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Small Intestine
The small intestine is a convoluted tube that begins just distal to the
pyloric sphincter of the stomach and then loops through the central and
inferior region of the abdomen before ending at the ileocecal valve,
where it merges with the large intestine. The small intestine is the
primary digestive organ in the body. Not only is it the part of the GI tract
where digestion is completed, it is also where the majority of absorption
occurs.
Although the small intestine is the longest part of the GI tract, its
diameter is about half that of the large intestine, averaging a little over
one inch (2.5 cm). When we are alive, the small intestine is more than 3
meters (10 feet) long – the size of a one-story building. Its length
provides expansive surface area necessary for digestion and absorption.
However, circular folds, villi, and microvilli add even more surface area.
With loss of muscle tone after death, the folds of the small intestine
relax, extending it to about 20 feet in length.
This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States
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Small Intestine Anatomy
The duodenum precedes the jejunum and ileum and is the shortest
part of the small intestine; it is less than 1 foot of the 10 foot intestine
(30 cm of the 3 m). The duodenum receives the stomach contents,
pancreatic juice and bile. Chemical digestion continues in the
duodenum. The jejunum is the next portion (3.5 - 5.5 ft or 110-170 cm)
of the small intestine and is responsible for the absorption of a majority
of nutrients. In the last section of the small intestine, the ileum,
vitamin B12 and bile salts are absorbed as well as materials not
absorbed by the jejunum.
The term mucosa or mucous membrane always refers to the
combination of the epithelium plus the lamina propria. The lamina
propria layer of the small intestinal mucosa is composed of areolar
connective tissue and quite a bit of mucosa-associated lymphoid tissue.
Most solitary lymphatic nodules are located in the distal portion of the
ileum. The blood supply which runs through the lamina propria and
drains the villus is part of the the hepatic portal systems and not general
systemic circulaions
In the ileum, the lamina propria also has aggregated lymphatic
follicles (groups of lymphatic nodules) called Peyer's patches. There
are more Peyer's patches toward the end of the small intestine, possibly
because of the high amounts of bacteria in that area must be prevented
from entering the bloodstream. The submucosa of the duodenum is the
only site of the complex mucus-secreting duodenal (Brunner's)
glands, which produce bicarbonate-rich alkaline mucus that helps
buffer the acidic chyme coming in from the stomach.
Like in the rest of the GI tract, the muscularis externa layer is made of
two layers of smooth muscle – an outer, thinner layer of longitudinal
fibers and an inner, thicker layer of circular fibers. The serosa
completely enshrouds the small intestine, with the exception of a large
region of the duodenum.