Friday, January 3 (2020); Respiratory system
The "oxygen cascade" is the stepwise decrease in oxygen partial pressure between the atmospheric air and the mitochondria. Partial pressure drops from atmospheric (159 mmHg at sea level) to 149 due to humidification in the respiratory tract, and then to 99 due to dilution by CO2. Diffusion into the capillary blood and the effect of venous admixture further drop the PaO2 to about 92 mmHg. From there, because of diffusion distance, the PaO2 drops by a variable amount in the tissues (ranging from 70 mmHg in the kidneys to 8 mmHg in the skin). Ultimately, the partial pressure of oxygen in the mitochondria is usually between 1 and 10 mmHg.