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During isovolumetric contraction, ventricular pressure lies between the venous and arterial pressures, $p_v<p_{LV}<p_a$. Both valves are closed, so $Q_v=Q_a=0$ and ventricular volume remains at its end-filling value $V^{+}$. ConsequentlyHere $V^{+}$ is end-diastolic ventricular volume and $V_0$ is the zero-pressure/unstressed volume.

The arterial and venous reservoirs still exchange systemic blood:If contraction is short compared with both $R_c{C}_a$ and $R_c{C}_v$, the fractional changes are small andwhere the superscript values refer to the beginning of contraction. Thus pressure can rise rapidly in the left ventricle through increasing elastance while the other pressures are approximately fixed. This stage ends when $p_{LV}$ reaches $p_a$ and the aortic valve opens.

Time-dependent elastance models the pressure generated by a cardiac ventricle at its current stressed volume. The reference $V_0$ is its zero-pressure volume. The inverse of elastance is compliance at fixed time. Rising elastance increases pressure without volume change during isovolumetric contraction; an approximately constant systolic value $E_S$ gives compliance $C_s=1/E_S$ during a simplified ejection stage.