ABSTRACT
Atmospheric air normally contains some water vapour which dilutes the other gases so that the actual fraction of inspired oxygen is lower than the dry value by a variable amount. The partial pressure of water vapour in a gas mixture is determined by the temperature of the gas and the relative humidity. Diffusion of oxygen across the alveolar-capillary membrane is a passive process, driven by the partial pressure gradient between the alveolar gas and pulmonary capillary blood. In normal adult haemoglobin there are two alpha chains and two beta chains. The haem group contains an iron atom in the ferrous form and this is where oxygen binds. When oxidized to the ferric form, the haem group can no longer bind oxygen. Pulse oximeters measure arterial oxygen saturation indirectly by measuring the pulse-related absorption of different wavelengths of light passed though the finger or earlobe.
