Oxygen Release

When arterial blood arrives at the tissues, it enters an environment where the partial pressure of oxygen is much lower. Consequently, despite the affinity of haemoglobin for oxygen, there is movement of oxygen molecules from haemoglobin through the plasma and into the tissues, down the partial pressure gradient.

Under normal circumstances however, not all of the oxygen is removed from the blood passing through the tissues.

For example, at rest the body’s requirement for oxygen is approximately 250 ml per minute. However as the total volume of oxygen available at any time is 1000 ml, only 25% of available oxygen is removed from the haemoglobin. Therefore venous blood is still 75% saturated, even though the partial pressure of oxygen has fallen to just 5.3 kPa. This represents another reserve which allows the body to adapt to conditions where the oxygen demand is increased e.g. during exercise or illness.

Learning bite

The oxygen content of venous blood can be calculated as follows:
= (Hb x 1.34 x SvO2) + (0.23 x PvO2)
= (150 x 1.34 x 75) + (0.23 x 5.3)
= 150.8 + 1.2
= 153 ml/L