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Motility. The small intestine has two patterns
of movement (motility): peristalsis and segmentation. Peristalsis
is contractions that begin at a particular point and move down the
small intestine for some distance. Peristalsis is the primary mechanism
by which the contents of the lumen are pushed along the intestine
toward the colon during the digestive process. Segmentation refers
to the process in which a number of contractions occur simultaneously
at uniform intervals along the intestine. This is the primary mechanism
for mixing the contents of the lumen with secretions and increasing
the contact between the lumenal contents and the mucosa, thereby
increasing absorption.
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Figure
6 – Short animation showing a comparison of peristalsis
without segmentation
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Figure
6 – Short animation showing a comparison of peristalsis
with segmentation
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Secretion. Specialized
cells in the mucosa lining the small intestine produce and deliver
(secrete) a variety of enzymes, mucus, and other substances into the
lumen to regulate digestion. These cells can also produce substances
that function like hormones, and that are secreted into the blood
rather than into the lumen. |
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Digestion. Food
must be broken down into smaller particles in order for the body to
build and nourish cells and provide energy. The digestive process
breaks down carbohydrates, proteins, and fats into small molecules
that are capable of being absorbed by the body. This is done by chemical
substances, such as pancreatic juices, and by mechanical means, such
as the mixing together of nutrients and enzymes in the lumen that
increases the exposure of food to digestive enzymes. |
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Absorption. The
nutrients in the lumen come in contact with the epithelial cells and
are absorbed through this cell membrane. In addition, the small intestine
also absorbs water and electrolytes. |
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Elimination. This is the process in which
waves of peristalsis move down the small intestine, pushing waste
material towards the colon, where it is stored. The waste is then
transported to the rectum, and eliminated through the anus.
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