© 2011 pearson education, inc. 3 the human body: are we really what we eat?

© 2011 Pearson Education, Inc.

3The 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



• Hypothalamus triggers 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



• 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



• 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



• Foods stimulate our senses:• Sight• Smell• Taste• Texture (mouth feel)• Hearing

• Social and cultural cues• Learned experiences

Control of Appetite: Hunger and Satiety


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


What Happens to the Food We Eat?

• 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

Overview of Absorption


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



• 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



• 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



• 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


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


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


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


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


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


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


Hormones Assist in Digestion

• Key hormones involved in digestion:• Gastrin• Secretin• Cholecystokinin (CCK)• Gastric inhibitory peptide (GIP)

• Hormones with potential digestive roles:• Somatostatin• Ghrelin


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


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


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


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


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


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


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


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


Muscles of the GI Tract

• Muscles mix food and ensure efficient digestion and absorption of nutrients• Peristalsis moves intestinal contents• Segmentation is 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


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


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


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


Digestive Disorders

• The lining of the stomach is designed to cope with hydrochloric acid, but other regions of the GI tract are not

• Heartburn is caused by hydrochloric acid in the esophagus

• Gastroesophageal reflux disease (GERD) is painful, persistent heartburn


Digestive Disorders

• Causes of GERD include• Hiatal hernia • Cigarette smoking or alcohol use• Being overweight• Pregnancy• Chocolate, citrus, spicy or fried foods• Large high-fat meal• Lying down soon after a meal


Digestive Disorders

• Peptic ulcers are areas of the GI tract that have been eroded by HCL and pepsin

• Bacterium Helicobacter pylori plays a key role in development of both gastric and duodenal ulcers


Digestive Disorders

• Food intolerance is a cluster of GI symptoms (gas, pain, diarrhea) that occur following consumption of a particular food

• Food allergy is a hypersensitivity reaction of the immune system to a particular component (usually a protein) in a food


Digestive Disorders

• Celiac disease (celiac sprue)• Damaged small intestine lining (decreased villi

and enzymes) reduces nutrient absorption • Offending component is gliadin (a protein in

gluten), which is found in wheat, rye, and barley• Malabsorption can lead to malnutrition (poor

nutrient status) • Early diagnosis avoids delayed growth in children

and malnutrition in adults


Digestive Disorders

• Crohn’s disease • This inflammatory bowel disease can affect any

area of the GI tract• Possible cause is immune system reaction to a

virus or bacteria• Symptoms include diarrhea, abdominal pain,

rectal bleeding, weight loss, fever, anemia• Children can experience delayed growth


Digestive Disorders

• Ulcerative colitis• Chronic disease characterized by inflammation

and ulceration of the mucosa, the innermost lining of the colon

• Possible cause is an immune response to a virus or bacteria

• Symptoms are similar to Crohn’s disease• Although not caused by food, patients may have

to avoid foods that cause GI discomfort


Digestive Disorders

• Diarrhea• Frequent passage of loose, watery stools• Usually caused by infection of the GI tract, stress,

food intolerances, reactions to medications, or bowel disorders

• Can lead to severe dehydration• More dangerous for children and the elderly• Traveler’s diarrhea—common concern


Digestive Disorders

• Constipation• Infrequent hard, small, difficult-to-pass stools• Prevention includes increasing fiber-rich foods

and fluids, and regular exercise


Digestive Disorders

• Irritable bowel syndrome (IBS) is a disorder that interferes with normal colon function• Symptoms include abdominal cramps, bloating,

and either diarrhea or constipation• Associated with stress, caffeine, large meals,

chocolate, alcohol, dairy, wheat• Treatment includes stress management, smaller

meals, high-fiber diet, fluids, and regular physical activity

© 2011 pearson education, inc. 3 the human body: are we really what we eat?

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