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Lecture notes study book Introducing Food Science of Robert L. Shewfelt, Alicia Orta-Ramirez, Andrew D. Clarke - ISBN: 9781498766647 (HEALTH)
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MODULE 3: DIGESTION, ABSORPTION AND ASSIMILATION OF FOODKnowledge outcomes
After completion of this theme, the learner should be able to describe:
- How the digestive system works.
- How food are digested and absorbed.
- How the absorbed nutrients are transported and used in the body.
- How food are metabolized in the body.
- How waste products are excreted from the body.
Skills outcomes
After completion of this theme, the learner should be able to:
Understand digestive system problems and give advice to handle such
problems.
,Content
The human digestive system consists of the gastrointestinal tract plus the
accessory organs of digestion (the tongue, salivary glands, pancreas, liver,
and gallbladder). Digestion involves the breakdown of food into smaller and
smaller components, until they can be absorbed and assimilated into the
body. The process of digestion has many stages. The first stage is
the cephalic phase of digestion which begins with gastric secretions in
response to the sight and smell of food. The next stage starts in the mouth.
Chewing, in which food is mixed with saliva, begins the mechanical process
of digestion. This produces a bolus which can be swallowed down
the esophagus to enter the stomach. Here it is mixed with gastric acid until it
passes into the duodenum where it is mixed with a number
of enzymes produced by the pancreas. Saliva also contains
a catalyticenzyme called amylase which starts to act on food in the mouth.
Another digestive enzyme called lingual lipase is secreted by some of
the lingual papillae on the tongue and also from serous glands in the main
salivary glands. Digestion is helped by the chewing of food carried out by
the muscles of mastication, by the teeth, and also by
the contractions of peristalsis, and segmentation. Gastric acid, and the
production of mucus in the stomach, are essential for the continuation of
digestion.
Peristalsis is the rhythmic contraction of muscles that begins in the esophagus
and continues along the wall of the stomach and the rest of
the gastrointestinal tract. This initially results in the production of chyme which
when fully broken down in the small intestine is absorbed as chyle into
the lymphatic system. Most of the digestion of food takes place in the small
intestine. Water
and
some minera ls are
reabsorbed back
into the blood in
the colon of the large
intestine. The waste
products of digestion
(feces)
are defecate d from
the anus via
the rectum.
, 3.1 DIGESTION AND ABSORPTION
To be used by the body, food must be eaten and digested and the nutrients
must be absorbed and transported to the cells of the body. Activity in the
digestive tract begins before food even enters the mouth. As the meal is
prepared, sensory inputs, such as the sight of the food, the clatter of the table
being set and the smell of the food make your mouth become moist and your
stomach begin to secrete digestive substances. This response occurs when
the nervous system signals the digestive system to ready itself for a meal.
3.1.1 The mouth
The mouth is the first part of the upper gastrointestinal tract and is equipped
with several structures that begin the first processes of digestion. These
include salivary glands, teeth and the tongue. The mouth consists of two
regions; the vestibule and the oral cavity proper. The vestibule is the area
between the teeth, lips and cheeks, and the rest is the oral cavity proper.
Most of the oral cavity is lined with oral mucosa, a mucous membrane that
produces a lubricating mucus, of which only a small amount is needed.
Mucous membranes vary in structure in the different regions of the body but
they all produce a lubricating mucus, which is either secreted by surface
cells or more usually by underlying glands. The mucous membrane in the
mouth continues as the thin mucosa which lines the bases of the teeth. The
main component of mucus is a glycoprotein called mucin and the type
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