Figure 50-40 An enlarged view of the ionization window showing the range of intracellular muscle pH values of ectotherms at 25°C, which is close to the neutral pH of water at that temperature. The typical range for the blood value is also shown, as well as a mean value of the dissociation curve for the protein buffer imidazole in histidine. At intracellular pH, its dissociation has a value of about 0.55. (Redrawn from Rahn H, Reeves RB, Howell BJ: Hydrogen ion regulation, temperature, and evolution. Am Rev Respir Dis 112:165–172, 1975.)