Dynamics of urea transport between blood and cerebrospinal fluid

Abstract

Blood and cisternal cerebrospinal fluid urea levels during and after a two hour infusion of a 30 per cent urea solution were studied in normal dogs. A three compartment mathematical model was used to describe the transport dynamics of urea between the blood, intracellular fluid and cerebrospinal fluid. Blood urea levels were increased by infusion and reduced by normal renal function in the same animal during a single experiment. This yielded measures of the transport rate in both directions (bi-directional). The values for the blood to intracellular fluid ( K 1) and blood to CSF ( K 2) transport coefficients were obtained by finding the solution of the model equations which gave the best fit to the experimental data. The best fit was found using a combination of analog and digital computer methods. The magnitude of K 1 was independent of the magnitude of K 2. The transport coefficients can be interpreted as the product of the inherent membrane permeability for urea and the surface area available for mass transfer. The average values obtained in this study are 17·5 ± 2·8 l./hr for the extracellular to intracellular space urea transport and 8·6 × 10 −3 ± 4·1 × 10 −3 l./hr for extracellular to CSF urea transport. These values and their significance are discussed.