Anatomy and Physiology of the Small Intestine

The human small intestine is approximately 2-6 m and is loosely divided into three sections: duodenum, jejunum and ileum, which comprise 5, 50, and 45% of the length. The biological and physical parameters of human intestinal tract are listed in Table 5.1 (Carr and Toner, 1984; Daugherty and Mrsny, 1999). Approximately 90% of all absorption in the gastrointestinal tract occurs in the small intestinal region. The small intestine has projections that increase the potential surface area for digestion and absorption. Macroscopic valve like folds, called circular folds, encircling the inside of the intestinal lumen is estimated to increase the surface area of the small intestine threefold. Villi increase the area 30-fold and the microvilli increase it by a factor of 600. Thus, such unique structures lead to a tremendous increase in surface area available for absorption in the small intestine.

The major role of the small intestine is the selective absorption of major nutrients and to serve as a barrier to digestive enzymes and ingested foreign substances. The epithelial cells in the intestinal region are a heterogeneous population of cells which include enterocytes or absorptive cells, goblet cells which secrete mucin, endocrine cells, paneth cells, M cells, tuft, and cup cells. The most common epithelial cells are the enterocytes or the absorptive cells. This cell is responsible for the majority of the absorption of both nutrients and drugs in the small intestine. It is polarized with distinct apical and basolateral membrane that are separated by tight junctions. Thus, the bulk of absorption takes place in the small intestine by various mechanisms such as passive diffusion (paracellular and transcellular) and carrier-mediated process (facilitated and active).

Table 5.1. Biological and physical characteristics of human intestinal tract_

Gastrointestinal segment

Surface area (m2)

Segment length (cm)

pH of the segment

Stomach

3.5

0.25

1-2

Duodenum

1.9

-35

4.0-5.5

Jejunum

184

-280

5.5-7.0

Ileum

276

-420

7.0-7.5

Colon and Rectum

1.3

-150

7.0-7.5

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