The components of semen come from two sources: sperm, and seminal plasma. Seminal plasma, in turn, is produced by contributions from the seminal vesicle, prostate, and bulbourethral glands. Seminal plasma of humans contains a complex range of organic and inorganic constituents.

Appearance and consistency

Most semen is white in colour, but grey or even yellowish semen can be normal as well. Blood in the semen can cause a pink or reddish colour, known as hematospermia, and may indicate a medical problem which should be evaluated by a doctor if it does not readily disappear.

After ejaculation, semen first goes through a clotting process and then becomes more liquid. It is postulated that the initial clotting helps keep the semen in the vaginal canal, but liquefaction frees the sperm to make their long journey to the ova. Immediately after ejaculation semen is typically a sticky, jelly-like liquid often forming globules. Within 5 to 40 minutes it will become more watery and liquid before finally drying.

Semen quality

Semen quality is a measure of the ability of semen to accomplish fertilization. Thus, it is a measure of fertility in a man. It is the sperm in the semen that is the fertile component, and therefore semen quality involves both sperm quantity and sperm quality.

The average volume of semen produced at ejaculation is 2 to 5 ml. Systematically lower volumes of 1.5 ml (hipospermia) or over 5.5 ml. (hiperespermia) are probably abnormal. Lower volumes may occur after very frequent ejaculation and higher volumes were observed after a prolonged abstinence.

The World Health Organization (WHO, 1999) provides a definition of sperm "normal", but has been modified over the years to the present:

* The concentration of spermatozoa should be at least 20 million per ml.

* The total volume of semen should be at least 2 ml.

* The total number of sperm in the ejaculate should be at least 40 million.

* At least 75% of the spermatozoa should be alive (it is normal for up to 25% are killed).

* At least 30% of sperm should have a normal shape and contour.

* At least 25% of the sperm must swim with a rapid movement forward.

* At least 50% of sperm should be swimming forward, even if slowly.

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