Renal Mechanisms in Maintenance of Fluid and Electrolyte Balance
MAINTENANCE of fluid and electrolyte balance by the kidney implies conservation at normal levels of the volumes, osmotic concentrations, compositions, and reactions of both extracellular and intracellular fluids. To do this the kidney must have means of adjusting appropriately to all possible stresses of diet and disease. The fluid in contact with the kidney and capable of modifying its functions directly is the blood. Consequently, appropriate renal mechanisms are responsive to changes in blood volume, in osmotic or water concentration, to changes in electrolytes, and also to blood pH.
The nephron, the unit of renal structure which subserves these varied functions, is a composite of vascular and epithelial tissues. The vascular element comprises the afferent and efferent arterioles, and the glomerular and peritubular capillaries. This element is concerned with the rate of renal blood flow, the volume of fluid filtered from the blood through glomerular capillaries and with picking up the fluids reabsorbed by the tubules and restoring them to the circulating systemic blood. The epithelial portion acts on the one hand by selective reabsorption of substances from tubular fluid and on the other by the secretion into it of certain specific materials.
The function of renal maintenance has been summarized by Homer Smith in the axiom that “the composition of blood and body fluids is determined less by what the mouth ingests than by what the kidneys keep.” This “keeping” involves both the vascular and the epithelial portions of the nephron. The vascular portion is primarily concerned with adaptation. . .