Human Physiology GI System Lecture Notes PDF

Lecture 17 Human Physiology Gastrointestinal System Objectives: Overview of GI System Processes Functional Anatomy - Brief Overview Digestion and Absorption of Nutrients and Water Required Reading: Section 18.6 Regulation of the Digestive...

Lecture 17 Human Physiology Gastrointestinal System Objectives: Overview of GI System Processes Functional Anatomy - Brief Overview Digestion and Absorption of Nutrients and Water Required Reading: Section 18.6 Regulation of the Digestive System (pp. 645-651) – reminder: this is testable material! Reference Sections: Sections 18.1, 18.2 and 18.7 – selected portions BIOL2010 Human Physiology 1 Overview of Gastrointestinal System Processes Consumed nutrients are too large to enter directly into the bloodstream Nutrients undergo digestion (mechanical and chemical) before absorption Motility and secretion aid digestion and absorption BIOL2010 Human Physiology 2 Functional Anatomy of the GI System Organs of GI tract Accessory Glands Figure 20.2 Major structures of the gastrointestinal system. BIOL2010 Human Physiology 3  The Gastrointestinal Tract Wall of GI tract Figure 20.3 Generalized structure of the gastrointestinal wall, depicting the four major layers and the tissues within them. BIOL2010 Human Physiology 4  The Gastrointestinal Tract Mouth – Mastication – Saliva (salivary amylase) – Tongue Pharynx – Common passage for air/food – Glottis and epiglottis Esophagus – Upper esophageal sphincter – Lower esophageal sphincter Figure 20.5 Mouth and esophagus BIOL2010 Human Physiology 5  The Gastrointestinal Tract Stomach – Contains gastric pits with different secretory cells Neck cells Chief cells Parietal cells G-cells – Stomach lining protected by gastric mucosal barrier – Chyme Figure 18.6 (Fox 16th Ed.) Gastric pits and gastric glands of the mucosa BIOL2010 Human Physiology 6  The Gastrointestinal Tract Small intestine – Primary site of digestion and absorption – Three divisions Duodenum Jejunum Ileum Figure 20.6 Anatomy of the small intestine. BIOL2010 Human Physiology 7  The Gastrointestinal Tract Small intestine Structure relates to absorptive function – Villi – Absorptive cells with microvilli = brush border Carrier proteins and digestive enzymes are located on microvilli Enzymes = brush border enzymes Figure 20.6 Anatomy of the small intestine. BIOL2010 Human Physiology 8  The Gastrointestinal Tract Colon – Ascending colon – Transverse colon – Descending colon All three for absorption of water and ions from chyme – Sigmoid colon storage >700 species of bacteria live in lumen of colon – Digestion of fibre, production of vitamin K and vitamin B7 BIOL2010 Human Physiology 9  Digestion and Absorption of Nutrients and Water Carbohydrates Proteins Lipids Absorption of water BIOL2010 Human Physiology 10  Carbohydrate Digestion Typical diet: 500 grams/day carbohydrates Most consumed as disaccharides (sucrose, lactose, maltose) or polysaccharides (starch, cellulose (indigestible), glycogen) Carbohydrate digestion – Only monosaccharides are absorbed – Disaccharides and polysaccharides must be digested to monosaccharides BIOL2010 Human Physiology 11  Carbohydrate Digestion Digestion of starch Enzymes: – Salivary amylase – Pancreatic amylase Amylases are limited in ability to digest polysaccharides – Cannot break bonds at branch points or between glucose monomers at end of polysaccharide chain End Products: – Maltose (disaccharide) and limit dextrins Figure 20.13 Digestion of starch by salivary or pancreatic amylase. BIOL2010 Human Physiology 12  Carbohydrate Digestion Digestion of carbohydrates to monosaccharides is completed by brush border enzymes of small intestine Brush border enzymes: – Dextrinase: limit dextrins → glucose – Glucoamylase: polysaccharides → glucose – Sucrase: sucrose → fructose + glucose – Lactase: lactose → galactose + glucose – Maltase: maltose → 2 glucose BIOL2010 Human Physiology 13  Carbohydrate Absorption Absorption: transport of nutrients from lumen to blood Absorption of Carbohydrates: – Glucose and galactose are absorbed in two ways: Via co-transport with sodium across apical membrane Facilitated diffusion across basolateral membrane – Fructose is absorbed in one way: Facilitated diffusion across both membranes BIOL2010 Human Physiology 14  Carbohydrate Absorption Figure 20.14 Digestion and absorption of maltose. BIOL2010 Human Physiology 15  