If Relim is expressed as amount per unit time and the concentration as amount per unit volume, the clearance has the units of a volume flow, e.g. for rate of elimination in mol min−1 and concentration in mol mL−1 the units of clearance are mL min−1. If instead the rate of elimination is expressed per unit mass of tissue, e.g. mol min−1 g−1, then the units of clearance would be mL min−1 g−1. For many substances provided the concentration isn’t too large, the rate of elimination is proportional to the concentration and the ratio used to define clearance is actually a constant. For higher concentrations, the rate of elimination may approach a limiting value and the clearance then decreases as the concentration increases (see Fig. 22).