Incorporation of venous resistance in Togawa's four quadrant diagram for Guyton's circulatory equilibrium.

Guyton indicates that the contribution of venous resistance to venous return resistance is magnified by the compliance ratio (30) of the venous compartment to the arterial compartment. Therefore, even a slightly increased venous resistance sensitively decreases venous return and hence cardiac output in the circulatory equilibrium. However, Guyton's diagram does not allow an easy and explicit evaluation of the effect of venous resistance on cardiac output. The circulatory equilibrium can be visualized graphically by Togawa's four quadrant diagram. However, Togawa's diagram does not include venous resistance. Therefore, I modified Togawa's diagram to incorporate venous resistance in the first quadrant for the cardiac output curve. The new diagram shows easily and explicitly that venous resistance sensitively tilts down either the venous return curve or the cardiac output curve and markedly decreases venous return and cardiac output.