HUN1201 LECTURE NOTESCHAPTER 3: The Human Body: Are We Really What We Eat?Why do we want to eat?• Hunger: physiological drive for food • Nonspecific• Can be satisfied by a variety of different foods• Appetite: psychological desire to consume specific foods • Aroused by environmental cues• Anorexia: physiological need for food yet no appetite What tells us we are hungry?• Hypothalamus: triggers feelings of hunger or satiation (fullness) • Located above the pituitary gland and the brain stem in an area of the brain that regulates involuntary activities• Integrates signals from nerve cells, chemical messengers (hormones), and the types of food consumedThe role of nerve cells in hunger: • Special cells lining the stomach and small intestine• detect changes in pressure which indicate whether the organ is emptyor distended with food• These cells then send signals to the hypothalamus.The role of hormones in hunger:• Hormones: Chemical messengers secreted into the bloodstream by endocrine glands to help regulate body functions • Pancreatic hormones, insulin and glucagon, maintain blood glucose levels • Feeling full (satiated) results from signals from the stomach and a risein blood glucoseThe role of food type on hunger:• Foods have differing effects on our feelings of hunger• Proteins have the highest satiety value (will feel satiated for longer)• High-fat diets have a higher satiety value than high-carbohydrate diets• Bulky meals (high in fiber and water) distend the stomach and promote a sense of satiety• Solid foods are more filling than semisolid foods or liquidsWhat happens to the food we eat?• Food undergoes three processes:• Digestion:• large food molecules are broken down to smaller molecules, mechanically and chemically• Absorption:• process of taking these products through the intestinal wall• Elimination:• undigested portions of food and waste products are removed from the bodyThe Gastrointestinal Tract• A series of organs arranged in a long tube that work together to process foods• Sphincters: muscles that control the passage of food material from one GIT organ to the nextCephalic phase of Digestion• Hunger and appetite work together to prepare the GI tract for digestion• First thought of food (nervous system) stimulates the release of digestive juicesDigestion begins in the mouth• Chewing moistens the food and mechanically breaks it into smaller pieces• Exposes more surface area to digestive juices• Saliva contains digestive juices secreted by the salivary gland in the mouth• Taste receptors detect distinct tastes: • Bitter, sweet, salty, sour, and umami• Olfactory receptors detect aromas of foods Components of Saliva• Enzymes are complex chemicals that induce chemical changes in other substances to speed up bodily processes • Salivary amylase begins starch digestion • Bicarbonates neutralize acids• Mucus moistens the food and oral cavity• Antibodies and lysozymes fight oral bacteriaChewing and Swallowing• The mass of food chewed and moistened in the mouth is called bolus• The epiglottis covers the opening to the trachea (windpipe) during swallowing• Food travels from the mouth to the stomach through the esophagus• Peristalsis is the muscular contractions that move food through the GI tractDigestion in the stomach• Gastrin:• a hormone secreted by stomach lining cells that stimulates the gastric glands to produce gastric juice• Gastric Glands:• Parietal Cells – secrete HCl and intrinsic factor• Chief Cells – secrete pepsinogen and gastric lipase• Gastric juice contains:• Hydrochloric acid (HCl) denatures proteins and activates pepsin• Pepsin—enzyme to digest protein• Gastric lipase—enzyme to digest fat• Intrinsic factor —protein to absorb vitamin B12• Mechanical Digestion: occurs as the stomach mixes and churns the food and gastric juices• Chyme: liquid product of mechanical and chemical digestion in the stomach• Mucus layer protects the stomach lining from the acid in gastric juices• Bicarbonate neutralizes the acid Digestion in the small intestine:• Small intestine is composed of three sections• Duodeum, jejunum, and ileum• Ileocecal valve (sphincter) connects the small intestine to the large intestine• Most digestion and absorption take place in the small intestineDigestion in the Large Intestine• The large intestine is also referred to as the colon • Bacteria assist with final digestion• Its main function is to store undigested food material and absorb water, short-chain fatty acids, and electrolytesEnzymes speed up digestion• Enzymes guide digestion through hydrolysis, a chemical reaction that breaks down substances by addition of water• Enzymes are specific to carbohydrate, protein, and fat digestionHormones assist in Digestion• Released into the bloodstream and act on specific target cells that contain the receptor protein for that given hormone• Hormones bind to the receptor on the cell membrane and activate a second messenger system within the cell to achieve the targeted response• Key hormones involved in digestion:• Gastrin• Secretin• Cholecystokinin (CCK)• Gastric inhibitory peptide (GIP)• Hormones with potential digestive roles:• Somatostatin• GhrelinAccessory Organs• Produce, store, and secrete enzymes and other chemicals that are involved indigestion.• Gallbladder• Pancreas• LiverAccessory Organ: Gallbladder• Gallbladder stores bile, a greenish fluid, produced by the liver • CCK signals the gallbladder to release bile• Bile emulsifies the lipids • Lipids are dispersed into smaller globules and become more accessible to digestive enzymesAccessory Organ: Pancreas• Manufactures, holds, and secretes digestive enzymes• Stores enzymes in the inactive form • Pancreatic amylase digests carbohydrates• Pancreatic lipase digests fats• Protease digests proteins• Insulin and glucagon (hormones) are produced to regulate blood glucose• Bicarbonate is secreted to neutralize chimeAccessory organ: Liver• The liver is one of the most important organs in the body• Synthesizes chemicals for metabolism• Bile for emulsification of fats• Receives the products of digestion via the portal vein• Releases glucose from glycogen stores• Stores vitamins • Manufactures blood proteins• Filters the blood, removing wastes and toxins.Absorption• Process of taking molecules
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