Pulmonary Vascular Pressures

Table II shows that the pressures in the pulmonary circulation are generally lower than in the systemic circulation, for at least two reasons. First, the pulmonary circulation supplies only a single organ, so a large pressure head is not necessary to distribute blood flow to multiple organs at different distances from the heart. The right ventricle only needs to generate sufficient pulmonary artery pressure to lift blood to the top of the lung. Low pressures mean that the pulmonary artery and its branches can have relatively thin walls, and they have much less connective tissue and smooth muscle compared with systemic arteries and arterioles. Pulmonary vascular pressures are so low that they are often measured with units of cm H2O, instead of mm Hg (1.3cmH2O = 1 mm Hg). Second, pulmonary capillaries are not supported on the outside by tissue. Pulmonary capillaries are exposed to open gas spaces in the alveoli, so they are more susceptible than systemic capillaries to stress failure, or bursting open if their internal hydrostatic pressure is too high. All capillaries must be extremely thin to allow effective diffusion of gases.

Table II also shows that the pressure drop from artery to vein is more uniform in the pulmonary circulation than in the systemic circulation. Direct and indirect measurements indicate that pulmonary capillary pressure is near the mean of the average pulmonary arterial and venous pressures. Pulmonary capillaries contribute to more of the total pressure drop from artery to vein than do systemic capillaries. This means that capillaries are more important determinants of total

TABLE II Mean (or Systolic/Diastolic) Pressures in the Pulmonary and Systemic Circulations (mm Hg)

Pulmonary

Systemic

Ventricle

Artery

Arteriole

Capillary

Venule

Atrium

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