Calcium absorption in the small intestine occurs by both active transport and diffusion. The relative contribution of each process varies with the region and with total calcium intake. Uptake of calcium by active transport predominates in the duodenum and jejunum,- in the ileum, simple diffusion predominates. The relative importance of active transport in the duodenum and jejunum versus passive diffusion in the ileum depends on several factors. At very high levels of calcium intake, active transport processes are saturated and most of the uptake occurs in the ileum, partly because of its greater length, compared with other intestinal segments. With moderate or low calcium intake, however, active transport predominates because the gradient for diffusion is low.
Active transport is the regulated variable in controlling calcium uptake from the small intestine. Metabolites of vitamin D provide a regulatory signal to increase intestinal calcium absorption. Under the influence of 1,25-dihydrox-ycholecalciferol, calcium-binding proteins in intestinal mu-cosal cells increase in number, enhancing the capacity of these cells to transport calcium actively (see Chapter 27).
The small intestine is also a primary site for phosphate absorption. Uptake occurs by active transport and passive diffusion, but active transport is the primary mechanism. As indicated in Figure 36.2, phosphate is efficiently absorbed from the small intestine,- typically, 80% or more of ingested phosphate is absorbed. However, phosphate absorption from the small intestine is regulated very little. To a minor extent, active transport of phosphate is coupled to calcium transport. Therefore, when active transport of calcium is low, as with vitamin D deficiency, phosphate absorption is also low.
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