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During ejection the mitral valve is closed, so $Q_v=0$. Adding the arterial and ventricular balances cancels aortic flow:where the last approximation assumes $p_v\ll p_a$.

Model the rapid contraction as a switch to approximately constant systolic elastance $E_S$, with $C_s=1/E_S$ the systolic ventricular compliance. A sufficiently small aortic-valve resistance makes $p_{LV}\simeq p_a$ after a short pressure-equalization transient. The systolic pressure must be high enough to open the valve, $E_S(V^{+}-V_0)>p_a^{+}$. During that transient neglect systemic runoff. Conservation of combined arterial and stressed ventricular volume givessoThis is an approximation across a rapid transient, not a discontinuity at finite valve resistance. The subsequent common pressure obeys $R_c(C_a+C_s){\dot{p}}_a\simeq -p_a$, givingThe source's $R_C$ in this expression denotes the same systemic resistance $R_c$.

At the end of firing, the remaining ventricular volume is $V_{\mathrm{end}}\simeq V_0+C_s{p}_a(\Delta t_F)$. The stroke volume is $V^{+}-V_{\mathrm{end}}$, henceThe assumptions are a closed mitral valve throughout ejection, negligible venous pressure, rapid valve equalization relative to systemic runoff, and nearly constant systolic elastance during the interval measured by $\Delta t_F$. These give the ventricular ejection pressure and stroke-volume approximation.

Stroke volume is the volume expelled by a ventricle in one contraction. In a time-dependent ventricular elastance model, if end-ejection pressure is $p$ and systolic compliance is $C_s$, the remaining ventricular volume is $V_0+C_{s}p$. Subtracting that from the end-filling volume gives the ejected volume.

After a rapid switch to systolic compliance $C_s$, negligible valve resistance equalizes arterial and ventricular pressures. Neglecting runoff during this short stage conserves combined stored volume and gives the displayed pressure. Thereafter, with closed mitral valve, negligible venous pressure and constant $C_s$, the pressure decays as ${\hat{p}}_a{e}^{-t/[R_c(C_a+C_s)]}$. At firing duration $\Delta t_F$, the stroke volume is $V^{+}-V_0-C_s{\hat{p}}_a{e}^{-\Delta t_F/[R_c(C_a+C_s)]}$. Each approximation follows from an explicit valve or time-scale separation condition.