Fick principle: the amount of a substance taken up (or released) by an organ or the whole body per unit time equals the arteriovenous concentration difference of that substance multiplied by blood flow.
Applied to oxygen, the Fick method calculates cardiac output as O2 consumption divided by the arteriovenous O2 difference; normal cardiac output is about 5 L/min.
Procedure
Step 1 Oxygen consumption of the body is measured by spirometry.
Step 2 Arterial blood is sampled from any peripheral artery, since arterial O2 content is the same throughout the body.
Step 3 Venous (mixed) blood is sampled from the pulmonary artery, by passing a catheter into it through the heart.