The obstacle course: A sperm's journey

The best way of maximizing the chances of your sperm fertilizing an egg is to get them off to a flying start, and this means getting a good hard erection.

Penile erection is the result of a complicated process of raised blood pressure and altered blood flow, and it can be influenced by a whole range of physical and emotional factors. Lifestyle is also important.

If you're very tired or depressed, or have simply drunk too much alcohol, you may have trouble getting stiff. Embarrassment is a real turn-off too, as Zeron found while being filmed! Most men suffer at least a few episodes of erection failure during their lives. Usually it's nothing to worry about.

Once ejaculation has happened, how sperm find their way to the egg is not accurately known. The first thing to bear in mind is that the woman's vagina is not a dead end. There is an exit into the womb, called the cervix, that provides the way out.

The channel that runs through the cervix is not empty; it is filled with mucus. Surprisingly, unlike the snot that comes from your nose when you have a cold, the mucus within the cervix is very complex.

Because it is the last line of defence between the womb and the outside world, the mucus must provide protection against bacteria, but at the same time allow sperm to enter the womb and menstrual flow to find its way out.

To accomplish this, the mucus contains thousands of fibres that help to make an intricate structure permeated by countless narrow channels.

Sperm cells can make their way through the channels, but bacteria (which are far less motile) and viruses (which can't move at all) find this impossible.

The chemical composition of the cervical mucus varies with the woman's cycle, affecting the ease with which sperm can penetrate it.

'The Race Is On' - Visual metaphor by Blyzz under CC-BY-ND licence

Spermatozoa which are healthy and take the right route are rare. Many take the wrong channel and never get near to their goal.

To achieve fertilisation, a sperm must travel up the oviducts and then do far more than just run into an egg. This is because the egg arrives in the oviducts surrounded by a triplicate coat of armour.

The sperm cells literally have to fight their way through the three layers, first using chemicals contained in their acrosome, and then using a spike on their head to puncture a hole as the sperm forces its way forward by thrashing its tail.

Finally, should one sperm cell get this far and make contact with the egg membrane, it is engulfed and can then go about fertilisation: all other sperm cells reaching the egg are then repelled.

This is because once a single sperm has penetrated the three lines of defence, the egg passes a chemical message across its surface making it impenetrable. Of the 40 to 300 hundred million sperm produced by the male, only one gets to do its job.

Where do the other sperm go?

Once sperm cells reach the end of the oviducts they are free to swim out of the end of the tube and into the body cavity, where they are eventually destroyed. So many women walking around today will have sperm cells swimming around the interstitial fluid that surrounds their body organs. The female reproductive tract does not finish in a dead end.

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Content last updated: 22/06/2005

About our expert

Mike Leahy left school by 'mutual agreement' whilst studying for his A levels and began an apprenticeship as a motor mechanic. At the age of 26, armed with a single A level, he left mechanics to study for a degree in Environmental Biology at Oxford Brookes University and gained a First Class Honours degree within two years. Mike moved on to Oxford University to study for a DPhil in Virology. As well as being considered a leader in the field of influenza virus replication, he's also appeared regularly on TV and Radio, including Rough Science, Horizon and Bodysnatchers.

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