Fick's principle

Fick's principle,

a method for making indirect measurements, based on the law of conservation of mass. It is used specifically to determine cardiac output, in which the amount of oxygen uptake of each unit of blood as it passes through the lungs is equal to the oxygen concentration difference between arterial and mixed venous blood. Cardiac output is calculated by measuring the uptake of oxygen for a given period, noted as milliliters per minute, then dividing that ratio by the difference in oxygen saturation of arterial and mixed venous blood samples in milliliters per 100 mL of blood and multiplying the total by 100.