Download Digestion

Chapter 3 Lecture
The Science of Nutrition
Third Edition
The Human Body:
Are We Really
What We Eat?
© 2014 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
© 2014 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
© 2014 Pearson Education, Inc.
© 2014 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
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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
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Why Do We Want to Eat?
• Foods stimulate our senses:
– Sight
– Smell
– Taste
– Texture (mouth feel)
– Hearing
• Social and cultural cues
• Learned experiences
© 2014 Pearson Education, Inc.
© 2014 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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
© 2014 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
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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
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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
© 2014 Pearson Education, Inc.
© 2014 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
• The esophagus propels food from the mouth to
the stomach
• Peristalsis is the muscular contractions that
move food through the GI tract
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
The pH Scale
• pH = potential of hydrogen
• Measures the potential of a substance to release
or take up hydrogen ions in solution
• Put another way, pH is a measure of a
compound's acidity or alkalinity
• Pure water is neutral/human blood is close to
neutral
• Tissues lining the stomach are generally
protected from the effects of acidity
© 2014 Pearson Education, Inc.
© 2014 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
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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
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 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
© 2014 Pearson Education, Inc.
© 2014 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
© 2014 Pearson Education, Inc.
© 2014 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
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© 2014 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
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Hormones Assist in Digestion
• Key hormones involved in digestion:
– Gastrin
– Secretin
– Cholecystokinin (CCK)
– Gastric inhibitory peptide (GIP)
• Hormones with potential digestive roles:
– Somatostatin
– Ghrelin
© 2014 Pearson Education, Inc.
© 2014 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
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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
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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 (also called the portal venous
system)
– Releases glucose from glycogen stores
– Stores vitamins
– Manufactures blood proteins
© 2014 Pearson Education, Inc.
© 2014 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
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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
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© 2014 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
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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
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© 2014 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
© 2014 Pearson Education, Inc.
© 2014 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
© 2014 Pearson Education, Inc.
© 2014 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
© 2014 Pearson Education, Inc.
© 2014 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
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Digestive Disorders
• Belching (burping) is primarily caused by
swallowed air, eating too fast, improperly fitted
dentures, and chewing gum
• Flatulence (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
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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
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
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
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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
© 2014 Pearson Education, Inc.
© 2014 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
PLAY
© 2014 Pearson Education, Inc.
ABC News Video: Food Allergy Myths
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
– Vomiting reflex is triggered when substances
stimulate brain cells to signal "backwards"
peristalsis
– Cyclic vomiting syndrome (CVS) is a
chronic condition of recurring cycles that can
last for hours or days
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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
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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
© 2014 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; also
called dysentery
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Digestive Disorders
• Constipation
– Infrequent hard, small, difficult-to-pass stools
– Prevention includes increasing fiber-rich foods
and fluids, and regular exercise
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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
© 2014 Pearson Education, Inc.