The biochemical consequences of chronic renal failure involve a disturbance in one of the fundamental mechanisms of the body's self-regulatory control systems. The concept of self-regulation in biological systems is both old and well established. The idea held by Hippocrates that disease was cured by natural powers, by a 'vis medicatrix naturae', implied the existence of systems ready to cooperate in a corrective manner when the natural state of the body was disturbed. In 1878 Claude Bernard made the distinction between the 'milieu ext6rieur', in which the whole organism exists, and the 'milieu interieur', in which the cells live. After considering some of the mechanisms involved he wrote in his now classical sentence that: ' . . . tous les mecanismes vitaux, quelque varies qu'ils soient, n'ont toujours qu'un but, celui de maintenir l'unite des conditions de la vie dans le milieu interieur.' The concept of an internal environment that was maintained in a steady state was further developed during the next 50 years and the term homeostasis was introduced by Walter B. Cannon to describe the physiologically constant state of the body. In 1929 he wrote: 'The coordinated physiological reactions which maintain most of the steady states in the body are so complex, and so peculiar to the living organism, that it has been suggested (Cannon, 1925) that a specific designation for these states be employed-homeostasis'. The role of the kidney in homeostasis is perhaps best expressed in the sentence of J. P. Peters (1935) who wrote: 'The kidneys appear to serve as the ultimate guardians of the constitution of the internal environment.' The kidney carries out its homeostatic functions by the processes of glomerular filtration and of tubular reabsorption and secretion, and thus regulates the concentration of metabolic end products, the osmotic pressure, the volume and the ionic composition of the internal environment. In chronic renal failure, the end results of the disturbances of renal function are alterations in the constitution of the internal environment which are of fundamental biochemical significance. UREA AND OTHER ORGANIC METABOLIC PRODUCTS
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