The Fick principle states that blood flow to an organ can be calculated using a marker substance if the following information is known: * Amount of marker substance taken up by the organ per unit time * Concentration of marker substance in arterial blood supplying the organ * Concentration of marker substance in venous blood leaving the organ Developed by Adolf Eugen Fick (1829–1901), the Fick principle has been applied to the measurement of cardiac output. Its underlying principles may also be applied in a variety of clinical situations.