Maintenance of the Body
proximal part of the small intestine, so these specializations de-
crease in number toward its distal end.
are deep, permanent folds of the mucosa
. Nearly 1 cm tall, these folds
force chyme to spiral through the lumen, slowing its movement
and allowing time for full nutrient absorption.
i; “tuFs of hair”) are ﬁngerlike projections of the mu-
cosa, over 1 mm high, that give it a velvety texture, much like the
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
Structural modiﬁcations of the small intestine
that increase its surface area for digestion and absorption.
Enlargement of a few circular folds, showing associated
ﬁngerlike villi (muscularis and serosa layers not indicated).
Structure of a villus. Enlargement shows one and part of two
other absorptive cells that exhibit microvilli on their free (apical)
Photomicrograph of the mucosa, showing villi (250
(For a related image, see
A Brief Atlas of the Human Body