Small intestine
 |
Diagram showing the small intestine |
In
biology the
small intestine is the part of the
gastrointestinal tract between the
stomach and the
large intestine (colon). In
humans over 5 years old it is 5-6m long. It is divided into three structural parts:
duodenum, jejunum and ileum. Food from the stomach is allowed in to the duodenum by a muscle called the
pylorus, or
pyloric sphincter, and is then pushed through the small intestine by a process of muscular contractions called
peristalsis.
The small intestine is the site where most of the nutrients from ingested food are absorbed. There are microscopic finger-like projections called
villi covering the small intestinal walls which increase surface area for absorption. Each villus contains a lacteal and capillaries. The
lacteal absorbs the digested
fat into the
lymphatic system which will eventually drain into the
circulatory system. The capillaries absorb all other digested
nutrients.
Although the small intestine is much longer than the
large intestine (typically 4-5 times longer), it is referred to as such due to its comparatively smaller diameter. On average, the diameter of the large intestine of an adult human measures approximately 3 times the diameter of the small intestine. Upon viewing accurate diagrams of the digestive system, non-medically trained personnel often initially express bemusement as to why the obviously longer portion of the digestive system is referred to as the small intestine.
The digestion of proteins into peptides and amino acids principally occurs in the stomach but some also occurs in the small intestine. The small intestine is where the most chemical digestion takes place; peptides are degraded into
amino acids;
lipids (fats) are degraded into
fatty acids and
glycerol; and
carbohydrates are degraded into simple sugars (e.g.,
glucose).
The largest amount of water from the
chyme (the substance that comes from the stomach) is absorbed in the large intestine.
(see also
gastroenterology)
* small intestine obstruction ("high" mechanic ileus)
** obstruction from external pressure
** obstruction by masses in the lumen (
foreign bodies,
bezoar,
gallstones)
* paralytic
ileus*
Crohn's disease*
celiac disease*
carcinoid*
Meckel's Diverticulum*
gastric dumping syndrome*
infectious diseases**
giardiasis**
ascariasis**
tropical sprue**
tapeworm infection*
mesenteric ischemia*
short bowel syndrome*
inguinal herniaSolomon et. al (2002) Biology Sixth Edition, Brooks-Cole/Thomson Learning ISBN 0-03-033503-5
*
Intestinal juice*
Colon (anatomy)*
Small Intestine from the Gray's Anatomy eBook*
Peyer's patches*
Normal Histology of the Small Intestine Simple:Small intestine
