[2] The jejunum is specialized for the absorption through its lining by enterocytes: small nutrient particles which have been previously digested by enzymes in the duodenum.

The length of the small intestine can vary greatly, from as short as 3 metres (10 feet) to as long as 10.5 m (34+1⁄2 ft), also depending on the measuring technique used.

[8][10] The surface area of the human small intestinal mucosa, due to enlargement caused by folds, villi and microvilli, averages 30 square metres (320 sq ft).

[18] By the fifth week of embryological life, the ileum begins to grow longer at a very fast rate, forming a U-shaped fold called the primary intestinal loop.

Secretin, another hormone produced in the small intestine, causes additional effects on the pancreas, where it promotes the release of bicarbonate into the duodenum in order to neutralize the potentially harmful acid coming from the stomach.

From the circular folds project microscopic finger-like pieces of tissue called villi (Latin for "shaggy hair").

The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries (amino acids and carbohydrates) and lacteals (lipids).

Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions: The small intestine supports the body's immune system.

Peyer's patches, located within the ileum of the small intestine, are an important part of the digestive tract's local immune system.

It commonly has a number of pyloric caeca, small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food.

There is no ileocaecal valve in teleosts, with the boundary between the small intestine and the rectum being marked only by the end of the digestive epithelium.

The length of the small intestine is typically longer in tetrapods than in teleosts, but is especially so in herbivores, as well as in mammals and birds, which have a higher metabolic rate than amphibians or reptiles.

The lining of the small intestine includes microscopic folds to increase its surface area in all vertebrates, but only in mammals do these develop into true villi.

[22] The boundaries between the duodenum, jejunum, and ileum are somewhat vague even in humans, and such distinctions are either ignored when discussing the anatomy of other animals, or are essentially arbitrary.

In this type of gut, the intestine itself is relatively straight but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns.