Oxygen cascade refers to the movement of oxygen down the concentration gradient from a region of relatively high level, that is, air to the region of relatively low level, that is, the cell. Mitochondria has the lowest level of partial pressure of oxygen, which is 4 - 20 mm Hg. The major steps involved in oxygen cascade are uptake of oxygen in the lungs, transport of oxygen by the blood on the basis of its carrying capacity, delivery of oxygen to capillaries, interstitium and then to individual cells where it is used for various cellular processes.