What is arterial oxygen content?

CaO2 is the total amount of oxygen carried in 100 mL of arterial blood, measured in millilitres of oxygen per decilitre. It combines the large amount bound to haemoglobin with the small amount dissolved in plasma, and it is the key input to oxygen delivery.

CaO2 equals (haemoglobin times 1.34 times saturation) plus (0.0031 times PaO2). The 1.34 is Hufner's constant, the millilitres of oxygen one gram of haemoglobin can bind, and 0.0031 is the solubility coefficient for dissolved oxygen per mmHg of PaO2.

Why is the dissolved fraction so small?

Oxygen is poorly soluble in plasma, so at a normal PaO2 of 100 mmHg only about 0.31 mL per decilitre is dissolved, against roughly 20 mL bound to haemoglobin. This is why anaemia reduces oxygen content far more than a modest drop in PaO2 does.

How does this relate to oxygen delivery?

Oxygen delivery, DO2, equals cardiac output times CaO2 times 10. Content tells you how much oxygen each unit of blood carries; multiplying by flow gives the rate at which oxygen reaches the tissues. A low content cannot be fully compensated without raising flow.

Which constant should I use for haemoglobin binding?

Common values are 1.34, 1.36, or 1.39 mL of oxygen per gram. This tool uses 1.34, the most frequently taught figure. The differences arise from in-vivo versus theoretical measurements and change the result only slightly.

Arterial Oxygen Content (CaO2) Calculator

Arterial oxygen content tells you how much oxygen is actually packed into each decilitre of blood. Saturation and PaO2 describe oxygen’s pressure and binding, but content is what determines, together with cardiac output, how much oxygen reaches the tissues. It is a cornerstone calculation in critical care and anaesthesia.

How it works

Oxygen travels two ways in blood: bound to haemoglobin and dissolved in plasma. The content formula adds both:

CaO2 = (Hb x 1.34 x SaO2) + (0.0031 x PaO2)

Haemoglobin is in g/dL, saturation is a fraction, and PaO2 is in mmHg. The constant 1.34 is the oxygen-carrying capacity of one gram of haemoglobin in mL, and 0.0031 is the plasma solubility of oxygen per mmHg.

Example and notes

For a patient with Hb 15 g/dL, SaO2 98 percent, and PaO2 100 mmHg, the bound term is 15 x 1.34 x 0.98 ≈ 19.7 mL/dL and the dissolved term is 0.0031 x 100 ≈ 0.31 mL/dL, for a total CaO2 of about 20 mL/dL. The dissolved fraction is tiny, which is why anaemia, a low Hb, drops content sharply while a small fall in PaO2 barely moves it. Multiply CaO2 by cardiac output and 10 to obtain oxygen delivery, DO2. This is an educational tool and not a substitute for clinical assessment.