Oxygen cascade
In respiratory physiology, the oxygen cascade describes the flow of oxygen from air to mitochondria, where it is consumed in aerobic respiration to release energy. Oxygen flows from areas with high partial pressure of oxygen (PO2, also known as oxygen tension) to areas of lower PO2. Air is typically around 21% oxygen, and at sea level, the PO2 of air is typically around 159 mmHg. Humidity dilutes the concentration of oxygen in air. As air is inhaled into the lungs, it mixes with water and exhaust gasses including CO2, further diluting the oxygen concentration and lowering the PO2. As oxygen continues to flow down the concentration gradient from areas of higher concentration to areas of lower concentration, it must pass through barriers such as the alveoli walls, capillary walls, capillary blood plasma, red blood cell membrane, interstitial space, other cell membranes, and cell cytoplasm. The partial pressure of oxygen drops across each barrier. Table 
Table 1 gives the example of a typical oxygen cascade for skeletal muscle of a healthy, adult male at rest who is breathing air at atmospheric pressure at sea level. Actual values in a person may vary widely due to ambient conditions, health status, tissue type, and metabolic demands. See also 

 Alveolar–arterial gradient
 Alveolar gas equation
 Blood gas tension

References 

Respiratory physiology
Cell biology