Chapter 21 Digestion

About this Chapter Overview of the digestive system & how it is organized How products are moved and the role of digestive secretions How and where food is broken down and absorbed How digestive wastes are concentrated and eliminated How digestion is regulated in the gastrointestinal (GI) tract

Processes of the Digestive System: Overview Ingestion Digestion: enzymatic Motility Secretion Absorption Elimination (Self protection) Figure 21-1: Processes of the digestive system

Digestive Anatomy: Overview Oral cavity Salivary glands Esophagus Stomach Fundus Body (rugae) Antrum pyloris

Chemical and mechanical digestion in the mouth Saliva Lubricates and softens food Salivary amylase Breaks down starches into smaller carbs Mechanical digestion; mastication Protection Lysosomes and immunoglobulins kill viruses and bacteria.

Figure 21-17: Integration of secretion in the stomach

Intestinal Phase: ENDOCRINE RESPONSE The pyloric sphincter sends chyme into the duodenum in spurts. Acidic chyme in the duodenum stimulates hormone release into the blood of: Secretin, cholecystokinin, GIP

Digestive Anatomy: Overview Figure 21-2a: ANATOMY SUMMARY: The Digestive System

More Digestive Anatomy Small intestine Duodenum Jejunum Ileum Pancreas Liver Large intestine Colon Rectum Anus

Digestive Anatomy: Histological Overview Stomach wall Mucosa Gastric glands Muscularis mucosa Submucosa Muscularis Externa Serosa Small intestine wall Vili & microvilli Peyer's patches

Digestive Anatomy: Histological Overview Figure 21-2c: ANATOMY SUMMARY: The Digestive System

Digestive Anatomy: Histological Overview Figure 21-2e: ANATOMY SUMMARY: The Digestive System

Motility: Smooth Muscle Contractions Tonic – support Phasic – move products Parastalsis – moves  Segmentation – mixes Figure 21-4: Contractions in the GI tract

Digestive Secretions: (7 L / Day From Tissues into Lumen) Salivary glands Pancreas Water Enzymes Mucus Ions: H+, K+, Na+ HCO3-, Cl- Mass Balance (H2O) Figure 21-5: Daily mass balance in the digestive system

Chemistry of Digestion: Carbohydrates Complex carbohydrate foods Long polymers Enzyme hydrolysis Amylase Maltase Sucrase Lactase

Chemistry of Digestion: Carbohydrates Figure 21-6: Carbohydrate digestion

Chemistry of Digestion: Carbohydrates Disccharides Monosccharides (Absorption)

Chemistry of Digestion: Proteins Proteins  enzyme hydrolysis  amino acids Enzymes: endopeptidases & exopeptidases Figure 21-7: Endopeptidases and exopeptidases

Chemistry of Digestion: Fats Bile emulsification to small fat droplets Enzymes: lipases, colipases & phospholipases Triglycerides  monoglycerides & free fatty acids

Chemistry of Digestion: Fats Figure 21-8: Fat digestion

Regulating Digestion: CNS and Enteric Nervous System (ENS) Long (cephalic) reflexes: CNS, feed forward & emotional reflexes Short reflexes – ENS ("little brain")  integration, motility & secretion: enzymes & hormone/paracrine GI peptides

Regulating Digestion: CNS and Enteric Nervous System (ENS) Figure 21-9: The enteric nervous system

Phases of Digestion: Overview Figure 21-11: Overview of functions in different regions of the digestive system

Cephalic and Oral Phases of Digestion Cephalic: anticipation of food CNS  ANS long reflex Enteric  cells short reflex GI motility GI secretions Mouth: starts digestion Grind, mix & liquefy Saliva: water, enzymes, mucus & lysozyme

Cephalic and Oral Phases of Digestion Figure 21-12: Long and short reflexes in the stomach

Swallowing reflex: Soft Palate & Esophagus Deglutition Epiglottis Upper esophageal sphincter Lower esophageal sphincter (heartburn)

