The zones of the lung divide the lung into four vertical regions, based upon the relationship between the pressure in the alveoli (PA), in the arteries (Pa), in the veins (Pv) and the pulmonary interstitial pressure (Pi): This concept is generally attributed to an article by West et al. in 1964,[1] but was actually proposed two years earlier by Permutt et al.[2] In this article, Permutt suggests "The pressure in the pulmonary arteries and veins is less at the top than at the bottom of the lung.
Local venous pressure falls to -5 at the apexes and rises to +15 mmHg at the bases, again for the erect lung.
Flow is determined by the Ppa-Ppv difference (Ppa - Ppv), which is constant down this portion of the lung.
However, transmural pressure across the wall of the blood vessels increases down this zone due to gravity.
Flow in zone 4 is governed by the arteriointerstitial pressure difference (Pa − Pi).
This is because as Pi rises, the arterial caliber is reduced, thereby increasing resistance to flow.
However, ventilation and perfusion are highest in base of the lung, resulting in a comparatively lower V/Q ratio.