CHAPTER
12
Gastrointestinal Digestion
and Absorption
The gastrointestinal (or alimentary) system (or tract) is
responsible for digestion and absorption of nutrients and
fluids. The tract extends from the mouth to the anus and in
an adult human is about 10 m long.
Digestion
is the hydro-
lysis of complex food substances into simpler units such as
monosaccharides, amino acids, fatty acids, and glycerol.
Absorption
is the transport of the products of digestion
and of vitamins, minerals, and water across the intestinal
epithelium to the lymphatic or blood circulatory systems.
Digestion and absorption involve not only the gastroin-
testinal (GI) tract but also secretions from salivary glands,
liver (and gallbladder), and pancreas. These processes in
the GI system are interrelated. The system provides highly
selective, efficient, and elaborate absorptive surfaces that
contain enzymes and secretions of enzyme-containing flu-
ids, electrolytes, and other substances required for diges-
tion and absorption. Secretory and absorptive activities are
regulated by hormonal and neural mechanisms.
12.1 Anatomy and Physiology of the GI Tract
The GI tract consists of mouth and esophagus, stomach,
small intestine (duodenum, jejunum, and ileum), and large
intestine (colon and rectum).
Mouth and Esophagus
In the mouth, food is mixed with saliva, chewed to break
up large particles, and propelled into the esophagus by
swallowing. Saliva is secreted by three pairs of glands
(parotid, submaxillary, and sublingual) and by numerous
small buccal glands and is under autonomic nervous sys-
tem control. Parasympathetic nervous stimulation causes
profuse secretion of saliva. Thus, atropine and other an-
ticholinergic agents diminish salivary secretion and make
the mouth dry. The presence of food in the mouth and the
act of chewing stimulate secretion of saliva by reflex ner-
vous stimulation. Salivary secretion can also be a condi-
tioned response, so that the sight, smell, and even thought
of food can elicit salivary secretion; however, in humans,
this type of conditioned response is weak.
The composition, pH, and volume of salivary excretion
for different salivary glands vary. Usually 1-2 L of saliva
are secreted per day. Saliva is a hypotonic solution with a
pH of about 7.0; principal cations are Na+ and K+, and
anions are Cl- and HCOJ. Aldosterone (a steroid hormone
of the adrenal cortex) modulates Na+ and K+ levels in
saliva as it does in the kidney (Chapter 32). Aldosterone in-
creases reabsorption of Na+ and secretion of K+ by direct
action on the salivary gland ducts. In
hypoaldosteronism
(e.g.,
Addison’s disease),
the salivary Na+/K+ ratio is high,
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