The theoretical maximum oxygen carrying capacity is 1.39 ml O2/g Hb, but direct measurement gives a capacity of 1.34 ml O2/g Hb. 1.34 is also known as Hüfner’s constant.
The oxygen content of blood is the volume of oxygen carried in each 100ml blood.
It is calculated by: (O2 carried by Hb) + (O2 in solution) = (1.34 x Hb x SpO2 x 0.01) + (0.023 x PaO2)
SO2 = percentage saturation of Hb with oxygen
Hb = haemoglobin concentration in grams pre 100 ml blood
PO2 = partial pressure of oxygen (0.0225 = ml of O2 dissolved per 100 ml plasma per kPa, or 0.003 ml per mmHg)
For a normal adult male the oxygen content of arterial blood can be calculated.
Given arterial oxygen saturation (SpO2) = 100%, Hb = 15 g/100 ml and arterial partial pressure of oxygen (PaO2) = 13.3 kPa, then the oxygen content of arterial blood (CaO2) is:
CaO2 = 20.1 +0.3 = 20.4 ml/100 ml
Similarly the oxygen content of mixed venous blood can be calculated. Given normal values of mixed venous oxygen saturation (SvO2) = 75% and venous partial pressure of oxygen (PvO2) = 6 kPa, so:
CvO2 = 15.2 + 0.1 = 15.2 ml/100 ml
Oxygen delivery (DO2) and oxygen uptake (VO2)
Oxygen delivery is the amount of oxygen delivered to the peripheral tissue, and is obtained by multiplying the arterial oxygen content (CaO2) by the cardiac output (Q).
For CaO2 = 20.1 ml/100 ml and Q = 5 l/min:
Oxygen delivery (DO2) = 1005 ml/min
Oxygen uptake is the amount of oxygen taken up by the tissues that can be calculated from the difference between oxygen delivery and the oxygen returned to the lungs in the mixed venous blood. The oxygen returned is given by the product of the mixed venous oxygen content (CvO2) and the cardiac output. For CvO2 = 15.2 ml/100 ml and Q = 5.0 l/min:
Oxygen return = 760 ml/min
Oxygen uptake (VO2) = (oxygen delivery) – (oxygen return) = 1005 – 760 = 245 ml/min
The primary goal of the cardio respiratory system is to deliver adequate oxygen to the tissues to meet their metabolic requirements, a balance between VO2 and DO2.
The balance between oxygen uptake by the body tissues and oxygen delivery to them is assessed by:
The oxygen content of mixed venous blood CvO2, which is normally about 15 ml/100 ml
The extraction ratio, which is the ratio of VO2 to DO2 expressed as a percentage. Normally the extraction ratio is about 25% but can double to 50% if tissue demand increases
Both of the above indices are dependant on mixed venous saturation (SvO2), and cardiac output.
Oxygen delivery and haemoglobin, BJACEACCP; 2004