INTRODUCTION
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.
