The amount of breathable liquid eliminated from a mammal through volatilization in the lungs and/or through skin transpiration is detected by measuring the amount of saturation of the expiratory gas by vapors or the breathable liquid. Instantaneous saturation values are employed to gauge the amount of interaction in the lungs between the breathable liquid and respiratory gas flowing therein and to control selected feedback operations to maintain the maximum possible amount of interaction therebetween. The saturation level of expiratory gas is also employed to optimize operating parameters or a system for recovering the breathable liquid from the expiratory gas, directly from the patient, or from a gas or liquid ventilator. The saturation level or expiratory gas is also employed to perform functional residual capacity studies and to correct for errors in conventional functional residual capacity measurements performed while a patient undergoes partial liquid ventilation. When breathable liquid is employed as a blood substitute, the quantification or the loss of the breathable liquid from volatilization and transpiration helps to determine when to replenish the breathable liquid in the bloodstream. Vapors of one form of breathable liquid, perfluorocarbon, are employed to determine the functional residual capacity of a mammal's lung.