Outline Week 12 - Digestive System I Major Organs Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Digestive Tract Function GI Tract Structure Regulation of the GI structure From Mouth to Stomach Mouth and Esophagus Stomach Functions of stomach Gastric mucosa HCl production and the regulation of HCl secretion No figures are included in this lecture note. Outline Stomach (cont.) Protein digestion in stomach Protection of stomach against HCl and pepsin Peptic ulcers Small Intestine Large surface area Villi and microvilli Intestinal contractions and motility Large Intestine Structure and function Overview of Digestive System From food, humans must get basic organic molecules to make ATP, build tissues, and serve as cofactors and coenzymes. Digestion breaks polymers (carbohydrates, fats, and proteins) into monomers via hydrolysis reactions Absorption takes these monomers into the bloodstream to be used by the cells
Digestive System GI tract (alimentary canal) Oral cavity Pharynx Esophagus Stomach Small intestines Large intestines Anus Accessory organs Teeth, tongue, salivary glands, liver, gallbladder, and pancreas Digestive Tract Function Motility Movement of food through the digestive system Secretion Release of substances to enhance breakdown of food Digestion Physical and chemical breakdown of food Digestive Tract Function Absorption Passage of digested end products into blood or lymph Excretion Indigestible components of food that are temporarily stored in the large intestine are eliminated as feces. Gastrointestinal(GI) Tract Structure Mucosa (lumen side) Epithelial tissue; inner secretary and absorptive layer; highly folded to increase surface area Submucosa A thick, highly vascular layer of connective tissue; contains blood and lymphatic vessels; contains glands and nerve plexuses (submucosal plexus)
Gastrointestinal(GI) Tract Structure Muscularis externa Smooth muscle layers; responsible for peristalsis and segmentation; myenteric plexus for control by the ANS Serosa Outermost layer of connective tissue Regulation of the GI Tract Parasympathetic division, arising from vagus nerve and spinal nerves, Stimulates motility and secretions of GI tract. Sympathetic division Inhibits peristalsis and secretion Stimulates contraction of sphincters Regulation of the GI Tract Parasympathetic division, arising from vagus nerve and spinal nerves, Stimulates motility and secretions of GI tract. Sympathetic division Inhibits peristalsis and secretion Stimulates contraction of sphincters GI tract contains an intrinsic system that controls its motility and secretions via enteric nervous system The motility and secretions of GI tract are influenced by paracrine and hormonal signals. From Mouth to Stomach
Mouth Mastication(chewing): breaks food down into smaller pieces for deglutition and mixes it with saliva. Saliva: contains mucus, an antimicrobial agent, and salivary amylase to start digestion of starch. Deglutition(swallowing): Involves coordinated contraction of 25 pairs of muscles Esophagus Connects pharynx to stomach (~25cm long) Upper third contains skeletal muscle Middle third contains mixture of skeletal and smooth muscle Terminal portion contains only smooth muscle Pass through diaphragm via esophageal hiatus Esophagus Moves food toward the stomach by a wave like contraction known as peristalsis After food passes into stomach, the lower esophageal sphincter is closed, preventing acidic gastric juice from being pushed through into the esophagus (preventing reflux) Stomach
Stomach Is the most distensible part of GI tract Is enclosed by lower esophageal sphincter on the top and pyloric sphincter on the bottom Empties into duodenum Consists of a cardia, fundus, body and pylorus Inner surface of stomach is highly folded into rugae Functions of Stomach Stores food Churns food to mix with gastric secretions Begins protein digestion Kills bacteria in the food with high acidity Move food into small intestine in the form of chyme Contractions of stomach churn chyme, mixing it with gastric secretions Eventually these will propel food into small intestine. Gastric Mucosa The gastric mucosa contains gastric glands with different cells that secrete different products that form gastric juice. Mucous neck cells(goblet cells) secrete mucus to help protect stomach lining from acid. Parietal cells secrete hydrochloric acid(hcl) and intrinsic factor (needed for the absorption of vitamin B 12 in the intestine). Chief cells secrete pepsinogen, the inactive form of a protein-digesting enzyme pepsin Gastric Mucosa ECL(enterochromaffin-like) cells secrete histamine and serotonin (paracrine signals) G cells secrete the hormone gastrin into the blood D cells secrete the hormone somatostatin into the blood. Gastric cells that secrete ghrelin, a hormone that promotes hunger when the stomach is empty.
