Digestion Tract: GIT

Small Intestine is the longest component (6 meters). It is the principal site of digestion & absorption. There are three segments:

The small intestine has finger-like projections called villi, which completely cover it and create a velvety appearance. Core of a villus contains lamina propria, fenestrated capillaries, lymphatic capillaries (lacteal), smooth muscles (associated with lacteal), and myofibroblasts (which bridge the diameter of the villus and are involved in villus contraction).

The intestinal glands (aka crypts of lieberkuhn) are simple tubular in shape and extend from the muscularis mucosae. They are open to the surface at the base of the villi. The glands are composed of simple columnar epithelium continuous with surface cells. The lamina propria surrounds the glands containing cells of the immune system (GALT), including plasma cells, lymphocytes, macrophages, and eosinophils. The lamina propria also contains lymphatic nodules, which are an important part of GALT. The nodules are especially large in the ileum (called Peyer’s patches). The muscularis mucosae has two thin layers of smooth muscle, inner circular and outer longitudinal.

There are five types of cells in the small intestine:

Submucosa consists of dense connective tissue and aggregates of adipose cells. Its conspicuous feature of the duodenum is the submucosal glands (Brunner’s glands) and branched tubulo-alveolar.