Atmospheric air is a mixture of gases; typical dry air contains about 79% nitrogen (N2) and 21% O2, with almost negligible percentages of CO2, H2O vapor, other gases, and pollutants. Altogether, these gases exert a total atmospheric pressure of 760 mm Hg at sea level. This total pressure is equal to the sum of the pressures that each gas in the mixture partially contributes. The pressure exerted by a particular gas is directly proportional to the percentage of that gas in the total air mixture.
Because 79% of air consists of N2 molecules, 79% of the 760 mm Hg atmospheric pressure, or 600 mm Hg is exerted by N2 molecules. Similarly, because O2 represents 21% of the atmosphere, 21% of the 760 mm Hg atmospheric pressure, or 160 mm Hg, is exerted by O2.
The individual pressure exerted independently by a particular gas within a mixture of gases is known as its partial pressure, designated by Pgas. Thus, the partial pressure of O2 in atmospheric air, PO2, is normally 160 mm Hg. The atmospheric partial pressure of CO2, PCO2, is negligible at 0.23 mm Hg. For practical purposes, we say that in dry inspired air PCO2 is zero.
For gases, the rate of transfer by diffusion (VX) is directly proportional to the driving force, a diffusion coefficient, and the surface area available for diffusion; it is inversely proportional to the thickness of membrane barrier.
The diffusion coefficient for CO2 is 20 times that of O2 because CO2 is much more soluble in body tissues than O2 is. The rate of CO2 diffusion across the respiratory membranes is therefore 20 times more rapid than that of O2 for a given partial pressure gradient. This difference in diffusion coefficient is normally offset by the difference in partial pressure gradients that exist for O2 and CO2 across the alveolar– capillary membrane. The CO2 partial pressure gradient is 6 mm Hg (PCO2 of 46 mm Hg in the blood; PCO2 of 40 mm Hg in the alveoli), compared to the O2 gradient of 60 mm Hg (PO2 of 100 mm Hg in the alveoli; PO2 of 40 mm Hg in the blood.
PA - Alveolar pressure; Pa - arterial pressure; PV – venous pressure.
The distribution of pulmonary blood flow within the lung is uneven and the distribution can be explained by the effects of gravity. When a person is supine, blood flow is nearly uniform because the entire lung is at the same gravitational level. However, when a person is upright, gravitational effects are not uniform and blood flow is lowest at the apex of the lung (zone 1) and highest at the base of the lung (zone 3). (Gravitational effects increase pulmonary arterial hydrostatic pressure more at the base of the lung than at the apex.)
Zone 1. As a result of the gravitational effect, arterial pressure (Pa) at the apex of the lung may be lower than alveolar pressure (PA), which is approximately equal to atmospheric pressure. If Pa is lower than PA, the pulmonary capillaries will be compressed by the higher alveolar pressure outside of them. This compression will cause the capillaries to close, reducing regional blood flow. Normally, in zone 1, arterial pressure is just high enough to prevent this closure, and zone 1 is perfused, albeit at a low flow rate.
Zone 2. Because of the gravitational effect on hydrostatic pressure, Pa is higher in zone 2 than in zone 1 and higher than PA. Alveolar pressure is still higher than pulmonary venous pressure (PV), however. Although compression of the capillaries does not present a problem in zone 2, blood flow is driven by the difference between arterial and alveolar pressure, not by the difference between arterial and venous pressure (as it is in systemic vascular beds).
Zone 3. In zone 3, the pattern is more familiar. The gravitational effect has increased arterial and venous pressures, and both are now higher than alveolar pressure. Blood flow in zone 3 is driven by the difference between arterial pressure and venous pressure, as it is in other vascular beds. In zone 3, the greatest number of capillaries is open and blood flow is highest.