Proteins Digestion and absorption of proteins – Consumed in diet – Secreted into lumen of intestinal tract – Sloughed off with cells lining intestinal tract Before absorption, proteins must be digested to: – Tripeptides – Dipeptides – Amino acids BIOL2010 Human Physiology 16  Protein Digestion Two types of enzymes digest proteins: – Endopeptidases Split polypeptides at interior peptide bonds into small peptide fragments – Exopeptidases Cleave amino acids from one end of polypeptide – These enzymes are stored in the secretory cells as inactive form - zymogens Stored in zymogen granules Secreted by exocytosis Activated by proteolytic activation BIOL2010 Human Physiology 17  Protein Digestion Protein digestion in stomach Protein digestion begins in stomach Enzyme: pepsin Inactive (secreted) form: pepsinogen Partially activated by acid Pepsin can start digestion of proteins but can’t complete digestion to amino acids Figure 20.15 Activation and activity of pepsin in the stomach. BIOL2010 Human Physiology 18  Protein Digestion Protein digestion in small intestine Pancreatic proteases (secreted as zymogens) – Trypsinogen  Trypsin – Chymotrypsinogen  Chymotrypsin – Procoarboxypeptidase  Carboxypeptidase Brush border enzyme – Enterokinase Exopeptidases (carboxypeptidase and aminopeptidase) finish protein digestion  amino acids, di- and tri-peptides Figure 20.16 Activation of proteases in the small intestine. BIOL2010 Human Physiology 19  Protein Absorption Absorption of amino acids and small peptides Amin dipeptid tripeptid o es es acids Apical membrane Active transport Sodium-linked secondary active transport or Amin facilitated diffusion Amin Amin o o o acids acids acids Basolateral membrane Facilitated diffusion Facilitated diffusion BIOL2010 Human Physiology 20  Lipids Typical diet: 50 grams/day lipids – Consumed primarily as 90% triglycerides; some phospholipids and cholesterol Lipids face special problem in digestion and absorption – Not water soluble – Do not mix with stomach or intestinal contents – Form fat droplets BIOL2010 Human Physiology 21  Lipid Digestion Most lipid digestion happens in small intestine – Enzymes of digestion: lipases Lingual lipase – in saliva and remains active in stomach Gastric lipase – secreted into gastric lumen Pancreatic lipase – mixes with chyme in small intestine – Lipases can only act on molecules near the edge of a fat droplet – Bile salts increase the surface area of droplets by emulsification Figure 20.17 A representative bile salt. BIOL2010 Human Physiology 22  Lipid Digestion Action of Bile Salts Synthesized in liver from cholesterol; secreted in bile to duodenum Amphipathic molecule with many hydrophilic polar groups on one side of molecule (other side is nonpolar) – Emulsification increases total surface area of droplets and the amount of lipid exposed to water – Polar “coating” allows better mixing with water Figure 20.18 Emulsification of a fat globule by bile salts. BIOL2010 Human Physiology 23  Lipid Digestion Action of Pancreatic Lipase Figure 20.19 Liberation of fatty acids and monoglycerides from fat droplets by lipases. BIOL2010 Human Physiology 24  Lipid Absorption Absorption of monoglycerides and fatty acids Free fatty acids and monoglycerides are absorbed via simple diffusion into intestinal cells Enter smooth ER and re-form triglycerides and other lipids Lipids enter Golgi apparatus  chylomicrons Chylomicrons secreted by exocytosis from basolateral membrane into interstitial fluid Chylomicrons enter lymphatic system via lacteal and eventually enter circulation Figure 20.21 Events in the absorption of fats. BIOL2010 Human Physiology 25  Lipid Absorption Fat droplets shrink as they move through intestine (due to action of pancreatic lipase) – Gone by the time chyme reaches colon In ileum, bile salts are absorbed into circulation Eventually recycled by liver via enterohepatic circulation Figure 20.20 The enterohepatic circulation. BIOL2010 Human Physiology 26  Absorption of Water Water in GI tract – Secretions: 7 liters/day – Intake: 2 liters/day Water absorption is passive Water follows absorption of solutes (especially sodium) by osmosis 95% of water initially present in duodenum is absorbed by the time chyme reaches colon BIOL2010 Human Physiology 27  Course Feedback Survey Please complete a course feedback survey See instructions in the module on Canvas Thank you!  BIOL2010 Human Physiology 28