The primary function of the kidneys is the regulation of fluid and electrolyte composition of the body. The kidneys have a high blood flow, and from this a very large volume (180 L/day) of ultrafiltrate of plasma is produced in the renal corpuscles (glomerular capillaries and
Bowman’s capsule). This large glomerular filtrate is necessary for the excretion of waste products of metabolism in the urine. The filtrate passes along the nephron, where the specific processes of tubular reabsorption and secretion occur. Most of the filtered fluid is reabsorbed. The proximal tubule alone reabsorbs 60% of the water and sodium filtered into Bowman’s capsule, and the normal urine volume is only 1.5 L/day. Substances can also be removed from
the peritubular capillary blood into the nephron lumen by specific tubular secretory mechanisms; many drugs are handled in this way. The final volume and composition of urine are modulated to maintain normal body fluid and electrolyte balance by factors governing the processes of glomerular filtration, tubular reabsorption and tubular secretion. The kidneys have an integral role in the long-term regulation of body water and electrolyte composition, and therefore renal function is an important determinant of the long-term regulation of blood volume and arterial blood pressure.