Mammalia – Placental Mammals – young develop in placenta before birth
VI.The Human Digestive System Consists of muscular tube extending from mouth to anus, and associated organs (e.g., salivary glands, gall bladder, liver, pancreas).
B.Overview—Food is ingested, digested, nutrients absorbed, waste expelled; different types of foods (proteins, fats) acted upon in different parts of system. 1.Ingestion—entry of food through mouth 2.Mechanical processing—physical manipulation of solid food by teeth, mixing by tongue 3.Digestion—chemical breakdown of food into small molecules 4.Secretion—release of water, acids, enzymes, buffers from lining of tract and accessory organs, aid in digestion 5.Absorption—movement of small organic molecules, electrolytes, vitamins, water, across epithelium into capillaries and lymphatic vessels 6.Excretion—elimination of waste after compaction
VII.Components of the Digestive System Digestive (or alimentary or gastrointestinal) tract begins in oral cavity, ends in rectum and anus.
VII.Components of the Digestive System Oral cavity—Food tasted, chewed, partially digested here.
Salivary glands—produce almost half a gallon (1.5 L) of saliva each day; saliva is 99 percent water, also ions, buffers, waste products, antibodies, & digestive enzymes.
Teeth break down connective animal tissues and plant fibers in food.
Pharynx and esophagus 1.Pharynx, or throat, transmits solid food, liquids, air; muscular contractions in pharyngeal muscles initiate swallowing. 2.After swallowing from throat, food moves through all other structures by the process of peristalsis 3.Esophagus is muscular tube, about 0.8 in. diameter; begins in pharynx, ends in stomach.
E.Stomach—Dimensions vary; can be narrow tube when empty or expanded into a sac that can hold almost 0.5 gallon; accordion-like folds in muscle wall (rugae) allow expansion. Functions include: a) Temporary storage of ingested food b) Mixing of ingested food c) Digestion of proteins through acids and enzymes c) Absorption of some substances (e.g., alcohol)
Digestion by gastric juices a)Epithelium secretes gastric juices containing HCl and pepsin from glands in gastric pits; HCl reduces pH to level suitable for protein-degrading activity of pepsin. b) Epithelium also secretes mucous, which coats inner lining of stomach and protects it from corrosive action of gastric juices.
Small intestine is almost 10 ft long, 1.6 to 1 inch wide; about 80 percent of absorption occurs here; has three regions:
1.Duodenum—closest to stomach; connects with it through pyloric sphincter; receives chyme from stomach, secretions from pancreas and liver. 2.Jejunum—about 3 ft long; most chemical digestion and absorption of nutrients and water occurs here. 3.Ileum—about 6 ft long; connects with large intestine through sphincter muscle.
Pancreas extends laterally from duodenum; about 6 in. long; functions: 1.secretes, digestive enzymes - lipases, carbohydrases, proteases) and buffers. 2.produce s a) Insulin—causes cells to take up sugars from bloodstream. b) Glucagon—releases glucose from storage in skeletal muscles, liver.
Liver and gallbladder 1.Liver is largest organ in body (not counting skin); has two lobes;
Functions of the liver: a) Regulating body metabolism; extracts nutrients absorbed into bloodstream; monitors and adjusts levels of key nutrients in blood. b) Removes and destroys some toxins (e.g., alcohol). c) Serves as large reservoir of blood; phagocytes here constantly remove cell debris, pathogens, damaged RBCs. d) Produces bile, which aids in digestion of fats.
2.Gallbladder is muscular sac that stores bile produced by liver; bile produced in liver may flow directly into small intestine through common hepatic duct (as during mealtimes), or may back up through cystic duct to be stored in gallbladder.
Large intestine: is about 5 ft long 1.Functions—holds and compacts undigested material; absorbs water and vitamins produced by intestinal bacteria.
2.Structure—has three main regions: a)Cecum—expanded chamber, receives chyme from small intestine through ileocecal valve; vermiform appendix is slender, hollow extension containing lymphatic tissue. b) Colon—longest portion; has characteristic series of muscular pouches; water is reabsorbed here; bacterial activity may produce hydrogen sulfide (produces odor of feces), flatus (gas) due to bacterial action on undigested plant polysaccharides. c) Rectum—chamber at end of tract; usually empty, except when peristaltic contractions force feces into it from colon; triggers defecation reflex, excretion through opening, the anus.
VIII.Different Digestive Processes for Different Foods and Nutrients A.Large organic molecules broken down to simple building blocks prior to absorption; digestive enzymes split bonds between basic units through hydrolysis. 1.Carbohydrates a)Digestion begins in mouth; salivary amylases break complex carbohydrates down to 2–3 carbon sugars. b) In duodenum, pancreatic amylases digest 2–3 carbon sugars to single- carbon sugars; these pass into interstitial fluid of microvilli, enter intestinal capillaries, then are carried to liver via hepatic portal vein.
2.Proteins a)Digestion begins in stomach; low pH, activity of pepsin split proteins into small fragments. b) In duodenum, pancreatic proteases break protein fragments into amino acids; amino acids absorbed by microvilli, carried to liver.
3.Lipids (mostly triglycerides) a) Emulsified by bile to form small droplets suitable for enzyme action. b) Pancreatic lipases break lipids into fatty acids. c) Fatty acids enter intestinal cells, bind to lipid-transport proteins, then enter lymphatic vessels that underlie microvilli and eventually enter venous circulation.
B.Water and vitamins 1.Water: in addition to 2 L to 2.5 L of water that enters as food and drink, secretions of tract contribute 6–7 L more; most of it is reabsorbed, only about 150 mL leaves in fecal waste.
2.Vitamins are organic compounds, needed in small quantities a) Fat-soluble vitamins (A, D, E, K) are stored in body; excessive intake leads to toxic effects, liver damage. b) Water-soluble vitamins (C, B) not readily toxic; excess excreted.