Semen is a white or grey liquid, emitted from the urethra (tube in the penis) on ejaculation. Usually, each ml of semen contains millions of spermatozoa (sperm), but the majority of the volume consists of secretions of the glands in the male reproductive organs.

The purpose of semen is purely for reproduction, as a vehicle to carry the spermatozoa into the female reproductive tract. Although ejaculation of semen accompanies orgasm and sexual pleasure, erection and orgasm are controlled by separate mechanisms and semen emission is not essential for enjoyable sex in most people.

The spermatozoa take over 70 days to develop and are produced solely in the testicles. Individual sperm develop within the testicles from a cell called a spermatogonium. The spermatogonium divides to produce spermatocytes, which then develop into spermatids. The spermatid develops its familiar tail and the cell gradually acquires the ability to move by beating its tail. The spermatid eventually develops into a mature spermatozoan. This process takes about 60 days and the sperm then takes a further 10 to 14 days to pass through the ducts of each testicle and its sperm-maturing tube, the epididymis, before it can leave the body in the semen, during ejaculation.

The fluid produced by the testicles contains several chemicals, but is particularly rich in testosterone.

The secretions of the prostate gland contain several chemicals, of which prostate specific antigen (PSA) is probably the most interesting. PSA is an enzyme, a catalyst that causes biochemical reactions, in this case a type of enzyme called a coagulase. The prime function of PSA is to liquefy the clotted semen, so that the spermatozoa can escape from it and swim off to fertilise the ovum (egg) in the woman's reproductive tract.

Prostate gland cells produce PSA and some of it leaks into the blood stream, as well as appearing in semen. The bigger and more active the prostate gland is, the more PSA appears in the blood. Men with big, benign (non-cancerous) prostate glands often have higher blood levels of PSA than average. Men with prostate cancer may have very high levels. However, PSA levels can also be normal in both conditions. It is a useful marker of prostate gland size and activity, but is not a specific test for prostate cancer.

Semen is usually white or grey, but can occasionally appear yellowish. Pink or red semen suggests that blood is present. Although this is only rarely due to a serious health problem, men with semen that seems bloodstained should seek advice from their family doctor.

Semen clots almost immediately after ejaculation, forming a sticky, jelly-like liquid. It will liquefy again in 5-40 minutes. It is quite normal for semen to form jelly-like globules and this does not indicate any health or fertility problem. Failure of clotting and subsequent liquefaction can cause fertility problems.

The average volume of semen produced at ejaculation is 2-5ml. Volumes consistently less than 1.5ml (hypospermia) or more than 5.5ml (hyperspermia) are probably abnormal. Lower volumes may occur after very frequent ejaculation and higher volumes are seen after prolonged abstinence.

The definition of a "normal" sperm count according to the World Health Organisation is:

1. The concentration of spermatozoa should be at least 20 million per ml.
2. The total volume of semen should be at least 2ml.
3. The total number of spermatozoa in the ejaculate should be at least 40 million.
4. At least 75% of the spermatozoa should be alive (it is normal for up to 25% to be dead).
5. At least 30% of the spermatozoa should be of normal shape and form. At least 25% of the spermatozoa should be swimming with rapid forward movement.
6. At least 50% of the spermatozoa should be swimming forward, even if only sluggishly. It is quite surprising how many dead and abnormal sperm can be present in a "normal" sample.

Results of a sperm count can be affected by many factors, including the length of time between ejaculation and semen sample analysis, and how the sample is stored.

There can be enormous variation in sperm count in an individual, even over a few days. It is important that at least two and preferably three or more samples are analysed, each at least two to three weeks apart. A single sample is inadequate to assess semen quality.

Sperm count is only an indication of fertilising capacity and a normal count does not guarantee success. In addition, more is not necessarily better, as too high a sperm count can also result in fertility problems.

• Endocrine disrupters.
• Toxic pollutants.
• Sexually transmitted infections.
• Zinc deficiency.
• Alcoholism.
• Smoking.
• Anabolic steroid use.
• Ionising radiation.
• Endocrine disrupters are chemicals present in the environment that, by virtue of their ability to adversely affect the endocrine (hormonal) system, cause health consequences. Several industrial pollutants can affect fertility, as can smoking and alcohol use. Some of these toxins can cause reproductive disorders, neurological disease, immune system disorders and cancer.

• Phthalates: Plastics industry
• Alkylphenols: Industrial and domestic detergents
• Bisphenol: A Lacquers to coat foods Dental treatments
• Organochlorine pesticides (Lindane, DDT, etc.):Lindane used on cereals, soft fruits, cabbage
• Dioxins: Paper production Transformer disposal
• Vinclozolin Fungicide used on foods
• Phyto-oestrogens
• Soya products

Here are some tips for a man to preserve his fertility:

• Adopting a healthy lifestyle, with a balanced diet and regular exercise.
• Not smoking or use recreational drugs.
• Not drinking alcohol, or keeping alcohol consumption within recommended limits.
• Trying to avoid exposure to industrial and occupational hazards by following safety procedures and wearing protective clothing when provided. Every employer has a legal responsibility to provide information on occupational health risks, and to ensure that employees have a safe and healthy working environment.
• Avoiding exposure to sexually transmitted diseases by adopting safer sexual practices, such as using condoms and being faithful to one partner.
• Existing evidence does not suggest that wearing of tight underclothes or immersing the testicles in cold water will improve semen quality.
• Some evidence suggests that stress reduces semen quality, probably due to hormonal changes in the body that result from stress.