The urinary system consists of the paired kidneys and ureters, the bladder, and the urethra. This system helps maintain homeostasis by a complex combination of processes that involves the following:
Filtration of cellular wastes from blood
Selective reabsorption of water and solutes
Excretion of the wastes and excess water as urine.
Urine produced in the kidneys passes through the ureters to the bladder for temporary storage and is then released to the exterior through the urethra. The two kidneys produce about 125 mL of filtrate per minute, of which 124 mL is reabsorbed in these organs and 1 mL is released into the ureters as urine. About 1500 mL of urine is formed every 24 hours. The kidneys also regulate the fluid and electrolyte balance of the body and are the site of production of renin, a protease that participates in the regulation of blood pressure by cleaving circulating angiotensinogen to angiotensin I. Erythropoietin, a glycoprotein that stimulates the production of erythrocytes, is also produced in the kidneys. The steroid prohormone vitamin D, initially produced in skin keratinocytes, is hydroxylated in kidneys to an active form (1,25-dihydroxyvitamin D3 or calcitriol) involved in regulating calcium balance.