of connective tissue connect the serosa to the abdominal wall,
supporting the gastrointestinal tract in the abdominal cavity.
Extending from the luminal surface into the lumen of
the small intestine are ﬁ ngerlike projections known as
(see Figure 15–7). The surface of each villus is covered with
a layer of epithelial cells whose surface membranes form small
(also known collectively as the
). The combination of folded
mucosa, villi, and microvilli increases the small intestine’s
surface area about 600-fold over that of a ﬂ at-surfaced tube
having the same length and diameter. The human small intes-
tine’s total surface area is about 250 to 300 m
, roughly the
area of a tennis court.
Epithelial surfaces in the gastrointestinal tract are con-
tinuously being replaced by new epithelial cells. In the small
intestine, new cells arise by cell division from cells at the
base of the villi. These cells differentiate as they migrate to
the top of the villus, replacing older cells that die and are
discharged into the intestinal lumen. These dead cells release
into the lumen their intracellular enzymes, which then con-
tribute to the digestive process. About 17 billion epithelial
cells are replaced each day, and the entire epithelium of the
small intestine is replaced approximately every ﬁ ve days. It is
because of this rapid cell turnover that the lining of the intes-
tinal tract is so susceptible to damage by agents that inhibit
cell division, like anticancer drugs and by radiation therapy.
The center of each intestinal villus is occupied both by a
single, blind-ended lymphatic vessel termed a
a capillary network (see Figure 15–7). As we will see, most
of the fat absorbed in the small intestine enters the lacteals.
Material absorbed by the lacteals reaches the general circula-
tion by eventually emptying from the lymphatic system into
the thoracic duct.
Other absorbed nutrients enter the blood capillaries. The
venous drainage from the small intestine, as well as from the
large intestine, pancreas, and portions of the stomach, does
not empty directly into the vena cava but passes ﬁ rst, via the
hepatic portal vein,
to the liver. There it ﬂ ows through a sec-
ond capillary network before leaving the liver to return to the
heart. Thus, material absorbed into the intestinal capillaries,
in contrast to the lacteals, can be processed by the liver before
entering the general circulation. This is important because the
liver contains enzymes that can metabolize (detoxify) harmful
compounds that may have been ingested, thereby preventing
them from entering the circulation. It also explains why cer-
tain drugs (e.g., testosterone) are given by injection or skin
patch (because oral administration may injure the liver).
The gastrointestinal tract also has a variety of immune
functions, allowing it to produce antibodies and ﬁ
tious organisms that are not destroyed by the acidity of the
stomach. For example, the small intestine has
and immune cells that secrete inﬂ ammatory mediators (e.g.,
cytokines; Chapter 18), that alter motility. These mediators
may play a role in causing inﬂ ammation in certain autoimmune
disorders such as
), which are described at the end of this chapter.
Digestion and Absorption
The average daily intake of carbohydrates is about 250 to 300 g
per day in a typical American diet. This represents about half
the average daily intake of calories. About two-thirds of this
carbohydrate is the plant polysaccharide starch, and most
of the remainder consists of the disaccharides sucrose (table
sugar) and lactose (milk sugar) (
). Only small
amounts of monosaccharides are normally present in the diet.
Cellulose and certain other complex polysaccharides found in
vegetable matter—referred to as
—are not broken down
Microvilli on the surface of intestinal epithelial cells.
From D. W. Fawcett,
J. Histochem. Cytochem,
13: 75–91 (1965). Courtesy of Susumo Ito.
Carbohydrates in Food