Therefore, changes in the pulmonary arterial tree and in pressure are considerably larger than in the systemic arterial tree. In addition, in pulmonary hypertension the changes in resistance and compliance are fundamentally and quantitatively different from those in systemic hypertension. In recent years it has become clear that in pulmonary hypertension not only the contribution of resistance is of importance but that the decrease in arterial compliance plays an equally important role. In pulmonary hypertension, the right ventricular load increases due to an increase in pulmonary vascular resistance and decrease in pulmonary vascular compliance, ultimately leading to right ventricular failure. Pulmonary pressure is lower as resistance is lower and the pulmonary vasculature is more compliant.
However, a major difference is that the pulmonary circulation works at much lower pressures than the systemic circulation. The common feature of pulmonary and the systemic circulation is that both transport an equal amount of blood.
The constant RC-time in the pulmonary bed results in proportionality of systolic and diastolic pressure with mean pressure and, in turn, in the constant ratio of oscillatory and mean power. In the systemic arterial tree the compliance is mainly located in the aorta (80% of total compliance in thoracic-abdominal aorta). This distribution depends on the number of peripheral vessels, which is ∼8–10 times more in the pulmonary system than the systemic tree. In the pulmonary arterial system compliance is distributed over the entire arterial system, and stands at the basis of the constancy of the RC-time. The anatomical differences between the pulmonary and systemic arterial system are the main cause of the difference in distribution of compliance.
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