Fig. 2.
A model of calcium metabolism in humans. Circles represent compartments, numbers in circles represent compartment numbers, arrows represent movement of calcium between compartments, and double-shafted arrows represent entry of calcium by way of the diet (Vi) or bone resorption (Vo−). The asterisks indicate entry of tracer, and triangles indicate sampled compartments. Compartment 1 contains blood, 2 extravascular space, and 3 exchangeable calcium on bone. Compartment 5 represents feces, and 6 urine, and 8 and 10 sites in the intestine. The box labeled 9 represents a delay in movement of calcium in the lower intestine. Fractional absorption was calculated as L(1,8)/(L(1,8)+L(10,8))