HUMAN BIOLOGY

DiGestioN aND NUtritioN 201

brush border The collective

array of microvilli on epithe-

lial cells lining the intestinal

mucosa.

microvillus Any of the

hundreds of microscopic

projections of the plasma

membranes of epithelial

cells that cover villi.

villus A fingerlike projection

from the mucosa (inner sur-

face) of the small intestine.

What is the role of the small intestine?

• The small intestine is where most nutrients in food are absorbed.


• The small intestine provides a vast surface area for nutrient
  absorption because it has a densely folded mucosa that in turn
  has millions of villi and hundreds of millions of microvilli.

taKe-Home message

the small intestine: a Huge surface

for Digestion and absorption

n The structure of the small intestine wall is the key to its

ability to absorb nutrients.

Your small intestine is about an inch and a half in diameter

and 6 meters (20 feet) long. It absorbs most nutrients in the

food you eat. Figure 11.8A and 11.8B show how densely

folded the mucosa is, and how the folds all stick out like

ruffles into the lumen. Each fold has even smaller, hairlike

projections (Figure 11.8C). Each “finger” is a villus (plural:

villi). Small blood vessels (an arteriole and a vein) and a

lymph vessel in each villus move substances to and from

the bloodstream. Gland cells in the mucosal lining release

digestive enzymes.

Most cells in the epithelium covering a villus have a

threadlike projection of their plasma membrane. This pro-

jection is called a microvillus (plural: microvilli). Each epi-

thelial cell has about 1,700 microvilli—a dense array that

gives the epithelium of villi its common name, the brush

border (Figure 11.8D and 11.8E).

What is the benefit of so many

folds and projections from the intes-

tinal mucosa? Together, they greatly

increase the surface area for absorb-

ing nutrients from chyme. Without

that huge surface area, absorption

would take place too slowly to sus-

tain life.

11.4

Fig u r e 11.8 The small intestine has a very large surface area. (© Cengage Learning)

circular

muscle

blood

vessels

lymph

vessel

villi

longitudinal

muscle

autonomic

nerves

B Structure of the wall

of the small intestine.

e Micrograph of microvilli

at the surface of a brush

border cell.

c One intestinal

fold with hairlike villi

at its surface.

D One villus

with brush

border cells

at its surface.

a Cross section through the small intestine

showing its highly folded lining.

Steve Gschmeissner/Science Source

© Samuel Borges Photography/Shutterstock.com

Biophoto Associates/Science Source

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