ABSTRACT
HOMEOSTASIS Homeostasis was originally defined as maintenance of the constancy of the internal environment. The ‘internal environment’ is the fluid that bathes the cells, the extracellular fluid (ECF). Homeostasis can therefore be redefined as the regulation of the volume and composition of the ECF. The kidney is the pre-eminent organ of homeostasis, although the lungs, gut and skin also contribute to it. One component of homeostasis is excretion of products of metabolism: failure of excretion leads to accumulation of these products with progressive pollution of the ECF. A second component is conservation of ECF solutes (mainly nutrients) that are too valuable to be allowed to escape into the urine. The third component is constant adjustment of the urinary excretion rate of water and inorganic solutes to maintain their concentrations in the ECF within the normal range, in the face of unpredictably changing input from the diet and from other body water compartments. This last might be called homeostasis proper. In order to achieve it, the kidney must:
1. detect small changes in ECF volume, or in the concentration of any of its many constituents, before such changes become serious
2. respond by altering the volume or composition of the urine so as to correct the tendency to abnormality.
