Calcium absorption--a paradigm for mineral absorption.

Intestinal calcium absorption proceeds by two mechanisms, an active transcellular process that takes place in the duodenum and a passive paracellular process throughout the small intestine. This article characterizes the three steps of transcellular calcium movement-entry, intracellular diffusion and extrusion-and identifies conditions that must be satisfied for other mineral ions to move transcellularly as part of a transepithelial transport process. Passive calcium movement is down a chemical gradient with the amount absorbed by this pathway determined in large measure by the sojourn time, most of which is spent in the ileum. Because transcellular movement of most mineral ions other than calcium, where measured, is either small or negligible, passive transport is likely to be the major route of intestinal absorption, the nature of which, however, has not been well established experimentally.