interstitial fluid K + K + Cl Cl 2 Cl Na + Na + ATP K + Na +, H 2 O Na +, H 2 O Na +, K +, and Cl enter by cotransport. Cl enters lumen through CFTR channel. Na + is reabsorbed. Negative Cl in lumen attracts Na + by paracellular pathway. Water follows.
Figure 21.14a The Pancreas Anatomy of the Exocrine and Endocrine Pancreas The exocrine pancreas secretes digestive enzymes and sodium bicarbonate. Pancreatic duct Pancreatic islet cells secrete hormones that enter the blood. Pancreatic acini form the exocrine portion of the pancreas. Pancreas Capillary Small intestine Acinar cells secrete digestive enzymes. Duct cells secrete NaHCO 3 that enters the digestive tract.
Figure 21.14b The Pancreas
Figure 21.14c The Pancreas Slide 1 Bicarbonate Secretion Bicarbonate secretion in the pancreas and duodenum Lumen of pancreas or intestine Pancreatic duct cell or duodenal cell Interstitial fluid Capillary H 2 O + CO 2 CO 2 CA HCO 3 Cl HCO 3 + H + Na + Cl CFTR channel Na + ATP K + Na + 2 Cl K + K + H 2 O, Na + Cells that produce bicarbonate have high concentrations of carbonic anhydrase (CA). Chloride enters cells by indirect active transport and leaves the apical side through a CFTR channel. Cl then reenters the cell in exchange for HCO 3. Leaky junctions allow paracellular movement of ions and water. Negative ions in the lumen attract Na + by the paracellular pathway. Water follows.
Figure 21.15d The Liver
The Intestinal Phase: Small Intestine Most digestion occurs in small intestine Carbohydrates and proteins absorbed as monosaccharides and short peptides Lipids digestion depends on bile salts and lipases absorbed by lacteal ducts Nucleic acids (DNA and RNA) are digested into nitrogenous bases and monosaccharides
The Intestinal Phase: Small Intestine Intestine absorbs vitamins and minerals Fat-soluble vitamins are absorbed with fats Water-soluble vitamins are absorbed by mediated transport Vitamin B 12 is absorbed when complexed to intrinsic factor and absorbed in the ileum Mineral absorption usually occurs by active transport Iron and calcium absorption is actively regulated Sodium is absorbed by enterocytes and colonocytes
Figure 21.16a Digestion and Absorption: Fats
Figure 21.17 Digestion and Absorption of Carbohydrates
Figure 21.18a Digestion and Absorption of Proteins
Figure 21.18b Digestion and Absorption of Proteins
Figure 21.19a Ion and water absorption
Figure 21.19b Ion and water absorption
Figure 21.19c Ion and water absorption
The Intestinal Phase: Large Intestine Large intestine concentrates waste for excretion Motility in large intestine Mass movement moves the bolus forward and triggers defecation Defecation reflex Digestion and absorption in large intestine Flatus Diarrhea can cause dehydration
Figure 21.21 Anatomy of the large intestine Hepatic portal vein Inferior vena cava Transverse colon Aorta Tenia coli Lymphoid nodule Intestinal glands are the site of fluid secretion. Muscularis mucosae Submucosa Ascending colon Food enters the large intestine through the ileocecal valve. Ileum Descending colon Haustra Longitudinal layer (tenia coli) Circular muscle Muscularis externa Cecum Appendix Sigmoid colon Rectum Rectum The defecation reflex begins with distension of the rectal wall. Internal anal sphincter External anal sphincter Anus
Immune Functions of the GI Tract M cells sample the gut contents of the gut Immune cells secrete cytokines Attract additional immune cells Cytokines trigger inflammatory response Increase in Cl, fluid, and mucus secretion Vomiting is a protective reflex
Summary Anatomy of the digestive system Digestive function and processes Regulation of GI function Integrated Function: The Intestinal Phase Immune functions of the GI tract