Inspired oxygen from the environment moves across the alveolar-capillary membrane into the blood. Most of the oxygen binds to hemoglobin in red blood cells, although a small amount dissolves into the plasma. The oxygen is then transported from the lungs to the peripheral tissues, where it is removed from the blood and used to fuel aerobic cellular metabolism. This process can be conceptualized as three steps: oxygenation, oxygen delivery, and oxygen consumption. In this topic review, oxygen delivery and consumption are reviewed. Oxygenation is discussed separately. (See "Oxygenation and mechanisms of hypoxemia".)
Oxygen content — The arterial oxygen content (CaO2) is the amount of oxygen bound to hemoglobin plus the amount of oxygen dissolved in arterial blood:
CaO2 (mL O2/dL) = (1.34 x hemoglobin concentration x SaO2) + (0.0031 x PaO2)
where SaO2 is the arterial oxyhemoglobin saturation and PaO2 is the arterial oxygen tension. Normal CaO2 is approximately 20 mL O2/dL.
Similarly, the mixed venous blood oxygen content (CvO2) is the amount of oxygen bound to hemoglobin plus the amount of oxygen dissolved in mixed venous blood: