Renal clearance equations can be used to determine whether a substance undergoes either net reabsorption or secretion by the nephron. The first step in the production of urine is the formation of an ultrafiltrate of plasma by the glomerulus. Starling forces across the glomerular capillaries drive an ultrafiltrate of plasma from the glomerular capillaries into Bowman's space. The ultrafiltrate is devoid of cellular elements, contains very little protein, but otherwise is identical to plasma. Renal blood flow and glomerular filtration rate are maintained constant, despite changes in arterial blood pressure between 90 and 180 mm Hg, by the phenomenon of autoregulation.
Autoregulation is achieved by the myogenic reflex and tubuloglomerular feedback.
The four major segments of the nephron determine the composition and volume of the urine by the processes of selective reabsorption of solutes and water and secretion of solutes. Reabsorption allows the kidneys to retain those substances that are essential and to thereby regulate their levels in the plasma. However in the distal segments the final adjustments in the composition and volume of the urine are made and most of the regulation by hormones and other factors is effected. Many organic compounds are bound to plasma proteins and therfore are unavailable for ultrafiltration. Thus secretion is their major route of excretion in the urine.