small intestine
Meaning of small intestine: Small describes width rather than length, since this is by far the longest stretch of the digestive tract, several times the large intestine.
Definition of small intestine
The small intestine is the long, coiled tube of the digestive tract running from the stomach to the large intestine. It has three parts: the duodenum, the jejunum and the ileum. Almost all absorption of nutrients happens here, across a lining folded and fringed so heavily that its working surface is vastly larger than the plain tube would suggest.
What do the three sections do?
The sections run in sequence and each has its own job, which is why damage confined to one part produces a specific pattern of deficiency rather than a general one.
| Section | Position | Main role |
|---|---|---|
| Duodenum | First and shortest, curving around the pancreas | Receives stomach contents, bile and pancreatic enzymes; absorbs iron and much calcium |
| Jejunum | Middle section | Bulk of sugar, amino acid, fat and vitamin uptake |
| Ileum | Final section, ending at the valve into the colon | Reclaims bile salts and absorbs vitamin B12 |
How does it absorb so much?
Surface area is multiplied three times over. The lining is thrown into circular folds that run around the inside of the tube. Every fold is carpeted with villi, finger-shaped projections a fraction of a millimeter tall. Every cell covering a villus carries microvilli, a dense brush of even smaller projections holding the enzymes that finish digestion. The combined effect turns a tube of modest inner surface into an absorption surface measured in square meters. Each villus contains a small blood vessel network that carries sugars and amino acids to the liver, and a lymphatic channel that carries absorbed fat into the circulation by a separate route.
What happens when it is damaged?
Because the small intestine does nearly all the absorbing, damage to it shows up as nutrient loss rather than only as pain. Celiac disease flattens the villi in response to gluten, cutting the surface area and causing deficiencies of iron, folate and fat-soluble vitamins. Crohn disease commonly affects the last part of the ileum, which is precisely where vitamin B12 and bile salts are recovered, so deficiency and fatty stools follow. Infections of the digestive tract can strip the lining temporarily. Surgical removal of a long segment leaves the remaining intestine unable to keep up, a state called short bowel syndrome.
Also known as
Used in a sentence
Imaging of the small intestine was performed to evaluate the cause of the patient's abdominal symptoms.
Good to know
The small intestine is named for its narrower diameter rather than a shorter length, since it is considerably longer overall than the large intestine.