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 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
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.