Assessment of oxygenation

Measurement of oxygenation is a crucial assessment in medicine. Assessment of arterial oxygenation evaluates the overall integrity of pulmonary gas exchange, rather than looking at the constituent processes such as the muscle pump, airway function, and gas transfer, as are measured by conventional lung function. P_aO₂ is traditionally measured by the analysis of a sample of arterial blood, taken from a superficial site such as the radial artery. The composition of arterial blood is the same throughout the systemic circulation. Sampling of capillary blood provides a less invasive means of oxygenation assessment, which is increasingly employed, particularly in an outpatient or domiciliary setting. Oxygen makes a journey from the atmosphere, through the upper airways, alveoli, arterial blood, and capillaries to the mitochondria. A significant drop in oxygen tension occurs as inspired air enters the alveoli and mixes with Co₂ found in alveolar gas, which further dilutes the P_AO₂ to approximately 14 kPa.