Beyond Basics
The Normal Digestive Process in the Lower GI Tract
Anatomy –The Small Intestine
The small intestine is a hollow tube that runs from
the stomach to the ileocecal valve, which separates the small intestine
from the colon. Food and liquid move through the center of the tube, which
is called the lumen. The inner lining of the small intestine (the mucosa)
contains several types of specialized cells that perform different functions
such as the secretion of digestive juices, or the absorption of nutrients
from the lumen. The walls of the small intestine contain muscle, which
works to both mix the contents of the lumen to aid digestion and absorption,
and to move the undigested matter towards the colon and, in turn, the
rectum, where it is eliminated as feces.
The small intestine is divided into three functional
regions: the duodenum, jejunum, and ileum. The stomach empties its contents
into the duodenum, which is the first part of the small intestine. Here
the contents are mixed with pancreatic enzymes (juices) and bile. Bile
is manufactured in the liver and stored in the gallbladder until needed
for digestion. The major function of the duodenum is this initial phase
of digestion (see DIGESTION). This
digestive phase is highly regulated. Water, ions, enzymes, bile acids,
and other secretions are added to the lumenal contents in proportions
that create an optimal environment for nutrient digestion (Fig. 2). For
example, if the contents contain a lot of fat, more bile will be secreted
since this substance helps in the digestion of fat.
Figure 2 –
Duodenum
Rhythmic contractions in the wall of the small intestine
(see MOTILITY) move the lumenal
contents into the next part of the small intestine, the jejunum. While
the duodenum is primarily responsible for the digestive process, the jejunum
is responsible for most of the nutrient absorption in the small intestine.
To accomplish this absorption, the highly coiled jejunum contains extensive
circular folds in the mucosa, which increase its surface area. In addition,
the mucosa forms millions of small finger-like projections (villi) that
protrude into the lumen from the mucosa.
The villi are covered with absorptive epithelial
cells. The epithelial cells are, in turn, covered with microvilli
that further expand the absorptive surface of the jejunum (Fig. 3).
Figure 3 –
Microscopic Anatomy of the Small Intestine
It is estimated that if the small intestine were simply
a hollow tube, its lumenal surface area would be about one half of a square
meter (about 5.4 square feet). However, the circular folds, the villi,
and the microvilli combined provide an absorptive surface that is approximately
250 square meters (2,690 square feet).
The villi are separated by depressions called crypts,
which are lined with cells that are primarily involved in secretion (see
SECRETION). Basically, nutrients
are absorbed by moving from the intestinal lumen into the epithelial cells.
Most of the nutrients then leave the cell and diffuse into the capillary
network inside the villus (Fig. 4), which brings them into the bloodstream.
Figure 4 –
Illustration of Villus
The diameter of the lumen in the jejunum gradually diminishes
between the duodenum and the ileum. The height of the villi and the depth
of the crypts also gradually decrease from the beginning of the jejunum
to the end of the ileum. There is no distinct point at which the jejunum
becomes the ileum.
The ileum provides another site for nutrient absorption.
The terminal end of the ileum has a specific role in the absorption of
bile acids and vitamin B12. As stated earlier, bile acids are a critical
factor in the digestion of fats. Bile acids absorbed in the ileum are
returned to the liver to again be processed into more bile and stored
in the gallbladder. In this way, the pool of bile acids is continually
recycled and maintained in the body. The ileum terminates at the ileocecal
valve, which is a one-way valve into the cecum (a pouch that forms the
first portion of the colon). This prevents the backflow of colon contents
from entering the small intestine.
This information
is provided as a general educational service
and is not intended to recommend any particular treatment plan
or to replace the advice of physicians. It is important that patients
seek and rely on the advice of a healthcare professional
about their individual medical conditions.
