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Digestive Systems •Campbell, 6th edition, Chapter 41 •Animal Nutrition DIGESTIVE SYSTEMS I. Digestion II. Food Types III. Feeding Mechanisms IV. Compartmentalization V. Stages of Food Processing VI. Types of Digestive Systems VII.Vertebrate Digestive System & Regulation 2 I. Digestion Any nutritionally adequate diet satisfies three needs: • 1) fuel (chemical energy) • 2) raw organic materials (biosynthesis) • 3) essential nutrients • Homeostatic mechanisms manage these resources. • Know difference between undernourished and malnourished. Essential Nutrients • 8 Amino Acids: tyrptophan methionine valine threonine phenylalanine leucine isoleucine lysine (histidine is essential for infants) • Fatty acids: membrane structure unsaturated; linoleic acid • 13 Vitamins: coenzymes Fat-soluble: vitamins A, D, E, & K Water-soluble: B complex,Vitamin C • Minerals: building materials, cofactors Ca, P, S, K, Cl, Fe, Mg, Zn, I, Na, Essential amino acids from a vegetarian diet. I. Digestion •Chemical and mechanical breakdown of organic molecules into units small enough for the body to absorb. •These molecules provide: 1) Energy resources 2) Essential chemical elements 3) Raw materials for anabolism II. Food Types • Most animals are opportunistic feeders: 1) HERVIVORES – feed on autotrophs 2) CARNIVORES – eat other animals 3) OMNIVORES – both III. Feeding Mechanisms Four major groups: • suspension-feeders Baleen whale, clam, oyster • substrate-feeders Leaf miner, earthworm • fluid-feeders Mosquito, leech • bulk-feeders Python, lion, bear Suspension-feeding: a baleen whale Substrate-feeding: leaf miner Fluid-feeding: a mosquito Bulk-feeding: a python IV. Digestion is compartmentalized • Intracellular – digestive enzymes are secreted by cells & food is digested by enzymes within the cell – sponges (choanocytes) • Extracellular - digestive enzymes are secreted by cells into a digestive cavity – cnidarians (both intracellular & extracellular) Intracellular digestion in Paramecium Extracellular digestion in a gastrovascular cavity. V. Stages of Food Processing • Ingestion – the act of eating or ingesting • Digestion – process of breaking food into small enough molecules for the body to absorb • Absorption – process of absorbing small molecules from the digestive compartment into bloodstream • Egestion – act of eliminating undigested materials from the digestive compartment VI. Types of Digestive Systems • Channel network – Porifera • Incomplete (one-hole sac or gastrovascular cavity) – Cnidarians Platyhelminthes • Complete (two-hole sac or alimentary canal) – Nematoda Chordata VI. Types of Digestive Systems • Incomplete Digestive Tract or gastrovascular cavity – functions in digestion (gastro) & distribution (vascular) – dual role: mouth = anus – Cnidaria Platyhelminthes VI. Types of Digestive Systems • Complete Digestive Tract • Alimentary canal – food moves in one direction – tube is organized into specialized regions – Nematoda Chordata Alimentary Canal Complete digestive tract of a mollusk Earthworm Digestive Tract The Human Digestive System VII. Vertebrate Digestive System Oral cavity: mouth, tongue, teeth • digestion begins here • mastication= mechanical grinding action of teeth • food soften with saliva from salivary glands • bolus = moistened ball of food Accessory Organs: Salivary Glands Saliva • contains mucus for lubrication and swallowing (1 - 1.5 L per day) • contains salivary amylasehydrolysis of amylose • contains mucin, buffers, antibacterial agents • venom- secreted by the salivary glands of some vertebrates • Parotid, submaxillary, sublingual salivary glands Pharynx (throat region) • swallowing is accomplished by the pharynx • an intersection leads to both esophagus and trachea; cartilage flap, epiglottis, covers the glottis & prevents choking (Figure 41.12) • passes bolus from mouth to esophagus • originates from a groove in the floor of the lungs • acts as a muscular pump in some worms (proboscis w/ pharynx) From mouth to stomach: the swalling reflex and esophageal peristalsis Esophagus • 25 cm long • muscularized passageway to stomach • peristalsis begins here rhythmic waves of contraction by smooth muscles in the wall of the canal • ruminants (cud-chewers), ruminating pouches, chambers of esophagus where fermentation occurs (cows produce 60 L saliva per day & burp 2 L gas/minute) Layers of the Digestive Tube From esophagus to anus: • 1) mucosa – lines tube; glandular epithelium; villi; contains some smooth muscle; produces mucus • 2) submucosa – connective tissue, nerves, blood vessels, lymph • 3) muscularis externa – inner circular muscle & outer longitudinal muscle • 4) serosa – outer fibrous coating or visceral peritoneum Cardiac sphincter • ringlike valve of smooth muscle that functions like a drawstring • controls entrance of food into stomach from esophagus Stomach • collapsible muscular bag • suspended in abdominal cavity by folds of peritoneum called mesentery • functions in