![](https://www.endotext.org/wp-content/uploads/2018/01/etx-bone-ch2-fig1.png)
Figure 1. Schematic Representation of Calcium and Skeletal Metabolism. Abbreviations: 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).