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The Human Body:
Are We Really
What We Eat?
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Why Do We Want to Eat?
• 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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Why Do We Want to Eat?
• Foods stimulate our senses:
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Sight
Smell
Taste
Texture (mouth feel)
Hearing
• Social and cultural cues
• Learned experiences
Control of Appetite: Hunger
and Satiety
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Hormones Assist in Digestion
• Key hormones involved in digestion:
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Gastrin
Secretin
Cholecystokinin (CCK)
Gastric inhibitory peptide (GIP)
• Hormones with potential digestive roles:
• Somatostatin
• Ghrelin
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
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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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Digestive Disorders
• Causes of GERD include
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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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Digestive Disorders
• Constipation
• Infrequent hard, small, difficult-to-pass stools
• Prevention includes increasing fiber-rich foods
and fluids, and regular exercise
© 2011 Pearson Education, Inc.
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.