[respiratory Failure].

Though the equation appears complex, it simply means that the oxygen tension in the alveoli is predominantly determined by the concentration of inspired oxygen, the barometric pressure and the carbon dioxide tension in the alveoli. Because carbon dioxide is very freely diffusible across the alveolar membrane, often arterial and alveolar carbon dioxide tension will be equal. That is why PaCO2 which is easier to measure is used in the equation. Oxygen supplementation will increase alveolar oxygen content and hence the partial pressure. The oxygen tension in the alveoli will be low in higher altitudes like mountains and will be high in hyperbaric chambers. Conditions with increased carbon dioxide level in blood and hence in alveoli, as in hypoventilation, chronic obstructive airway diseases (COAD) etc. are associated with decreased oxygen tension in the alveoli.