HCl in Stomach The parietal cells secrete H + into the gastric lumen using active transport (H + /K + pump); Cl - is secreted by facilitated diffusion. The HCl makes gastric juice very acidic, with a ph of less than 2). Ingested proteins are denatured at low ph (to make them more digestible. Activate pepsinogen into pepsin Regulation of HCl Secretion Sight, smell and taste of food stimulates vagus nerve, which stimulate ECL cells (major) to secrete histamine. Distension of stomach stimulates vagus nerve. The presence of partially digested proteins and amino acids in the stomach lumen stimulates gastrin secretion. Gastrin stimulates ECL cells to secrete histamine. Histamine stimulates parietal cells to secrete HCl via H 2 histamine receptors Regulation of HCl Secretion Positive feedback mechanism As more HCl are secreted, more short polypeptides and amino acids are released from the ingested proteins. This stimulates additional secretion of gastrin and thus HCl. Negative feedback mechanism As ph drops (due to more HCl), the hormone somatostatin is released from D cell of the gastric mucosa, which inhibits gastrin secretion. Protein Digestion in Stomach Pepsinogen(precursor enzyme) is secreted by chief cells into the lumen The HCl in gastric juice helps activate pepsin, a protein-digestive enzyme that partially digests food proteins into shorter polypeptide chains. Pepsin is active at low ph
Digestion and Absorption in the Stomach Protein digestion begins in the stomach. Starch digestion begins in the mouth, but salivary amylase is not active at ph 2, so this activity stops in the stomach. Alcohol and NSAIDs (aspirin) are the only common substances absorbed in the stomach Protection of Stomach against HCl and Pepsin HCl and pepsin could damage the stomach lining and produce a peptic ulcer. Defenses that help prevent this: Adherent layer of mucus with alkaline bicarbonate (to neutralize HCl), a barrier to actions of pepsin Tight junctions between epithelial cells Rapid epithelial mitosis that replaces epithelium every three days. Peptic Ulcers Peptic ulcers are erosions of the mucosa of the stomach or duodenum caused by action of HCl. Helicobacter pylori infection is associated with ulcers. H. pylori - bacterium that reduces mucosal barriers to gastric acid Treatment for ulcers combines K + /H + pump inhibitors (Prilosec) and antibiotics. Table and Figure 1. HCl production 2. Three phases of gastric secretion
Small Intestine Small Intestine Starts at the pyloric sphincter and ends at the ileocecal valve. Is the longest part of GI tract(~ 3m long in living adults) Duodenum (first 20-30 cm) Jejunem (middle 2/5) Ileum (last 3/5); empties into large intestine Small Intestine Complete digestion of carbohydrates, proteins, and fats. Absorption of the digested foods Facilitated by large surface area Small Intestine Large surface area Plicae circularis circular folds in mucosa Villi Fingerlike projections Contain capillaries and central lacteal (lymphatic capillaries), which plays an important role in the absorption of food molecules.
Villi A simple columnar epithelium, containing mucussecreting goblet cells, coats the villus. Within the connective tissue of the villus is a network of blood capillaries and a lacteal Blood capillaries absorb monosaccharides and amino acids, and a lacteal absorbs fats. At the base of a villus, the epithelium pouches inward to form the intestinal crypts. Produces new epithelial cells by mitotic cell division. The new cells migrate out of the crypts and up to the tips of the villi, from which they are continuously shed. Microvilli (Brush Border) Folds of plasma membrane on the apical(top) surface of each epithelial cell Hold brush border enzymes Resides on the surface of the microvilli on the intestinal epithelial(mucosal) cell surfaces Intestinal Contractions and Motility Peristalsis Propels chyme through the small intestine (weak and slow) Segmentation Major type of contraction Simultaneous contractions of numerous segments of the intestine. Help to mix chyme with digestive secretions More frequently at the proximal end than at the distal end of the intestine. Intestinal Contractions and Motility Intestinal contractions occur automatically in response to pacemaker cells in the intestinal wall The pacemaker cells depolarize spontaneously (like the pacemaker cells of the heart), leading to the production of action potentials and contraction of the smooth muscle cells.
Intestinal Contractions and Motility Regulation of Motility Parasympathetic axons stimulates motility. Enteric nervous system(enteric brain) ~100 million neurons within the walls of the digestive tract Control digestive function independent of the CNS Table Intestinal(Brush Border) Enzymes Large Intestine Structure Large Intestine Extends from the ileocecal valve at the end of small intestine to anus Chyme from the small intestine enters cecum, then passes to ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal.
Large Intestine Structure Outer surface bulges to form pouches (haustra: 결장팽대부 ) Occasionally, the smooth muscle may become weakened and form more elongated outpouchings, or diverticular ( 게실 ). Tissue layers are similar to those of the small intestine, except that the mucosa of the large intestine does NOT have villi and microvilli. Large Intestine Function Has no digestive function, but absorb water, electrolytes(ions), several B vitamins, and vitamin K Storage of undigested materials (feces) Main function After water and electrolytes have been absorbed, waste material passes to rectum, creating urge to defecate. Defecation reflex normal response to the presence of feces in the rectum. Large Intestine Function Contains large population of microflora (more than 400 different species), which provides their hosts(us) several benefits: Synthesize vitamin K and some B vitamins Synthesize short chain fatty acids Used for energy by large intestine epithelial cells Help absorb electrolytes such as sodium, calcium, bicarbonate, magnesium, and iron. Large Intestine Function Reduce the ability of pathogenic bacteria to infect the large intestine. Disruption of normal microflora can lead to inflammatory bowel disease