Compartment models used for 82Rb and 13N-ammonia. Figure represents kinetic model that was used for analysis of the ammonia data. Ca(t) corresponds to activity concentration in arterial blood, CE(t) activity concentration in interstitial and free cellular space, and CG(t) in metabolic space. K1, k2, and k3 are kinetic parameters of the model in expressed in units of ml/min/g, 1/min, and 1/min, respectively.