Key to understanding whether the lung is involved in a particular case of<unk>  is the difference between the alveolar and the arterial oxygen levels; this A-a difference is often called the A-a gradient and is normally small. The arterial oxygen partial pressure is obtained directly from an arterial blood gas determination. The oxygen contained in the alveolar air can be calculated because it will be directly proportional to its fractional composition in air. Since the airways humidify (and so dilute) the inhaled air, the barometric pressure of the atmosphere is reduced by the vapor pressure of water.
Hypoxemia
Hypercapnia
Hypocapnia
Hyperaemia
Hypoventilation
Acidosis
Hypothermia
Hyperoxia