The Digestion and Absorption of Food
529
The adult gastrointestinal tract is a tube approximately 9 m (30 feet) in length, running through the
body from mouth to anus. The lumen of the tract, like the hole in a doughnut, is continuous with
the external environment, which means that its contents are technically outside the body. This fact is
relevant to understanding some of the tract’s properties. For example, the large intestine is inhabited by
billions of bacteria, most of which are harmless and even benefi cial in this location. However, if the same
bacteria enter the internal environment, as may happen, for example, in the case of a ruptured appendix,
they may cause a severe infection.
Most food enters the gastrointestinal tract as large particles containing macromolecules, such as
proteins and polysaccharides, which are unable to cross the intestinal epithelium. Before ingested food
can be absorbed, therefore, it must be dissolved and broken down into small molecules. This dissolving
and breaking-down process is called
digestion
and is accomplished by the action of hydrochloric acid
in the stomach, bile from the liver, and a variety of digestive enzymes released by the system’s exocrine
glands. Each of these substances is released into the lumen of the GI tract through the process of
secretion.
The molecules produced by digestion then move from the lumen of the gastrointestinal tract across a
layer of epithelial cells and enter the blood or lymph. This process is called
absorption.
While digestion, secretion, and absorption are taking place, contractions of smooth muscles in the
gastrointestinal tract wall serve two functions. They mix the luminal contents with the various
secretions, and they move the contents through the tract from mouth to anus. These contractions are
referred to as the
motility
of the gastrointestinal tract.
The functions of the gastrointestinal system can be described in terms of these four processes—
digestion, secretion, absorption, and motility (
Figure 15–2
)—and the mechanisms controlling them.
The gastrointestinal system is designed to maximize absorption, and within fairly wide limits, will
absorb as much of any particular substance as is ingested. With a few important exceptions (to be
described later), therefore, the gastrointestinal system does not regulate the amount of nutrients
absorbed or their concentrations in the internal environment. The regulation of the plasma
concentration of the absorbed nutrients is primarily the function of the kidneys (Chapter 14) and a
number of endocrine glands (Chapters 11 and 16).
Small amounts of certain metabolic end products are excreted via the gastrointestinal tract, primarily by
way of the bile. The lungs and kidneys are responsible for the elimination of most of the body’s waste
products. The material known as
feces
leaves the system at the end of the gastrointestinal tract. Feces
consists almost entirely of bacteria and ingested material that was neither digested nor absorbed—that is,
material that was never actually part of the internal environment.
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