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Chapter 3
Digestion, Absorption, and
Metabolism
© 2008 Thomson - Wadsworth
What Happens to the Food We Eat?
The food we eat undergoes three
processes:
1. Digestion
2. Absorption
3. Elimination
These processes occur in the (GI)
gastrointestinal tract.
Carbohydrates, proteins, and fats are
usually very large molecules.
The goals of digestion:
Break these large molecules down to
smaller molecules
Absorb the smaller molecules into the
cells of the body
Must overcome many challenges along the
way……..
© 2008 Thomson - Wadsworth
Digestion and Absorption
• The mouth is the entry point for food into
the digestive tract. Digestion in the mouth
involves mastication (chewing), the
stimulation of taste buds, and swallowing.
Saliva contains salivary amylase, which
helps to break down carbohydrates. Saliva
also has an enzyme called lysozyme, which
help to inhibit bacterial growth in the mouth
and helps prevent tooth decay.
• Chewing mechanically breaks food down,
increasing the surface area in contact with
digestive enzymes.
• The epiglottis closes to prevent food from
entering the pharynx. After swallowing the
food is called a bolus.
Copyright 2010, John Wiley & Sons, Inc.
Digestion and Absorption
• Esophagus - The esophagus is the tube that leads the bolus to
the stomach. There is a sphincter at the upper and lower (also
known as the cardiac sphincter) ends of the esophagus.
Copyright 2010, John Wiley & Sons, Inc.
Digestion
Anatomy of the Digestive Tract
 The stomach adds juices and grinds the
bolus to a semiliquid mass called chyme.
The pyloric sphincter regulates the flow of
partially digested food into the small
intestine.
 The small intestine receives digestive
juices from the gallbladder and the
pancreas. The three segments of the small
intestine are the duodenum (1 foot),
the jejunum (8 feet) and the ileum (11 feet).
Majority of absorption occurs here!
Digestion\Absorption
Large Intestine
• The large intestine does not participate in
digestion.
• The large intestine can absorb water and
some vitamins and minerals.
• Undigested residues, including some fibers,
continue through the digestive tract and form
waste.
• Recycling of usable materials
• Water, nutrients and fecal material may
spend up to 24 hours in the large intestine.
• Transit time: amount of time it takes food
to pass the length of the GI tract
• Feces: body waste, including unabsorbed
food residue, bacteria and dead cells
Copyright 2010, John Wiley & Sons, Inc.
Transverse
colon
Opening from
small intestine
to large intestine
Ascending
colon
End of small
intestine
Appendix
Descending
colon
Rectum
Anus
Sigmoid
colon
Stepped Art
Digestion
• The Muscular Action of Digestion – under
autonomic control
 Peristalsis pushes the digestive contents
along.
 Stomach action involves circular,
longitudinal, and diagonal muscles.
 Segmentation is contractions by circular
muscles that contract and squeeze contents
to promote mixing with digestive juices.
 Sphincter contractions open and close
passageways. This prevents reflux and
controls the passage of contents.
© 2008 Thomson - Wadsworth
Digestion
The Secretions of Digestion
 Saliva from the salivary glands moistens foods
 Gastric juice from the gastric glands includes
hydrochloric acid. The goblet cells of the
stomach wall secrete mucus to protect the
walls of the stomach from the high acidity levels
that are measured by pH units.
 Pancreatic juice contains intestinal enzymes
(carbohydrase, lipase, protease) and
bicarbonate to help neutralize acidic chyme.
 Bile is produced by the liver, stored in the gall
bladder, and acts as an emulsifier to suspend
fat.
The brain is
involved in
regulating
digestion and
absorption
through the
nervous
system and
through
hormones.
Homeostasis
is important in
the functioning
of the
digestive and
absorptive
systems.
The Regulation of the GI Tract
• Gastrointestinal Hormones and Nerve
Pathways
Hormones act as messengers.
Gastrin is secreted by the stomach.
Gastrin stimulates secretion of HCl
Secretin is secreted by the
duodenum. This stimulates the
pancreas to release bicarbonate
secretion into intestine.
Regulation of the GI Tract
• Gastrointestinal Hormones and Nerve
Pathways
Pancreatic secretions change based
on the content of the diet.
