ABSTRACT
If the composition of the fluid entering and leaving the kidneys is compared, it is apparent that many of the normal plasma constituents are handled individually, with some undergoing conservation within the tissue and others readily eliminated. For example to illustrate this point, there is a virtual absence of glucose and protein and the appearance of a high concentration of urea in the fluid eliminated from the kidneys. This vital organ not only eliminates urea, the end product of protein catabolism, but also exercises a major influence in regulating the composition of the blood to maintain the internal homeostatic mechanisms. Acid-base, salt and water balances in the body are controlled within the kidneys by the processes of filtration, reabsorption and secretion. Changes in any of these functions will be reflected by a change in the composition of the urine. The kidneys also have an endocrine function; this organ is a major site of synthesis of several hormones including erythropoietin and 1, 25-dihydroxycholecalciferol, which influence systemic metabolic functions of the renin-angiotensin and kallekrein-kinin systems (Scicli et al., 1976). These systems are involved in the regulation of blood pressure and vasopressor activity, and of the prostaglandins E2 and F2α, which exert a regulatory effect on sodium homeostasis (Fulgraff and Brandenbusch, 1974).
