Urine is formed through a process of filtration, reabsorption, and secretion in the kidneys. The glomerular filtration rate (GFR) is determined by the net filtration pressure across the glomerular capillaries, which is influenced by hydrostatic and oncotic pressures. The GFR is regulated by sympathetic activation as well as hormones and autocoids like norepinephrine, epinephrine, endothelin, angiotensin, nitric oxide, and prostaglandins. Autoregulation of GFR occurs through myogenic and tubuloglomerular feedback mechanisms.
