The kidneys are vital organs that are part of the urinary system of vertebrates. The kidneys have multiple functions: hormonal, blood pressure regulation and toxin elimination. They ensure, by filtration and excretion of urine, the hydroelectrolytic balance (homeostasis) of the blood and the body in general. Their hormonal functions include the synthesis of erythropoietin, calcitriol (active form of vitamin D) and renin. The kidney has two distinct regions, the cortex, where the glomeruli (key role in waste filtration) and the medulla (essentially tubules and vessels) are located. Each kidney is composed of about a million nephrons (glomeruli + tubules), whose role is to filter the different substances contained in the blood and then reabsorb what is still useful, leaving the waste to be eliminated through urine.

Urine is thus generated by the kidneys, it transits through the ureters to reach the bladder where it is stored and then eliminated during urination via the urethra.