Swallowing reflex: Soft Palate & Esophagus Figure 21-13: The swallowing reflex

Gastric Phase: The Stomach Storage Digestion HCl – parietal C. Lipase – chief C. Pepsin – chief C. Protect walls HCO3- Mucus

Secretion in the Stomach Parietal cells – pH-1 Chief cells – inactive pepsinogen  active pepsin D cells – somatostatin Enterochromaffin-like (ECL) cells – histamine G cells – the hormone gastrin. Mucuos cells – mucus and bicarbonate

Gastric Phase: The Stomach Figure 21-17: Integration of secretion in the stomach

Gastric Phase: The Stomach Figure 21-15: The mucus-bicarbonate barrier of the gastric mucosa

Regulation of the Stomach Digestive Activities CNS – cephalic input ENS – ANS integration w/hormones & paracrines

Regulation of the Stomach Digestive Activities Figure 21-17: Integration of secretion in the stomach

Intestinal Phase: Reflexes Direct Digestive Action Limit chyme entrance rate & motility Neutralize HCl, add bile & enzymes

Intestinal Phase: EXOCRINE RESPONSE Pancreatic bicarbonate secretions Secretin stimulates bicarbonate release. Neutralize HCl, add bile & enzymes Pancreatic exocrine enzyme secretions CCK stimulates pancreatic release of inactive enzymes Figure 21-20 Liver adds bile via gall bladder CCK stimulates gall bladder contraction Bile; a non-enzyme Bile salts Act as detergents Bilirubin cholesterol

Intestinal Phase: Pancreatic Secretions Enzymes HCO3- Insulin Glucagon Figure 21-18: The intestinal phase of gastric function

Intestinal Phase: Pancreatic Secretions Figure 21-19: The hepatic portal system

Intestinal Phase: Liver-Nutrient Storage & Conversions Bile secretion Hepatic portal system directs absorbed nutrients

Intestinal Phase: Liver-Nutrient Storage & Conversions Figure 21-24: Carbohydrate absorption

Intestinal Phase: Carbohydrate Digestion & Absorption Hydrolysis to simple sugars Absorption: transport Na+/ glucose symport Fructose GLUT To ECF  capillary Figure 21-23: Bile salts

Let’s remember what “fat” is. . . Phospholipids Triglycerides cholesterol

Intestinal Phase: Fat Digestion & Absorption Figure 21-26: Fat digestion and absorption

Intestinal Phase: Large Intestine, H2O Absorption & Defecation (Small intestine reabsorbs  7.5 L/day of water) Large Intestine reabsorbs 1.4 L/day Defecation Reflex: mass movement  rectal distension  internal sphincter (invol)  external sphincter (vol)

Intestinal Phase: Large Intestine, H2O Absorption & Defecation Figure 21-27: Anatomy of the large intestine

Intestinal Phase: Large Intestine Digestion & Absorption Bacterial digest significant amounts of complex carbs and proteins through fermentation. Most remaining water is reabsorbed Figure 21-28: NaCl reabsorption by colonocytes

Intestinal Phase: Large Intestine Digestion & Absorption Bacterial fermentation: Vit. K , lactate & buterate Water and electrolyte secretion &/or absorption Figure 21-28: NaCl reabsorption by colonocytes

Digestive Health: Protection & Problems Immune defense: M-cells, Peyer's patches, lymphocytes Irritable bowel disease – chronic inflammation Diarrhea: leads to dehydration (4 million deaths/yr) Osmotic-solutes prevent H2O reabsorption Secretory- bacterial toxins ("flush out' pathogens) Vomiting (emesis) can lead to alkalosis Ulcers- H. pylori "heart-burn"  acid reflux disease

Summary Processes of digestion: ingestion, digestion, absorption, secretion, motility, reabsorption & defecation Anatomy of digestion: mouth, esophagus, stomach, small & large intestines, rectum, anus (pancreas & liver) Enzyme hydrolysis of carbohydrates, proteins, lipids and nucleic acids provide the nutrients, absorbed & conducted to liver for storage Regulation from CNS (long reflex) and ENS (short reflex) integrate hormones & paracrines to coordinate digestion