The concentration C of a substance in the body depends on the quantity of the substance Q and the volume V through which it is distributed. For a static substance, this is given by
C = Q/V
For a situation like that in the kidneys, where the fluids are moving, the concentration is the ratio of the rate of change of quantity with respect to time and the rate of change of volume with respect to time.
(a) Formulate the equation for concentration of a moving substance.
(b) Show that this is equal to the rate of change of quantity with respect to volume.