Modeling bone mineral metabolism, with special reference to calcium and lead

Realistic modeling of lead kinetics requires both qualitative and quantitative understanding of bone metabolism. In this paper, bone metabolism is discussed based on the behavior of Ca, Sr, and Ra tracers. Apposition, the increase of mineral volume, is the chief feature of bone metabolism in the neonatal and young animal. Resorption, which together with apposition functions to model the growing skeleton, is also important during growth. In adulthood, with stabilization of the total bone mass, resorption balances apposition at a relatively low level. This continuing low-level resorption/apposition remodeling process maintains healthy bone and restructures the bone in response to changing functional demands. In addition to these processes, surface exchange and diffuse exchange occur in bone. Rapid exchange between blood and bone calcium (or tracer) takes place at all bone surfaces in intimate contact with blood. Diffuse exchange is a slow process by which bone-seeking elements can penetrate the entire bone volume. Particularly in longer-lived animals like humans, slow exchange of tracer with bone Ca is the dominant mechanism of tracer uptake during adulthood. These three mechanisms of uptake and loss (apposition/resorption, rapid exchange, and diffuse exchange) appear to be the essential mechanisms determining metabolism of bone-seeking elements. The magnitudes of accretion plus rapid exchange, of resorption, and of diffuse exchange have been estimated with the aid of radiotracers and fluorescent markers.