Figure 3.

H⁺ and Ca²⁺ sinks can only sustain very small fluxes. H⁺ and Ca²⁺ concentrations in the immediate vicinity of their sinks were calculated at increasing fluxes (q values from 0 to 7 s⁻¹), using Eq. 8, case II, and the following values: a = 0.5 nm, b = 10 μm, D = 37.8 μm²/s (H⁺), and 13 μm²/s (Ca²⁺), u = 40 nM (H⁺, pH 7.4), and 100 nM (Ca²⁺). Note that negative concentrations would be required at the H⁺ and Ca²⁺ sinks to sustain fluxes greater than 6 and 5 s⁻¹.