The Work of the Kidney in the Production of Urine

Abstract

The work performed by the kidney in the production of urine appears to be more readily susceptible of analysis by means of the laws of thermodynamics than any other complete function of the animal body. The recognition of this possibility, of course, is not new. It has attracted a number of investigators whose essays have been collected and reviewed by Cushny [1]. The purpose and method were essentially the same in all of these studies: the computation of the theoretical minimum amount of work necessary to elaborate a solution such as urine from another such as blood. None of these computations was complete; and even in the last, made in 1914 [2], a number of processes were not taken into account, viz.: the suppression of ionization of the phosphates, and other weak acids by the greater acidity of the urine, and the production of ammonia from urea by the kidney. In the account presented below of the minimum work necessary for the production of the urine, an attempt has been made to appraise the factors omitted in previous studies; and an alternative method has been employed in the analysis of the work involved in the transport of water. It is felt that a clearer understanding of the energetics of water transport in the body in general is obtained from the alternative treatment. The minimum work has been considered as a quantity equal to the sum of the free energy changes for the transport of each constituent, including water, from blood to urine. It has been assumed, an assumption justifiable on practical grounds, that the substances concerned may be considered as perfect solutes.

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