Figure 1. Schematic Representation of Calcium and Skeletal MetabolismAbbreviations: A, absorption; S, secretion; ECF, extracellular fluid; GF, glomerular filtration; TR, tubular reabsorption.The dark vertical line between bone and ECF represents bone surface and bone-lining cells. Shaded area represents labile skeletal calcium. The various calcium compartments are not to scale. See text for discussion. (see Acknowledgements)