Cholecystokinin (CCK) targets the
gall bladder to release bile.
Motility slows for foods that take
longer to be digested. Nerves are
triggered by the presence of food
and cause the release of substances
that speed up or delay motility.
Gastrointestinal Bacteria
 A healthy GI tract has many different nondisease-causing bacteria known as flora or
microflora. (10 trillion bacteria, 400 species)
 Probiotics are bacteria found in the GI tract
that can be beneficial to health. An example
is the bacteria found in yogurt.
Absorption
Nutrients can be absorbed through
simple diffusion, facilitated diffusion,
or active transport.
Carrier loads
nutrient on
outside of cell . . .
Outside
cell
Carrier loads
nutrient on
outside of cell . . .
Cell
membrane
Inside
cell
. . . and then
releases it on
inside of cell.
SIMPLE
DIFFUSION
Some nutrients (such
as water and small
lipids) are absorbed by
simple diffusion. They
cross into intestinal
cells freely.
FACILITATED
DIFFUSION
Some nutrients (such as the watersoluble vitamins) are absorbed by
facilitated diffusion. They need a
specific carrier to transport them
from one side of the cell membrane
to the other. (Alternatively,
facilitated diffusion may occur
when the carrier changes the cell
membrane in such a way that the
nutrients can pass through.)
. . . and then
releases it on
inside of cell.
ACTIVE
TRANSPORT
Some nutrients (such as
glucose and amino acids)
must be absorbed actively.
These nutrients move
against a concentration
gradient, which requires
energy.
Stepped Art
Absorption
The enormous surface
area of the small intestine
facilitates nutrient
absorption.
• Anatomy of the Absorptive System
 Villi are the fingerlike projections within the
folds of the small intestine that move in a
wave-like pattern to trap nutrients.
 Microvilli are the microscopic hairlike
projections on each villi.
Absorption
Water-soluble nutrients
and small products of fat
digestion are released to
the bloodstream.
Fat-soluble vitamins and
larger fats form chylomicrons and are released
to the lymphatic
system.
The Health and Regulation
of the GI Tract
The System at Its Best:
A healthy digestive tract is essential.
Balance, moderation, and variety of
meals help promote a healthy GI
tract.
Heartburn and
“Acid Indigestion”
• Gastroesophageal reflux is the
backward flow of stomach contents
into the esophagus.
• Antacids and acid controllers may
help indigestion.
Ulcers
• Peptic ulcers can be
gastric or duodenal
and are regions of
the GI tract that
have been eroded
by HCL and pepsin.
• The bacterium
Helicobacter pylori
contributes to the
production of both
gastric and
duodenal ulcers.
Diarrhea
• Frequent, loose, watery stools
Can be caused by food intolerances,
toxins, infection of the GI tract,
stress, or bowel disorders
Can lead to severe dehydration
Is more dangerous for children and
the elderly
• Irritable bowel syndrome and/or
colitis are possible GI disorders.
Constipation
• Defecation habits are different among
people.
• Many causes are possible.
• Hemorrhoids may be a problem.
• Diverticulosis is a condition in which the
intestinal walls weaken and bulge. The
bulging pockets are called diverticula.
Diverticulitis is a worsened condition and
requires intervention.
• Use of laxatives, enemas and mineral oil
may not be necessary with lifestyle
changes.
Chemical Reactions in the Body
• During digestion, the energy-yielding
nutrients are broken down, then
absorbed, and taken up by the body
cells.
• In metabolism they are broken down
further into energy, water and carbon
dioxide.
Metabolism of Nutrients
• A series of reactions needed to
transform food into a final product
that can be used by the body.
• Energy is released from chemical
bonds.
• Energy from food is converted into
ATP.
• ATP is the energy currency of the
cell.
Copyright 2010, John Wiley & Sons, Inc.
Chemical Reactions in
the Body
• Metabolism takes place inside of
cells. The process is the basis of life,
allowing cells to grow and reproduce,
maintain their structures, and respond
to their environments.
Breaking Down Nutrients for
Energy occurs in 3 stages:
The breakdown of glucose to energy
starts with:
1)glycolysis to pyruvate.
2)TCA cycle or tricarboxylic acid
cycle (or Kreb’s cycle)
3)electron transport chain