Chris - This is a very good question; healthy testes make sperm at the rate of between 1000 and 5000 sperm cells per second. These collect in a long coil of tubes above and behind each testicle called the epididymis, where they are stored until needed. The testes hang external to the body within the scrotum to achieve an optimal temperature for sperm production. This is because sperm are made most efficiently at a slightly lower temperature than body temperature.

In the epididymis the sperm are nourished and make their way to the vas deferens, which is a muscular tube connecting the testis to the urethra up inside the male body. When sperm are ejaculated they are pushed along the vas deferens by rhythmic contractions of the musculature. Once inside the body, secretions from the prostate gland and seminal vesicles are added, producing the semen that is ejaculated from the penis.

However, although they can survive there for quite a long period within the male anatomy, a fraction of the sperm that are made are never ejaculated. Instead, they will senesce and are broken down. And of course all the things that you take into your body, cigarette smoke, other toxins and things will damage the sperm potentially. So they have a sort of recycle time. And sperm that have reached their sell-by-date get broken down in the same way that, let's say, blood cells get broken down. And, basically, any of the nutrients and goodies in the sperm just get recycled back inside the body, and new sperm are produced to make up for the shortage.

Helen: - So it's a kind of continual, sort of, replacement, really...?

Chris: - Exactly. Those that don't leave the body eventually break down, and their components are recycled.