Nutrition and the Digestive System
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Figure 16- 13 Absorptive features of the small
intestine.
(A) Plicae circulares; (B) villi; (C) microscopic
anatomy of a -villus; and (D) microvilli.
look like microscopic eye dropper bulbs approximately 0.5
to 1 mm long. A tremendous number of villi line the
intestine, about 4 to 5 million. These villi vastly in-crease
the surface area of the epithelium for absorption
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of nutrients. The structure of each villus contains a cap-
illary network where blood picks up nutrients, a venule or
small vein to transport the nutrients, an arteriole or small
artery, and a lacteal of the lymphatic system to pick up fats.
In addition, the individual epithelial cells that cover the
surface of a villus have a brush border of microvilli to
further increase the absorptive capability of the small
intestine. Nutrients that pass through the epi-thelial cells
covering the villus are able to pass through the endothelial
cells of the capillary walls and through the lacteals to enter
the blood and lymphatic circulatory systems. From there
they are transported to the billions of cells in the body.The Large Intestine
The functions of the large intestine are the absorption of
water, the manufacturing and absorption of certain
vitamins, and the formation and expulsion of the feces
(FEE-seez). The large intestine is about 5 feet in length and
averages 2.5 inches in diameter (Figure 16-14). It is also
referred to as the bowel. It is attached to the pos-terior
wall of the abdomen by extensions of its visceral
peritoneum known as the mesocolon. It is divided into
four principal regions: (1) the cecum (SEE-kum), the
pouchlike first part of the large intestine; (2) the colon
(KOH-lon), the largest part; (3) the rectum; and (4) the
anal canal.
The opening from the ileum of the small intestine into
the cecum of the large intestine is a fold of mucous
membrane known as the ileocecal valve. This valve al-lows
material to pass from the small intestine into the large
intestine. The cecum, a blind pouch (one end is closed), is 2
to 3 inches long and hangs below the ileo-cecal valve.
Attached to the closed end of the cecum is the twisted tube
known as the vermiform (VER-mih-form) appendix,
about 3 inches in length. The open end of the cecum
merges with the long tube called the colon.The colon looks like a tube of consecutive pouches.
The pouches are called haustrae (HAW-stree). The first
part of the colon is known as the ascending colon. It
rises on the right side of the abdomen, reaches the under-
surface of the liver, and turns to the left at the right colic
(hepatic) flexure. The right colic flexure continues
across the abdomen to the left side as the transverse
colon. It then curves beneath the lower end of the spleen
to the left side as the left colic (splenic) flexure. Next,
it passes downward as the descending colon. The last
part of the colon is called the sigmoid (SIG-moyd)
colon, where the colon joins the rectum.