Absorption Anatomy and Histology

The small intestine can be divided into three sections: the duodenum, which has important regulatory functions and receptors, with bile and pancreatic ducts leading into it; the jejunum (1.52.5 m in length); and the ileum (2-3 m in length). The intestinal tube is 4 cm in diameter and ~5 m long, which would normally give it a surface area of 0.5 m2. Through the addition of folds, villi, and microvilli the absorptive surface increases to 200 m2.

Each villus makes up a functional unit, together with its neighboring crypt (A). Initially 16 stem cells per crypt divide into about 150 proliferating cells, which run through a few additional cell cycles. During their migration to the villus tip, they differentiate into various cell types, with secretory or absorptive functions. The tip cells are sloughed off and replaced every 3-6 days. Hence, at any given time, some of the "nutrients" and enzymes found in the intestinal lumen are actually sloughed-off cells of the intestinal lining. Each villus contains one central blood vessel with subepithelial branching. About 10-15% of the cardiac output is allocated to the small intestine. This blood flow can be increased postprandi-ally to more than twice its original volume, surrounding the chyme after meals.

Like the stomach's, the small intestine's motility is determined by slow waves. The frequency of the associated changes in potential decreases from the oral to the anal area, given the orally located sections a trigger function. Also, peristaltic waves, and with them the movement of intestinal contents, can only travel in one direction, i. e., from oral to anal. The intestinal tube is also able to actively mix its contents by separating individual segments.

Analysis of the histology of enterocytes (B) reveals a multitude of transport pathways within and between the cells, as well as to the outside. Consequently, absorption is not a unified process, but an overarching term encompassing many interlacing processes.

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