Extracting nutrients: the small intestine The
small intestine is where nutrients move from the
gut to the blood. The nearly 18 feet of small intes-
tine loop back and forth within the abdominal
cavity, framed inside the COLON. Two layers of
smooth muscle, the outer longitudinal and the
inner circular, form the walls of the small intes-
tine. These muscles rhythmically contract to move
digestive matter through the gastrointestinal tract.
The intestinal mucosa, a thin mucous membrane,
forms the inner lining. It produces a number of
digestive enzymes and digestive hormones.
In the first segment of the small intestine, the
duodenum, the intestinal mucosa is fairly smooth.
Only 10 inches long, the duodenum handles the
majority of digestive activity. In addition to receiv-
ing chyme from the stomach, the duodenum
receives bile and pancreatic juices via the common
bile duct, which enters the duodenum at a small
port called the ampulla of Vater. These solutions
complete the breakdown of foods into end-prod-
uct nutrients. Bile transforms fats into fatty acids.
Pancreatic and intestinal enzymes convert proteins
to amino acids and polysaccharides (compound
sugars) to monosaccharides (simple sugars). Fur-
ther chemical interactions separate out vitamins
and minerals. Monosaccharides and some elec-
trolytes (salts and minerals) enter the bloodstream
through the duodenum, which is also the major
site for absorption of iron and calcium.
The watery mixture containing the remaining
nutrients moves on to the middle and end seg-
ments of the small intestine for absorption. In
these segments, the JEJUNUMand the ILEUM, mil-
lions of tiny projections called villi extend from
the intestinal mucosa to vastly expand the
mucosal surface area. A network of capillaries
(tiny blood vessels) weaves through the villi.
Nutrients pass through the mucosal membrane
and into the capillaries, which transport them into
the bloodstream and throughout the body. The
jejunum, about seven feet long, absorbs the
remaining monosaccharides and many amino
acids, additional electrolytes, water-soluble vita-
mins (the B vitamins and vitamin C), folic acid,
and minerals such as iron. The final segment of
the small intestine, the ileum, is about 10 feet
long. It absorbs fatty acids and the remaining
amino acids, as well as vitamin B 12 and the fat-sol-
uble vitamins (vitamins A, D, E, and K). The
ileum also reabsorbs bile salts. The journey
through the small intestine takes 8 to 10 hours.
Compacting waste: the colonThe colon, also
called the large intestine, collects and compacts
the remnants of digestion for their elimination
from the body, a process it accomplishes primarily
by absorbing water. Like the small intestine, the
colon’s wall contains two layers of muscle, the
outer running lengthwise and the inner circling
around, that rhythmically contract. The inner lin-
ing is flat mucous membrane. The CECUM, the first
segment of the colon, is a pouchlike structure that
receives digestive material from the ileum, which
enters near its floor. The ileocecal valve maintains
the passage for one-way movement. The cecum
absorbs about a third of the water from the diges-
tive material it receives. Peristalsis then carries the
intestinal content from the cecum through the
rest of the colon.
The main colon loops around the outer edge of
the abdominal cavity. It contains five segments.
The ascending colon rises from the cecum and
travels up the body’s right side. At the gallbladder
the colon takes a turn; the next segment is the
transverse colon. The transverse colon extends
across the top of the abdomen, with the liver and
stomach above and the small intestine beneath. It
takes a downward turn at the base of the stom-
ach’s fundus, becoming the descending colon. The
descending colon drops along the left perimeter of
the abdominal cavity. The descending colon makes
a staggered turn inward toward the midline of the
body, becoming the sigmoid colon. These four seg-
ments of the colon are functionally contiguous,
progressively dehydrating and compacting the
digestive residue that moves through them. The
final segment of the colon is the RECTUM, by which
point digestive waste has reached the solid form
known as feces or stool. The rectum stores stool
until its expulsion from the body via the anus
(bowel movement). The journey through the
colon generally takes four to six hours, though
can take longer.Health and Disorders of the
Gastrointestinal System
The gastrointestinal system represents an intricate
balance of mechanical and chemical functions.4 The Gastrointestinal System