Oxygen Delivery
The total volume of oxygen delivered to tissues per minute is the product of the cardiac output and the oxygen content:
Oxygen delivery (DO2) = cardiac output x oxygen content.
In normal circumstances, cardiac output is about 5000 ml/min. Therefore:
DO2 = 5000 ml/min x 197 ml/litre
≈ 1000 ml/min (since circulating volume of blood is around 5000 ml).
This value will clearly be affected by changes in the cardiac output and oxygen content, either separately or in combination. In practice, increases in oxygen delivery are mainly achieved by augmenting the cardiac output (e.g. during exercise) because the potential for further increases in the oxygen content is small. As we have seen in the previous example, this is the case even if the inspired concentration is increased significantly.
Larger effects on the delivery of oxygen can occur if either value falls. For example, there is a reduction in the amount of oxygen which could be carried by each litre of blood in patients with anaemia. Although this would cause the DO2 to fall, the reduction would be limited by the body increasing the cardiac output as a means of compensation.
Scenario
A life-threatening fall in oxygen delivery can occur when both the cardiac output and the oxygen capacity is reduced, as occurs in a trauma patient who has lost 30% of his blood volume. The reduced circulating blood volume both reduces the cardiac output and the oxygen carrying capacity. A state of high oxygen demand usually drives an increased cardiac output to furnish the additional O2, but augmented O2 extraction may supervene if the quantity delivered is insufficient.
Learning bite
1g of Hb carries 1.34 ml of O2 if fully saturated. With a PO2 of 13.3 kPa Hb is normally about 97% saturated and at a Hb concentration of 150 g/L arterial blood will hold ≈ 200 ml/L of O2. Therefore with a cardiac output of 5/Lmin, the O2 in the circulation is 1 L/min.