Figure 3-2 A recirculatory model accounting for cardiac output (C.O.), transit delays and pulmonary uptake (Delay elements V_C ), and nondistributive mixing pathways (V_ND and CI_ND ). All components within the dashed circle are required to accurately model the central volume of distribution. In most situations, this complexity is not required, and the simpler approach of assuming instantaneous mixing within the central volume is an adequate approximation. CI_ND-F and CI_ND-S , fast and slow nondistributive clearances; CI_T-F and CI_T-S , fast and slow tissue clearances; V_ND-F and V_ND-S , fast and slow nondistributive volumes; V_T-F and V_T-S , fast and slow tissue volumes. (From Krejcie TC, Avram MJ, Gentry WB, et al: A recirculatory model of the pulmonary uptake and pharmacokinetics of lidocaine based on analysis of arterial and mixed venous data from dogs. J Pharmacokinet Biopharm 25:169–190, 1997.)