ABSTRACT
The oxygen cascade describes the sequential reduction in partial pressure of oxygen (PO2) from atmosphere to cellular mitochondria. Oxygen is present in the air at a concentration of 21%. By the time the oxygen has reached the alveoli the PO2 has fallen to about 15 kPa. This is because the PO2 of the gas in the alveoli is a balance between two processes: the removal of oxygen by the pulmonary capillaries and its continual supply by alveolar ventilation – thus hypoventilation will result in a lower alveoli. After oxygenation blood moves into the pulmonary veins, which return to the left side of the heart to be pumped to the systemic tissues. An increase in the size of any of the ‘steps’ in the oxygen cascade may result in hypoxia at the mitochondrial level. Oxygen is carried mainly in combination with haemoglobin and also dissolved in plasma.
