Zones of the lung
- 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 (PA) at end expiration is equal to atmospheric pressure (0 cm H20 differential pressure, at zero flow), plus or minus 2 cm H2O (1.5 mmHg) throughout the lung. 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). Overall, mean pulmonary venous pressure is ~5 mmHg. Local venous pressure falls to -5 at the apexes and rises to +15 mmHg at the bases, again for the erect lung. Pulmonary blood pressure is typically in the range 25 - 10 mmHg with a mean pressure of 15 mmHg. Regional arterial blood pressure is typically in the range 5 mmHg near the apex of the lung to 25 mmHg at the base.
Zone 1 is not observed in the normal healthy human lung. In normal health pulmonary arterial pressure exceeds alveolar pressure in all parts of the lung. It is generally only observed when a person is ventilated with positive pressure. In these circumstances, blood vessels can become 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 x(the base of lung.) because perfusion is nearly absent. However, ventilation and perfusion are highest in base of the lung, resulting in a comparatively lower V/Q ratio.
- West J, Dollery C, Naimark A (1964). "Distribution of blood flow in isolated lung; relation to vascular and alveolar pressures". J Appl Physiol 19: 713–24. PMID 14195584.
- Permutt S, Bromberger-Barnea B, Bane H.N (1962). "Alveolar Pressure, Pulmonary Venous Pressure, and the Vascular Waterfall". Med. thorac. 19: 239–269. doi:10.1159/000192224.
- Physiology at MCG 4/4ch4/s4ch4_28
- Description at anaesthetist.com
- Animated diagram at sites.fasthosts.com
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