876
UNIT 4
Maintenance of the Body
23
proximal part of the small intestine, so these specializations de-
crease in number toward its distal end.
Te
circular folds
are deep, permanent folds of the mucosa
and submucosa
(Figure 23.22a)
. Nearly 1 cm tall, these folds
force chyme to spiral through the lumen, slowing its movement
and allowing time for full nutrient absorption.
Villi
(vil
9
i; “tuFs of hair”) are fingerlike projections of the mu-
cosa, over 1 mm high, that give it a velvety texture, much like the
Microscopic Anatomy
Modifications for Absorption
Te small intestine is highly
adapted for absorbing nutrients. Its length alone provides a huge
surface area, and its wall has three structural modifications—
circular folds, villi, and microvilli—that amplify its absorptive
surface enormously (by a factor of more than 600 times). In fact,
the intestinal surface area is about equal to 200 square meters,
the size of a singles tennis court! Most absorption occurs in the
Vein carrying
blood to
hepatic portal
vessel
Muscle
layers
Circular
folds
Villi
(a)
(b)
(c)
Lumen
Absorptive
cells
Lacteal
Intestinal
crypt
Mucosa-
associated
lymphoid
tissue
Intestinal crypt
Muscularis
mucosae
Duodenal
gland
Submucosa
Enteroendocrine
cells
Venule
Lymphatic vessel
Absorptive cells
Microvilli
(brush border)
Goblet
cell
Blood
capillaries
Villi
Goblet
cells
Villus
Figure 23.22
Structural modifications of the small intestine
that increase its surface area for digestion and absorption.
(a)
Enlargement of a few circular folds, showing associated
fingerlike villi (muscularis and serosa layers not indicated).
(b)
Structure of a villus. Enlargement shows one and part of two
other absorptive cells that exhibit microvilli on their free (apical)
surface.
(c)
Photomicrograph of the mucosa, showing villi (250
3
).
(For a related image, see
A Brief Atlas of the Human Body
,
Figure 69b.)
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