However, transmural pressure across the wall of the blood vessels increases down this zone due to gravity. Flow is determined by the Ppa-Ppv difference (Ppa - Ppv), which is constant down this portion of the lung. There is no external resistance to blood flow and blood flow is continuous throughout the cardiac cycle. Zone 3 comprises the majority of the lungs in health. Flow here is sometimes compared to a starling resistor or waterfall effect. This dissipates the capillary pressure and returns to the start of the cycle. Pressure from the arterial side builds up until it exceeds alveolar pressure and flow resumes. At first there is no flow because of obstruction at the venous end of the capillary bed. Zone 2 is the part of the lungs about 3 cm above the heart. In these circumstances, blood vessels can become completely collapsed by alveolar pressure (PA) and blood does not flow through these regions.

It is generally only observed when a person is ventilated with positive pressure or hemorrhage. In normal health pulmonary arterial (Pa) pressure exceeds alveolar pressure (PA) in all parts of the lung. Zone 1 is not observed in the normal healthy human lung. Regional arterial blood pressure is typically in the range 5 mmHg near the apex of the lung to 25 mmHg at the base. Pulmonary blood pressure is typically in the range 25–10 mmHg with a mean pressure of 15 mmHg. Local venous pressure falls to -5 at the apexes and rises to +15 mmHg at the bases, again for the erect lung.

Overall, mean pulmonary venous pressure is ~5 mmHg. On the other hand, gravity causes a gradient in blood pressure between the top and bottom of the lung of 20 mmHg in the erect position (roughly half of that in the supine position). It is quite likely that there is a portion of the lung toward the top in an upright subject in which the pressure in the pulmonary arteries is less than alveolar pressure.'Īlveolar pressure (PA) at end expiration is equal to atmospheric pressure (0 cm H 2O differential pressure, at zero flow), plus or minus 2 cm H 2O (1.5 mmHg) throughout the lung. 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. in 1964, but was actually proposed two years earlier by Permutt et al.

This concept is generally attributed to an article by West et al. 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):

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