Understanding Nutrition, 8e

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Understanding Nutrition, 8e

• Chapter 3 - Digestion, Absorption, and Transport

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Peristalsis and Segmentation
The small intestine has two muscle layers that
work together in peristalsis and segmentation.
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Closeup of Muscles
The small intestine has two muscle layers that
work together in peristalsis and segmentation.
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Peristalsis 1 of 3
The inner circular muscles contract, tightening
the tube and pushing the food forward in the
intestine.
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Peristalsis 2 of 3
When the circular muscles relax, the outer
longitudinal muscles contract, and the intestinal
tube is loose.
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Peristalsis 3 of 3
As the circular and longitudinal muscles tighten
and relax, the chyme moves ahead of the
constriction.
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Segmentation 1 of 3
Circular muscles contract, creating segments
within the intestine.
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Segmentation 2 of 3
As each set of circular muscles relaxes and
contracts, the chyme is broken up and mixed with
digestive juices.
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Segmentation 3 of 3
These alternating contractions, occurring 12 to
16 times per minute, continue to mix the chyme
and bring the nutrients into contact with the
intestinal lining for absorption.
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The Salivary Glands
The salivary glands secrete saliva into the mouth
and begin the digestive process. Given the short
time food is in the mouth, salivary enzymes
contribute little to digestion.
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The pH Scale
A substances acidity or alkalinity is measured
in pH units. The pH is the negative logarithm of
the hydrogen ion concentration. Each increment
presents a tenfold increase in concentration of
hydrogen particles. For example, a pH of 2 is
1000 times stronger than a pH of 5.
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The Colon
The colon reabsorbs water and salts.
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Absorption of Nutrients
Absorption of nutrients into intestinal cells
typically occurs by simple diffusion, facilitated
diffusion, or active transport. (a) Some
nutrients, such as water and small lipids, are
absorbed by simple diffusion. They cross into
intestinal cells freely.
Click on the Video buttons to see animations of
digestion in the mouth esophagus stomach liver g
allbladder pancreas small intestine large
intestine sphincter
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Absorption of Nutrients (cont.)
Absorption of nutrients into intestinal cells
typically occurs by simple diffusion, facilitated
diffusion, or active transport. (b) Some
nutrients, such as the water-soluble 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.)
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Absorption of Nutrients (cont.)
Absorption of nutrients into intestinal cells
typically occurs by simple diffusion, facilitated
diffusion, or active transport. (c) Some
nutrients, such as glucose and amino acids, must
be absorbed actively. These nutrients move
against a concentration gradient, which requires
energy. 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 the the nutrients can
pass through.)
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The Vascular System
(1) Blood leaves the right side of the heart by
way of the pulmonary artery. (2) Blood loses
carbon dioxide and picks up oxygen in the lungs
and returns to the left side of the heart by way
of the pulmonary vein. (3) Blood leaves the
left side of the heart by way of the aorta, the
main artery that launches blood on its course
through the body. (4) Blood may leave the aorta
to go to the upper body and head or blood may
leave the aorta to go to the lower body. (5)
Blood may go to the digestive tract and then the
liver or blood may go to the pelvis, kidneys,
and legs. (6) Blood returns to the right side of
the heart. (7) Lymph from most of the body's
organs, including the digestive system, enters
the bloodstream near the heart.
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Nephron, One of the Kidneys Functioning Units
A nephron, one of the kidneys many functioning
units.
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Kidney Section
Kidney, sectioned to show location of nephrons.
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A Nephron
(1) Blood flows into the glomerulus, and some of
its fluid, with dissolved substances, is absorbed
into the tubule. (2) Then the fluid and
substances needed by the body are returned to the
blood in vessels alongside the tubule. (3) The
tubule passes waste materials on to the bladder.
The cleansing of blood in the nephron is
roughly analogues to the way you might clean your
car. First you remove all your possessions and
trash so that the car can be vacuumed (1). Then
you put back in the car what you want to keep (2)
and throw away the trash (3).
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The Liver
(1) Vessels gather up nutrients and reabsorbed
water and salts from all over the digestive
tract. Not shown here Parallel to these vessels
(veins) are other vessels (arteries) that carry
oxygen-rich blood from the heart to the
intestines. (2) The vessels merge into the
portal vein, which conducts all absorbed
materials to the liver. (3) The hepatic artery
brings a supply of freshly oxygenated blood (not
loaded with nutrients) from the lungs to supply
oxygen to the liver's own cells. (4) Capillaries
branch all over the liver, making nutrients and
oxygen available to all its cells and giving the
cells access to blood from the digestive system.
(5) The hepatic vein gathers up blood in the
liver and returns it to the heart. In contrast,
nutrients absorbed into lymph do not go to the
liver first. They go to the heart, which pumps
them to all the body's cells. The cells remove
the nutrients they need, and the liver then has
to deal only with the remnants.
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