The prostate gland, which makes about 10 per cent of the seminal fluid (semen — the ejaculated fluid that contains the sperms), lies at the base of the bladder and surrounds the first part of the urethra. The prostate produces substances which act as a vehicle for the sperms, supply them with nutrients and buffer them against attack from the acid vaginal secretions. It is because it needs to counteract acid vaginal secretions that semen is so alkaline.

Beyond the prostate is a pair of small glands called Cowper’s glands. These produce a small amount of lubricant which is added to the seminal fluid before ejaculation takes place. Rather less than 10 per cent of the volume of any ejaculate is composed of fluid from the testes and epididymes; about 80 per cent comes from the seminal vesicles; and the remaining 10 per cent from the prostate.

The whole process of maturation of a sperm from the day that it starts in the testis to the day it is ejaculated takes about three months, most of which is spent in the epididymis. Once a man becomes sexually excited, his penis enlarges and the vasa deferentia increase their muscular milking action and send more sperms into the ampullae. A combination of muscular contractions of the pelvic structures produces an ejaculation of the semen and when this happens the seminal vesicles and the prostate add their secretions to the sperms which are discharged as semen via the penis. Sperms form only 5 per cent of the volume of semen and a normal man ejaculates between two and five millilitres (or even more) of semen after a day or two of abstinence. Men who have large volumes of semen are not necessarily more fertile than those who do not, but volumes of less than 1 ml are sometimes associated with infertility because there is not enough fluid to maintain contact with the cervix.

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