Absorption refers to the movement of the products of digestion across the intestinal mucosa and into the vascular system for distribution.

To better understand the physiologically eloquent and clinically important processes of intestinal absorption, the reader might need to review the processes of diffusion across membranes, the difference in composition of intracellular and extracellular fluid (Chapter 1), the electrical polarity across cell membranes, the function of the sodium-potassium (Na*,K ) adenosine triphosphatase (ATPase) pump, and the function of selective ion channels (Chapters 1 and 4).

In considering intestinal absorption, keep in mind that molecules move across membrane barriers in response to chemical and electrical gradients. When molecules can freely penetrate a membrane, their movement across it is completely determined by the laws of diffusion and differences in chemical and electrical gradients: molecules flow to areas of lower concentration and charged particles move to areas of opposite charge. However, charged ions and most organic nutrient molecules do not freely penetrate the GI epithelium. Therefore, they do not move in accordance with the laws of diffusion unless there is some mechanism to facilitate their transport across membranes.