Why is mean systemic filling pressure used to calculate pressure differential for venous return?

Question

I'm really struggling to understand venous return curves and their relationship to mean systemic filling pressure.

I understand mean systemic pressure is the pressure that would be measured throughout the cardiovascular system if the heart stopped beating and hence pressure would be the same throughout the body and no blood flow because there is no pressure difference.

I'm very confused about the effect of gravity on this, surely if you are standing up the pressure of the blood pooled in your extremities is greater, so is this value just a mean of all the pressures? And I don't understand the usefulness of this parameter, this occurs when the heart has stopped functioning so why do we relate MSP to be a part of the pressure differential driving venous return in the equation:

Vr = (MSFP - P_RA)/venous resistance

Maybe my confusion would be resolved if this graph was explained better:

One might notice that during this entire process, somewhere in the circulatory system there is a points which remains at a stable pressure (which happens to be the MCFP), irrespective of the catastrophic events reverberating through the rest of the organism. Rothe (1993) thought that this probably happens in small venules (less than 1mm diameter), and that it is not constant - present at different points of different organs, and changing constantly depending on prevailing conditions.

I think this means that MSFP might be the high starting pressure in the venules that drives flow towards the right atria and so the MSFP being equilibrated pressure if a heart fails is just that. But I find that arbitrary and entirely confusing.

Answer 1

I think the words tripping you up are "mean" and "systemic". Specifically, it sounds like your intuition is that you should be thinking about an average (mean) across space, across all the systemic circulation, and hydrostatic pressures are dominating your thinking. Probably this is the fault of the person who strung these particular words together, but more generally I'd urge when learning terminology in biology and medicine to not let yourself get fooled by etymology and the meanings of specific words, but rather to treat terminology as terminology.

Instead, I'd think about this as it's usually measured, which is as a pressure at the venous return with the heart stopped. For "mean", I would not think of this as an actual average in the typical sense, but rather just as a steady-state, when there is no pressure differential between veins and arteries, rather than no pressure differential between head and foot.

Physically, MSFP represents the pressure of the compliance of the vasculature. Blood vessels are not hard plastic or metal pipes, they are flexible and muscular, and they push on the blood inside to create a pressure even without any pumping or anything. That's what MSFP is trying to measure. Don't forget that in physiology we are nearly always talking about relative pressures rather than absolute; we don't think of the approximately 760 mmHg of atmospheric pressure when we measure blood pressures in the range of 10 to 200 mmHg, there's always a reference point, usually the immediate vicinity outside the vessels/outside the body. That would also be a good approximate reference to the pressure in an "empty" heart, so if you measure the pressure right outside the heart at a steady state, it gives you an idea of on average how much tendency there will be for blood to flow into the heart if it were empty.

I hope that's helpful, you might try to read more from other sources if your textbook isn't quite clicking:

https://en.wikipedia.org/wiki/Mean_systemic_pressure

https://ccforum.biomedcentral.com/articles/10.1186/s13054-022-04024-x

As far as hydrostatic pressures, I think you're not seeing them discussed in this context because they just aren't that relevant to this particular static model of what is happening to blood at the level of the heart due to passive tension and blood volume in the vasculature. I'd set it aside and learn separately about how blood flow overcomes hydrostatic pressures particularly in the legs through valves and pressure generated by surrounding tissues.