The prostate is a compound tubuloalveolar exocrine gland of the male reproductive system in most mammals. It differs considerably among species anatomically, chemically, and physiologically. The function of the prostate is to secrete a slightly alkaline fluid, milky or white in appearance, that in humans usually constitutes roughly 30% of the volume of the semen along with spermatozoa and seminal vesicle fluid. Semen is made alkaline overall with the secretions from the other contributing glands, including, at least, the seminal vesicle fluid.