The Human Body: Are We Really What We Eat?

1. 3 The Human Body:Are We Really What We Eat?

2. 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

3. Why Do We Want to Eat? • Hypothalamustriggers feelings of hunger or satiation (fullness) • Integrates signals from nerve cells in other body regions and from chemical messengers • Special cells lining the stomach and small intestine send signals to the brain to indicate if they are full or empty

5. Why Do We Want to Eat? • 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 rise in blood glucose

6. Why Do We Want to Eat? • Foods have differing effects on our feelings of hunger • Proteins have the highest satiety value • 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 liquids

7. Why Do We Want to Eat? • Foods stimulate our senses: • Sight • Smell • Taste • Texture (mouth feel) • Hearing • Social and cultural cues • Learned experiences Control of Appetite: Hunger and Satiety

9. What 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 body

10. What Happens to the Food We Eat? Overview of Absorption • Gastrointestinal (GI) tract: • A series of organs arranged in a long tube that work together to process foods • The GI tract includes: • Organs such as the stomach, intestines • Sphincters: muscles that control the passage of food material from one organ to the next

12. Digestion Begins in the Mouth • Cephalic 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 juices • Chewing moistens the food and mechanically breaks it into smaller pieces

13. Digestion Begins in the Mouth • 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

14. Digestion Begins in the Mouth • 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 bacteria

16. Digestion Begins in the Mouth • The mass of food chewed and moistened in the mouth is called bolus • The epiglottis covers the opening to the trachea during swallowing • Food travels from the mouth to the stomach through the esophagus • Peristalsis is the muscular contractions that move food through the GI tract

19. Digestion in the Stomach • Gastrin—a hormone secreted by stomach lining cells that stimulates gastric juice • 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

20. Digestion in the Stomach • 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

24. 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 intestine

26. Digestion 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 electrolytes

28. Enzymes 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 digestion Role of Enzymes

30. Hormones Assist in Digestion • Hormones are released into the bloodstream to specific target cells that contain the receptor protein for that given hormone • Second messenger system: Hormones bind to the receptor on the cell membrane for activation

32. Hormones Assist in Digestion • Key hormones involved in digestion: • Gastrin • Secretin • Cholecystokinin (CCK) • Gastric inhibitory peptide (GIP) • Hormones with potential digestive roles: • Somatostatin • Ghrelin

34. Accessory 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 enzymes

35. Accessory 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 chyme

36. Accessory Organ: Liver • The liver is one of the most important organs in the body • Synthesizes chemicals for metabolism • Produces bile for emulsification of fats • Receives the products of digestion via the portal vein • Releases glucose from glycogen stores • Stores vitamins • Manufactures blood proteins

37. Absorption • Process of taking molecules across a cell membrane and into cells of the body • A small amount of absorption occurs in the stomach • Most absorption of nutrients occurs in the small intestine

38. Absorption • Mucosal membrane, the lining of the GI tract, has special structures to facilitate absorption • Villi are folds in the lining that allow it to absorb more nutrients • Enterocytes are absorptive cells in the villi • Capillaries and a lacteal (small lymph vessel) are inside each villus • The brush border is composed of microvilli,which greatly increase the surface area

40. Absorption • Passive diffusion—nutrients simply pass through the enterocytes and into the bloodstream without the use of a carrier or energy • Facilitated diffusion—requires a carrier protein

41. Absorption • Active transport —requires energy and a protein carrier to transport nutrients • Endocytosis (pinocytosis)—active transport by which a small amount of intestinal contents is engulfed by the cell membrane into the cell Basic Absorption Mechanisms

43. Transport of Nutrients and Wastes • Blood travels through the cardiovascular system • Lymph travels through the lymphatic system • Lacteals pick up most lipids and fat-soluble vitamins • Lymph nodes are clusters of immune cells that filter microbes and other harmful agents

45. Muscles of the GI Tract • Muscles mix food and ensure efficient digestion and absorption of nutrients • Peristalsis moves intestinal contents • Segmentationis a unique pattern of motility • Circular and longitudinal muscles contract and relax to mix the chyme and enhance its contact with digestive juices and enterocytes

47. Muscles of the GI Tract • Haustra are segmentations in the colon that contract sluggishly to move contents • Mass movement occasionally occurs to move wastes toward the rectum • Muscle contraction rate depends on its location in the GI tract and presence of food • Voluntary muscles are in the mouth • Involuntary muscles take over in the GI tract

49. Enteric Nervous System • Contraction and secretions of the GI tract are controlled by: • Enteric nervous system in the gut wall • Parasympathetic and sympathetic nerves of the autonomic nervous system, which is part of the peripheral nervous system (PNS) • Central nervous system (CNS), which includes the brain and spinal cord

50. Digestive Disorders • Belching (burping) is primarily caused by swallowed air, eating too fast, improperly fitted dentures, and chewing gum • Flatus(intestinal gas) is a normal process that may be caused by • Foods rich in fiber, starches, and sugar • Bacteria that act on partially digested carbohydrates • The fat substitute olestra and sugar alcohols • Beano can offer some relief