Transmural pressure is the pressure difference between the inside and the outside of a structure. For example, the pressure difference between the inside and the outside of the left ventricle, or the pressure difference between the inside and outside of a blood vessel.
Perfusion pressure on the other hand is the difference in pressure between two different sites in a system of tubes where fluid is flowing or has the potential to flow from one point to another. Perfusion pressure is also called the pressure head or the driving pressure. Perfusion pressure is equal to one transmural pressure minus a second transmural pressure; for example, one located at one point in a hydraulic system minus the transmural pressure at another point in a hydraulic system. Mean arterial (aortic) pressure minus mean venous pressure yields the perfusion pressure. It is largely responsible for the blood flowing through the systemic circulation from the aorta to the vena cavae.
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