I finally get it, so perhaps I can work better with Jack here.
The red dotted lines in FA on p. 282 are misrepresenting. The higher and lower red dotted lines should have increased and decreased magnitude of negative slope, respectively. They should not demonstrate a parallel relationship. The only reason they do is because it's apparent that FA was trying to depict multiple relationships all independent of one another on one graph, when in actuality, there should have been two graphs: one showing the interdependence of the variables and the other the independent value-changes.
To answer your question:
Since moving along a venous return curve to the right and down means increased venous compliance and moving to the left and up means decreased compliance, if the TPR is greater and the equilibrium point (point of intersection of the CO-VR curves) is of lower magnitude, all at constant volume/inotropic state, there must be arterial/arteriolar vasoconstriction such that a lesser proportion of blood is retained on the venous side of the circulation. This means that subsequent modulation of venous compliance would induce a lesser change in RA filling. This is why a decreased equilibrium point should result in a reduced slope of the venous return curve. Similarly, increased blood volume at this point would carry a lesser impact on CO because the veins are less distended (i.e. a greater available venous reservoir is available) and greater volume would need to be added in order to restore comparable RAEDV.
Theoretically, if increased TPR occurred, inotropic state would increase to compensate. The net result would be a maintenance of the equilibrium point, not a reduction of it.
Bottom line: increased TPR at constant volume/inotropic state results in an attenuated impact of added blood volume on CO or altered venous compliance on venous return; the converse is true for decreased TPR.
