Ganong's Review of Medical Physiology, 23rd Edition

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SECTION V
Gastrointestinal Physiology

relationship is mutually beneficial, because the bacteria perform

several metabolic functions that cannot be accomplished with

mammalian enzymes, and likely also provide some degree of

protection against subsequent infection with pathogenic micro-

organisms that might cause disease. Nevertheless, the constant

presence of bacterial and other stimuli, as well as the large sur-

face area that must be defended against potentially harmful sub-

stances, doubtlessly accounts for the fact that the intestine has a

very well-developed local immune system that comprises both

innate and adaptive immune effectors (see Chapter 3). Indeed,

there are more lymphocytes in the wall of the intestine than

there are circulating in the blood.

STRUCTURAL CONSIDERATIONS

The parts of the gastrointestinal tract that are encountered by
the meal or its residues include, in order, the mouth, esopha-
gus, stomach, duodenum, jejunum, ileum, cecum, colon, rec-
tum, and anus. Throughout the length of the intestine,
glandular structures deliver secretions into the lumen, particu-
larly in the stomach and mouth. Also important in the process
of digestion are secretions from the pancreas and the biliary
system of the liver. The intestinal tract is also functionally di-
vided into segments that restrict the flow of intestinal contents
to optimize digestion and absorption. These sphincters include
the upper and lower esophageal sphincters, the pylorus that re-
tards emptying of the stomach, the ileocecal valve that retains
colonic contents (including large numbers of bacteria) in the
large intestine, and the inner and outer anal sphincters. After
toilet training, the latter permit delaying the elimination of
wastes until a time when it is socially convenient.
The intestine is composed of functional layers (Figure 26–1).
Immediately adjacent to nutrients in the lumen is a single layer
of columnar epithelial cells. This represents the barrier that
nutrients must traverse to enter the body. Below the epithelium

is a layer of loose connective tissue known as the lamina pro-

pria, which in turn is surrounded by concentric layers of

smooth muscle, oriented circumferentially and then longitudi-

nally to the axis of the gut (the circular and longitudinal muscle

layers, respectively). The intestine is also amply supplied with

blood vessels, nerve endings, and lymphatics, which are all

important in its function.

The epithelium of the intestine is also further specialized in

a way that maximizes the surface area available for nutrient

absorption. Throughout the small intestine, it is folded up

into fingerlike projections called villi (Figure 26–2). Between

the villi are infoldings known as crypts. Stem cells that give

rise to both crypt and villus epithelial cells reside toward the

base of the crypts and are responsible for completely renewing

the epithelium every few days or so. Indeed, the gastrointesti-

nal epithelium is one of the most rapidly dividing tissues in

the body. Daughter cells undergo several rounds of cell divi-

sion in the crypts then migrate out onto the villi, where they

are eventually shed and lost in the stool. The villus epithelial

cells are also notable for the extensive microvilli that charac-

terize their apical membranes. These microvilli are endowed

with a dense glycocalyx (the brush border) that probably

FIGURE 26–1
Organization of the wall of the intestine into functional layers.
(Adapted from Barrett KE:
Gastrointestinal Physiology

. McGraw-Hill, 2006.)

Lumen

Epithelium

Basement memdrane

Lamina propria

Muscularis mucosa

Submucosa

Circular muscle

Myenteric plexus

Longitudinal muscle

Mesothelium (Serosa)

Mucosa

Muscularis

propria