Video narration
The oxygen cascade describes the progressive fall in oxygen partial pressure from the atmosphere to the mitochondria. Let's follow oxygen through the body. First, atmospheric air. Oxygen makes up approximately 21% of air. At sea level, atmospheric pressure is 101.3 kilopascals. Therefore, the partial pressure of oxygen in dry air is approximately 21.2 kilopascals. Second, humidification. As inspired air passes through the respiratory tract, it becomes saturated with water vapour at body temperature. Water vapour exerts a pressure of 6.3 kilopascals. The inspired oxygen partial pressure is calculated using this equation: P I O two equals inspired oxygen fraction multiplied by atmospheric pressure minus water vapour pressure. At sea level, this becomes 0.21 multiplied by 101.3 minus 6.3, giving approximately 20 kilopascals. Third, the alveoli. Oxygen mixes with alveolar gas while carbon dioxide enters the alveoli from the blood. Now, we use the alveolar gas equation. P A O two equals P I O two minus P a C O two divided by the respiratory quotient, R. Using an inspired oxygen partial pressure of 20 kilopascals, an arterial carbon dioxide partial pressure of 5.3 kilopascals, and a respiratory quotient of 0.8. This gives an alveolar oxygen partial pressure of approximately 13.4 kilopascals. Fourth, arterial blood. Oxygen diffuses from the alveoli into pulmonary capillary blood. Arterial oxygen partial pressure is usually slightly lower than alveolar oxygen partial pressure. The difference is called the alveolar-arterial oxygen gradient. It is calculated by subtracting arterial oxygen partial pressure from alveolar oxygen partial pressure. Fifth, the tissues. Oxygen-rich arterial blood reaches tissue capillaries, where oxygen diffuses down its partial-pressure gradient into cells. Tissue capillary oxygen partial pressure may be around 6 to 7 kilopascals. Finally, the mitochondria. Oxygen partial pressure falls further, commonly to around 1 to 5 kilopascals, supporting oxidative phosphorylation and ATP production.