Red cell permeability effect on the mean transit of an indicator transported through an organ by red cells and plasma.

When an indicator that permeates red blood cells with an equilibration time of the order of the mean transit time of the indicator through an organ is bolus injected solely into the plasma inflow to that organ, the mean transit time of the indicator is greater than it is when the indicator is injected preequilibrated between the plasma inflow and the red cell inflow. The mean transit time for equilibrated entry is in turn greater than that for indicator injected solely into the red cell inflow. A simplified method of calculation of this effect is given in the present paper; it requires only a model solution for the indicator steady state as distinguished from a more complicated time-dependent solution. The dependence of the effect on red cell membrane permeability allows this permeability to suitable indicators to be deduced. The method was applied to existing data on urea and thiourea passage through the dog kidney.