= Solution
The cytoplasmic <concentration> $c$ gains a constant influx $r$ from outside the cell and loses <calcium> through a first-order extrusion term $kc$. The pump <flux> $J_+$ moves <calcium> from <cytoplasm> into the <sarcoplasmic reticulum>, <Hill function>[saturating] with cytoplasmic <concentration> through a <Hill function> with scale $K_1$, maximal rate $V_1$ and exponent $n$. The release <flux> $J_-$ returns stored <calcium> to the <cytoplasm>. It requires both <calcium> in the store and calcium-dependent activation of release; its two <Hill function>[saturating] factors have scales $K_2,K_3$ and exponents $m,p$. This represents <calcium-induced calcium release>. The leak $k_sc_s$ is passive return from the store.
Every internal transfer appears with opposite signs in the two balances, so $d(c+c_s)/dt=r-kc$ in the model's compatible <volume> <normalization>. Only influx and extrusion change the total. The nonlinear pump and activated release can redistribute this total rapidly and generate <oscillations> even with constant external input.
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