Zones of The Lung

The zones of the lung divide the lung into three vertical regions, based upon the relationship between the pressure in the alveoli (PA), in the arteries (Pa), and the veins (Pv):

• Zone 1: PA > Pa > Pv
• Zone 2: Pa > PA > Pv
• Zone 3: Pa > Pv > PA

This concept is generally attributed to an article by West et al. in 1964, but was actually proposed two years earlier by Permutt et al. 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. 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."

The concept is as follows: Alveolar pressure remains at 0, plus or minus 2 cm H2O throughout the lung. In the highest parts of the lung, the effect of gravity causes a variation in blood pressure between the top and bottom of the lung of 20 mmHg in the erect position. Overall pulmonary venous pressure is 5 mmHg. Venous pressure falls to -5 at the apexes and rises to +15 mmHg at the bases. Overall pulmonary blood pressure is 25/10 mmHg with a mean of 15 mmHg. Mean arterial blood pressure is 25 mmHg at the apexes and 5 mmHg at the bases.

Zone 1 does not normally exist. In normal health pulmonary arterial pressure exceeds alveolar pressure in all parts of the lung. It generally only exists when a person is ventilated with positive pressure. Blood vessels are completely collapsed by alveolar pressure and blood does not flow through these regions. They become alveolar dead space.

Zone 2 is the part of the lungs about 3 cm above the heart. In this region blood flows in pulses. At first there is no flow because of obstruction at the venous end of the capillary bed. Pressure from the arterial side builds up until it exceeds alveolar pressure and flow resumes. This dissipates the capillary pressure and returns to the start of the cycle.

Zone 3 comprises the majority of the lungs in health. There is no external resistance to blood flow and blood flow is continuous throughout the cardiac cycle.

The ventilation/perfusion ratio is higher in zone #1 (the apex of lung when a person is standing) than it is in zone #3 (the base of lung.)