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After reading this passage from my physiology textbook (Vanders Physiology p. 384), I'm confused about how exactly the right and left ventricle can create markedly different pressures as they pump blood even if they have the same stroke volume, i.e. they push out the same amount of blood.

Typical pulmonary arterial systolic and diastolic pressures are 25 and 10 mmHg, respectively, compared to systemic arterial pressures of 120 and 80 mmHg. Therefore, the pulmonary circulation is a low-pressure system, for reasons to be described later. This difference is clearly reflected in the ventricular anatomy— the right ventricular wall is much thinner than the left. Despite the difference in pressure during contraction, however, the stroke volumes of the two ventricles are the same.

How can two ventricles push the same amount of blood but at different pressures? I always assumed that pressure was created by a change in volume of the ventricles, and so I expected that if both ventricles have the same "change in volume" as they contract, they should also push out blood at the same pressures. Why is this not the case?

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    $\begingroup$ Pressure doesn't just depend on the characteristics of the pump — it also depends on the system that is being pumped into! Think about what pressure you would expect if you took the heart out of an animal and allowed it to pump in a bucket of fluid — now what is the pressure? $\endgroup$
    – tyersome
    Dec 28 '20 at 19:10
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The key is vascular resistance.

Pumps like the heart don't move fluid, they exert force ("push") on it, increasing the pressure. The movement happens subsequently because there is somewhere for the fluid to go, so it moves down the pressure gradient from a place of high pressure to a place of lower pressure. If you clamped their outputs (increased resistance to infinity), neither ventricle would push any blood out.

We can say that the flow rate is proportional to the pressure difference and resistance:

Flow ~ ${\Delta}$P/R

If we assume that the two ventricles push out over the same time frame, you could easily get equal "Flow" with different "${\Delta}$P" if RLungs is different from RBody, and this is exactly the case: pulmonary vascular resistance is much lower than systemic vascular resistance. It's easier to pump the same volume through the lungs than through the rest of the body, so less pressure is needed.

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