mechanical mixing of food with HCl and enzymes • fully distended, human stomach holds 2-4 liters of food 3 regions of the Stomach: • 1) cardiac – upper • 2) fundus – deep, storage • 3) pylorus – lower, empties into small intestine Stomach • glucose and alcohol are absorbed in stomach • takes about 4 hours to empty stomach • chyme – semi-liquid mass; may back up in gastric pits and cause ulcers Stomach • Rugae – folds of stomach with deep pockets, or gastric pits, contain • 1) mucous cells – secrete mucus for protection • 2) parietal cells – secrete HCl (pH 1.5-2.5) HCL kills most bacteria & living cells; erodes plant materials; initiates change of pepsinogen to pepsin • 3) chief cells – secrete pepsinogen (inactive) which is converted by HCL into active pepsin Chemical Digestion Digestion in Stomach Digestion is regulated by hormones and the Autonomic Nervous System • Stomach hormone: Gastrin • produced in the presence of proteincontaining food in the stomach • stimulates the release of gastric juices and muscular contractions of stomach & intestine • Blood sugar is regulated by pancreatic hormones insulin and glucagon Accessory Digestive Organs Ruminant digestion Enzymatic Digestion in the human digestive system Digestion in Stomach Stimulation of epithelial cells of stomach mucosa increases secretion of gastric juice: • • • • • 1) 2) 3) 4) 5) mucous HCl pepsinogen renin (hydrolyzes milk) water Pyloric sphincter • sphincter separating the stomach and small intestine • regulates the passage of material from stomach to small intestine Small Intestine • digestion is completed here • most enzymatic hydrolysis and adsorption occurs here • surface area of small intestine is 300 m2; about the size of a double tennis court • mucosa has fingerlike projections, villi, which extend into the lumen • the villi have microvilli (cytoplasmic projections on the surface of epithelial cells) Figure 41.15 Structure of the small intestine Small Intestine • Small intestine is 20-23’ long: – 1) duodenum 8-10” – 2) jejunum 8’ – 3) ileum 12’ • duodenum – most active in digestion • jejunum & ileum – absorption • study Figure 41.13: Enzymatic digestion Digestion in Small Intestine Digestion is regulated by hormones and the Autonomic Nervous System • Duodenal hormones: • 1) secretin – stimulates pancreas & liver to secrete alkaline fluids • 2) cholecystokinin – triggers release of enzymes from pancreas and gall bladder (amylase, lipase, deoxyribonuclease, protease, etc.) Activation of protein-digesting enzymes in the small intestine Accessory Organs: Liver • largest internal organ = 3 lb chemical factory • processes food by the Hepatic Portal Vein delivered from digestive tract • variable nutrient levels in HPV while level in systemic circulation remains constant • though the liver performs many functions, cells of the liver function without division of labor Liver Functions: • 1) maintenance of a constant glucose level in bloodstream • 2) detoxification of drugs & alcohol • 3) production of bile • 4) destruction old RBC & converts hemoglobin to bilirubin • 5) production the plasma protein prothrombin • 6) regulation of cholesterol & other fats Bile • produced by liver • enters small intestine via duct from gall bladder • fat emulsifier • contains bilirubin (breakdown hemoglobin) • jaundice occurs when bilirubin is not removed • neutralizes stomach acid, contains sodium bicarbonate Accessory Organs: Pancreas • clusters of cells in pancreas called Islets of Langerhans secrete pancreatic hormones: • 1) Glucagon – increases blood sugar by converting stored liver glycogen to glucose • 2) Insulin – decreases blood sugar level by converting glucose to glycogen for storage • produces alkaline fluid to neutralize stomach acid Homeostatic regulation of cellular fuel Accessory Organs: Gall Bladder • stores bile produced by the liver • underneath side of liver • balloon-like structure Ileo-cecal valve • valve between small and large intestine • regulates the passage of material from small to large intestine Large Intestine or colon • main function is reabsorption of H2O, sodium, and minerals, vitamins • Escherichia coli symbiotic bacteria that escape digestion & absorption in the small intestine; chief source of vitamin K • 7 L of water per day enter stomach and large intestine Histology of intestine Large Intestine • fecal material – unusable material left in the digestive tract • mostly water, dead bacterial cells, cellulose fibers, other indigestible materials, lubricated by mucus • elimination through the anus after brief storage in the rectum • elimination of feces = egestion or defecation; NOT an excretory process since fecal material was in the intestine, but never in the body proper. Appendix/ Cecum • • • • • • blind pouch off large intestine evolutionary remnant may become inflamed, irritated, infected plays no role in human digestion may be involved in the immune response CECUM – well developed in herbivorus vertebrates; contains microbes that breakdown cellulose