The document discusses the Fick Principle, which states that cardiac output or blood flow to an organ can be calculated using the difference in concentration of a marker substance in the blood before and after it passes through the organ. Specifically: 1) Cardiac output is the volume of blood pumped by the heart per minute and can be determined using the Fick Principle by measuring the difference in oxygen concentration of arterial and venous blood from the lungs. 2) The Fick Principle involves determining the volume of the marker substance (e.g. oxygen) consumed per minute, the concentration of the marker in arterial blood, and the concentration in venous blood leaving the organ. 3) Using sample oxygen concentration values