Located behind the bladder, but in close proximity to it, are two smaller chambers called the seminal vesicles. These are the storehouses for the seminal reproductive fluid of the male. This contains sperms that have been manufactured in the testes in the scrotum. Mixed with fluid formed by the tubes through which it passes, as well as fluid produced by the prostate, the seminal fluid gradually builds up in these storage receptacles until required. A small tube leads from the vesicles to join the urethra after it has left the bladder.
Normally, the position of the urethra is such that urine passes directly to the exterior. But during an erection, created through the stimulus of sexual excitability, the erect penis tends to shut off the connection with the bladder, opening the connection with the seminal vesicles.
During intercourse at the moment of orgasm, the vesicles contract, sending a discharge of seminal fluid into the urethra to be discharged (or ejaculated, as it is termed) at the hole (or meatus) at the end of the penis. During sexual intercourse, this ensures a plentiful supply of seminal fluid and sperms being lodged high in the vaginal tract of the female partner. This will enable fertilisation to occur if the time is ripe and a female egg (ovum) happens to be in the Fallopian tube at that instant.
It is a very smoothly operating system, and the mechanics are a wonderful work of art. Seldom (if ever) do things go wrong under normal situations. The seminal fluid and urine never seem to become mixed up. Without anyone telling them where to go or what to do, they automatically do what they are supposed to.