Standard urea kinetic modeling (UKM) equations make the assumption that the blood entering the vascular access has the same composition as the blood leaving the systemic tissue compartment. While this assumption correctly describes the situation for a central venous access (Fig. 1A), such as a right atrial dialysis catheter, it is not adequate for the common peripheral access [1, 2]. The composition of the blood entering the access (c_art) during hemodialysis (HD) is different from the composition of the blood leaving the systemic tissue compartment (c_ven) (Fig. 1B). The manner in which the access and systemic circuits are connected has important, previously unrecognized consequences on the kinetic computation of urea distribution volume for blood-concentration-based UKM, and also affects the computation of access recirculation (AR). During HD, clearance of solute from the access loop dilutes the solute concentration of the blood entering the access. We have defined the ‘long-loop’ dilution effect on blood entering the access as ‘cardiopulmonary recirculation’; (CPR), because it is analogous to the ‘short-loop’; dilution effect which may take place through the vascular access (‘;access recirculation’;) [1]. It is the aim of this work to derive an analytical expression for CPR and to present a correction for the resulting reduction in HD efficiency in UKM equations.