Oxygen cascade

The oxygen cascade depicted in Figure 113 shows the decrease in oxygen tension at each stage of oxygen transport from the ambient air into the tissue.

Notice that alveolar oxygen tension, PAO2 = 13.3 kPa, is substantially lower than oxygen tension in ambient air. This is because alveolar gas has a different composition from ambient air: inhaled air is fully saturated with water vapour on inspiration and it then mixes in the alveoli with gas that already underwent gas exchange.

Alveolar oxygen tension can be calculated from the Nunn equation (see the chapter on Gas R line for more details).

The arterial–alveolar tension gradient will depend on the diffusing capacity of the alveolar membrane, cardiac output, rate of binding of oxygen with haemoglobin (these are the factors affecting gas transfer) and on shunt fraction. In the normal lung, and with the other conditions being normal, the most important factor is the physiological shunt.

Oxygen tension in the capillary is further reduced as a result of the uptake by the tissues. If oxygen extraction is poor (for instance, in low cardiac output states when oxygen delivery does not match oxygen demand, or in mitochondrial poisoning), capillary and venous oxygen tension is relatively high while the tissues suffer hypoxia.

Each step of oxygen transport down the cascade can be altered by abnormal conditions or pathological changes.