Oxygen Journey

For tissues to survive, oxygen has to be delivered to them from the atmosphere. This is achieved by the respiratory and cardiovascular systems working together so that a specific sequence of events can take place:

  • Oxygen delivery to the lungs
  • Transfer of oxygen into the blood
  • Oxygen carriage by the blood
  • Oxygen delivery to the tissues
  • Release of oxygen to the tissues


Oxygen will only pass from the atmosphere, to blood, to tissues, if it moves from a relatively high concentration to a lower one. Consequently there is a gradual fall in the partial pressure of oxygen in the different phases of the sequence.

This is referred to as the oxygen cascade.

Oxygen cascade

Table 1: The oxygen cascade
Oxygen journey Mean partial (kPa) pressure of oxygen
Inspired air 21.2
Trachea 19.9
Alveolar gas 14.6
Arterial blood 13.3
Peripheral tissues 3.3

Learning bite

Impairment of any of these processes can lead to hypoxaemia (a low level of oxygen in the blood). A break at any point in this sequence will lead to a deficiency in tissue oxygenation and this is called hypoxia.