The Small Intestine

Figure 1 - Multiple Diagrams of the Small Intestine in Different Scales

The small intestine is split into the duodenum, jejunum, and ileum. The small intestine has 2 special features to help with its main function, the absorption of nutrients. The plicae circulares and the villi are features the help increase the surface area of the small intestine. Plicae circulares are large wavy folds on the inner wall and villi are fingerlike projects protruding from the plicae circulares.

Figure 2 - Villi and Microvilli

Within the villi are blood capillaries that can help transport the monomers of carbohydrates and proteins - simple sugars and amino acids. Lipids are converted to chyle through emulsifications and lipases and moves through a lacteal and onto the lymphatic system to the thoracic duct. Muscles then contract to squeeze chyle through the lymphatic system. On each of the villi cells are little hairs that further increase the surface area, known as microvilli.

Between each villi are crypts called Liberkühn glands that are responsible for secreting intestinal juice. Liberkühn glands are primarily composed of 3 main cell types.

Enterocytes - Simple columnar epithelial cells that are focused on absorbing through an electrochemical gradient of sodium across epithelial cells (osmosis). Digestive enzymes are also secreted such as peptidase, sucrase, maltase, lactase, and intestinal lipase.
Goblet cells - Has a cup-like appearance, hence its name. It secretes mucin for a protective mucus layer. Mucus is able to help protect the inner lining of the small intestine from stomach acids and inhibit inflammation from bacteria.
Paneth cells - Secretes antimicrobial peptides and immunomodulating proteins that regulate the composition of intestinal flora.

Duodenum

Figure 3 - Histology of the Duodenum

The duodenum is the first part of the small intestine. It is the shortest segment of the small intestine and is where the pancreatic duct and bile duct connect with to collect bile and pancreatic juice. Plicae circulare are less frequent, villi are flatter, and it consists of lesser amounts of goblet cells. An important feature they have are the Brunner's glands found in the submucosa. They secrete a majority of the mucin that contains bicarbonate, an alkaline substance, through the crypts of Liberkühn to protect the duodenum from chyme.

Jejunum

The jejunum does not contain Brunner's glands or Peyer's patches. It is the longest segment of the small intestine and connects the duodenum to the ileum. Villi are longer, plicae circulares are more common, and it consists of larger amounts of goblet cells. Absorption of carbohydrates, proteins, and lipids primarily occur here.

Ileum

Figure 4 - Histology of the Ileum

The ileum is identified by its numerous Peyer's patches. Peyer's patches are ares that are free of projects and depressions such as villi and crypts. 30-40 patches are usually found in the ileum. Although the function is not completely known, it plays a role in immunologic responses. The ileum's main function is the absorb of B12 vitamins that was mentioned in the Stomach page. It can also continue digesting and absorbing any chyme that wasn't digested or absorbed by the jejunum.

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