The kinetics of sodium, movement into human red blood cells has been studied in vivo with 24Na. When human serum albumin-131I is used to measure the percentage of plasma trapped in the packed red blood cells after centrifugation, approximately 30 % of red blood cell sodium is found to equilibrate immediately with plasma. It is concluded that this immediately exchangeable compartment of red blood cell sodium is an experimental artefact, associated with the use of labeled albumin for measuring plasma trapping. This immediately exchangeable fraction disappears when sucrose-14C is used to measure plasma trapping. The experimental results were examined by compartmental analysis, using an analogue computer. The results obtained, when plasma trapping was measured with sucrose-14C could be simulated by the use of models containing two compartments, arranged in series or in parallel. The errors of the techniques used and the possible physical basis for the results are discussed.

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