Depiction of the alveolar-capillary unit representative of the alveolar-arterial gradient and easily and quickly
defined by the PaO2 / FiO2 ratio (P/F ratio).
As the distance between the alveolus and the capillary increases, it is referred to as an “widening ” of the A-a gradient.
Many things can occupy this potential space : water, blood, infection (bacteria) as well as thickening membranes.