Final answer:
Pulmonary vascular resistance decreases as right ventricular output and associated blood flow increase due to blood vessel distension (increased compliance) and a decrease in resistance.
Step-by-step explanation:
How does pulmonary vascular resistance decrease as vessel pressure OR flow rises? As the right ventricular output (also known as cardiac output or CO) increases, several physiological responses occur that contribute to a decrease in pulmonary vascular resistance (PVR). Increased CO leads to an increase in blood flow and vessel pressure. According to Poiseuille's law, resistance is inversely proportional to the fourth power of the vessel radius. Thus, when blood flow is increased, the vessels can distend (increased compliance), which effectively increases the diameter of the blood vessels and decreases the resistance.
Blood viscosity is another factor that can affect resistance; however, in this context, as CO increases, the more relevant factors are vessel diameter and compliance. Another physiological phenomenon is the recruitment and distension of previously under-filled capillaries, which also serves to lower overall PVR. Additionally, elevated flow can lead to a shear stress-induced release of vasodilators (such as nitric oxide) from the endothelium, further reducing vascular resistance. Furthermore, an increased blood volume enhances venous return, leading to a larger preload and thus a stronger heart contraction, contributing to a sustained increase in CO. Feedback mechanisms will adapt in response to changes in afterload - the pressure against which the heart must pump - to maintain flow.