Two-pool calcium model ($\dot{c}=r-kc-{\dot{c}}_s$)

A two-pool calcium model couples cytosolic calcium and an intracellular store. With compatible reference volumes, uptake $J_{+}$ moves calcium into the store, release $J_{-}$ and leakage $k_s{c}_s$ return it, and ${\dot{c}}_s=J_{+}-J_{-}-k_s{c}_s$. External supply $r$ and export $kc$ give $d(c+c_s)/dt=r-kc$. Thus an equilibrium has $c=r/k$ independently of the internal exchange law. Distinct physical reference volumes require their appropriate volume factors before adding the balances.