Expansion
Oxygen tension falls stepwise from atmospheric to about 1 kPa in the mitochondrion
Expansion
Approximate oxygen tensions breathing air at sea level:
| Stage | Tension |
|---|---|
| Dry atmospheric air | 21.2 kPa (159 mmHg) |
| Humidified tracheal gas | 19.9 kPa (149 mmHg) |
| Alveolar gas | 13.3 kPa (100 mmHg) |
| Arterial blood | 12.7 kPa (95 mmHg) |
| Capillary and tissue | 5.3 kPa (40 mmHg) |
| Mitochondrion | 0.5 to 3 kPa |
Each step has a cause:
- Humidification adds water vapour at 6.3 kPa, diluting oxygen
- Alveolar gas loses further tension to carbon dioxide, per the alveolar gas equation
- The small alveolar to arterial step reflects venous admixture and V/Q scatter
- The large tissue step is the diffusion gradient driving oxygen into cells
The mitochondrion functions at a remarkably low tension, the Pasteur point of about 0.13 kPa being the critical level below which oxidative phosphorylation fails. The whole cascade exists to maintain that small final gradient.