Pulmonary Blood Pressure
- Blood pressures within the pulmonary circulation are significantly lower than those of the systemic circulation across all vessels. For example, Pulmonary arterial pressures average 15 mm Hg and fluctuate between 25 mm Hg systolic pressure and 8 mm Hg diastolic pressure. The left atrial pressure is on average 2 mm Hg, meaning the entire blood pressure gradient for the pulmonary circulation is roughly 12 mm Hg. This stands in contrast to the blood pressure gradient for the systemic circulation which may average between 80 - 100 mm Hg.
- Although the pressure gradient for the pulmonary circulation is much lower than that of the systemic circulation, both circulations must carry a blood flow volume equivalent to the cardiac output. The capacity of the pulmonary circulation to carry a large blood flow volume using only a small blood pressure gradient is the result of the pulmonary circulation's much lower vascular resistance to blood flow. The low vascular resistance of the pulmonary circulation in part arises from the absence of muscular arterioles which are most responsible for the high resistance found in the systemic circulation.
- Additionally, at normal pulmonary arterial pressures many pulmonary capillaries are either closed or only semi-patent. However, when arterial pressures rise, these pulmonary capillaries open or further increase in diameter, thus drastically reducing pulmonary vascular resistance and thus preventing increases in the pulmonary blood pressure. Evolutionarily, the low pressures within the pulmonary circulation can be explained by considering that the maximum height which blood must reach in the pulmonary circulation is only a few centimeters, that is, to the lung apex. Because low arterial pressures are sufficient to perfuse these apical regions, the cardiac oxygen demand of the right heart is rendered quite low, thus allowing for significant energy conservation.