The prostate is an exocrine gland of the male mammalian reproductive system. Its main function is to store and secrete a clear, slightly basic fluid that constitutes up to one-third of the volume of semen. The prostate differs considerably between species anatomically, chemically, and physiologically. A healthy human prostate is slightly larger than a walnut. It surrounds the urethra just below the urinary bladder and can be felt during a rectal exam. Skene's glands found in many females are homologous to the prostate gland in males.