Cardiac/Venous function curves

[Transformers]

Looking at FA 2013, p 255)

For the venous return part of the curve, it appears that FA has change in blood volume AND change in TPR = shifting the curve left and right and without any change to the slope of the curves.

BRS Physio has change of TPR = changing only the SLOPE of the venous return curve without any effect on the mean filling pressure (x-intercept is constant). In contrast, change in blood volume should only affect mean systemic filling pressure and hence cause right and left shift with the SAME slope...sorry im a bit paranoid over this, but should I just listen to FA...which is technically right like in the case of a hemorrhage you do increase TPR and lose blood volume

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[SpecterGT260]

First aid has this worded funny... They have it stated as "before compensation" which... kinda depends on what is being compensated... are we getting reflex tachy? Are we increasing TPVR? There are umpteen reflex loops involved here so they really did a bad job in detailing this one.

So....
The venous part of the graph always throws me off because it is a little counter intuitive. It is displayed as atrial venous return as a function of atrial pressure. So what does that mean? If you have very high right atrial pressure, you get a decrease in venous return (impedance to flow into the right atrium). So we have to consider a balancing act between the right atrial pressure and the systemic venous pressure and how this affects blood entering the right side of the heart.

If we increase blood volume we have increased systemic venous pressure for all conditions (this is also similar to giving someone nitrates, I think). This means you have more power to overcome any atrial pressure which means the entire plot expands up and out to the right. This part you seemed OK with already.

If we increase TPVR we increase the MAP which results in increased afterload which means a % decrease in CO for all sarcomere lengths (EDV/P). This is why the CO function curve "tilts" downward. The other result of increased TPVR is decreased flow from the arterial side to the venous side. The capillaries act like little resistors and what you get is a lower pressure system on the venous side. But here is the thing about competing pressures, they always interact in a *relative* fashion. The right atrial pressure at the onset of filling is not dependent upon venous pressure (theoretically) so it remains constant in this situation. This means that the same total amount of venous flow, or "venous pressure" will be needed to balance atrial pressure in order to result in a net return of 0. So the X intercept on the venous function plot doesn't move and it is a change of slope. Technically.

I think FA left it out because it is lower yield. It may be a hard concept to directly ask on the test so just knowing that the plots drop with increased vascular resistance is sufficient.