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Cum. You've heard the word a million times. You know it's slang for semen, or the fluid ejaculated from the tip of your penis (urethra) during orgasm. But what exactly is semen?

If you said sperm, you're partly correct. But only around one percent of the average ejaculate (or "load") consists of sperm. The rest is made up of secretions from parts of your genital-urinary tract.

What's sperm got to do with it?
Biologically speaking, the main purpose of semen is to help sperm survive their journey to a female's egg, where fertilization occurs. In this respect, semen acts like a team of bodyguards; it carries sperm along, nourishes them and helps deliver them safely to their destination.

Sperm are produced in the testicles. They then spend 60 to 90 days in the epididymis (the coiled tubes draped above the testes), where they grow to maturity. Amazingly, the average ejaculate contains between 40 million to 600 million sperm, depending on the volume of cum.

What else is semen made of?
Besides sperm, semen is made of secretions from the seven lower urinary tract structures. These strangely named structures include: epididymis, vas deferens, ampullae of the vas, seminal vesicle, prostate, Cowper's glands and glands of Littre. Semen itself has high concentrations of potassium, zinc, citric acid, fructose, phosphorylcholine, spermine, free amino acids, prostaglandins and enzymes, which nourish and protect the sperm.

How much semen is produced?
The volume of your ejaculate can vary, depending on how well hydrated you are and how many times you've ejaculated within a short period of time. The average ejaculate measures around a half-teaspoon, though it may reach a little over a teaspoon. During a single sexual experience, the volume of ejaculate is greatest the first time you cum, and less for each subsequent load. "Remember the law of diminishing returns," says Franklin Lowe, M.D., M.P.H., F.A.C.S., associate director of Urology at St. Luke's/Roosevelt Hospital Center in New York, NY.

In general, volume decreases with age as the prostate enlarges. This does not mean potency necessarily decreases, however. "The greatest misconception about semen is that the amount of ejaculate fluid is correlated with virility and potency," says Dr. Lowe. "In fact, sperm count can be normal even in smaller volumes."

Despite anecdotal reports of herbs that can help increase the volume of ejaculate, there is no medically proven way to improve your volume, notes GayHealth.com's medical director, Dr. Stephen Goldstone.

Does semen have a typical look, smell and taste?
Semen usually appears as an opalescent white fluid that gets clear and runny within minutes after ejaculation. Often, semen has an odor that resembles chlorine. And the different parts of semen have different tastes. "For example, the component from the seminal vesicle, which is high in fructose [sugar], will be slightly sweeter than the rest of the fluid," Dr. Lowe explains. That said, semen characteristics do vary from person to person, month to month. Small changes in color, smell or taste are not necessarily cause for alarm, adds Dr. Goldstone.

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Blood in your semen can signal a broken blood vessel, which may result from the force of your ejaculation. If this is the case, the blood will usually subside after a couple of days. You should contact your doctor if your semen continues to look brown or red, because this may signal an infection of the prostate or seminal vesicle, says Dr. Lowe. A yellowish or greenish discharge from the urethra is frequently a sign of gonorrhea, a sexually transmitted disease (STD) that can be cured with antibiotics.

Also consult your doctor if your ejaculate has a strong, foul smell. This, too, could be a sign of a prostate infection.

What is 'pre-cum'?
Pre-cum is a normal part of the male sexual experience. It is a clear secretion from the glands of Littre that seeps from the urethra when you're in a sexually excited state. As the name suggests, pre-cum precedes ejaculation.

Like semen, the biological function of pre-cum is to protect the sperm. Pre-cum is an alkaline, which neutralizes the acid levels in urine. By lowering the acid levels, sperm have a better chance of surviving their trip through your urethra.

The quantity of pre-cum (if any) can widely vary, depending on the guy and his level of sexual stimulation.

Can I get HIV or STDs from pre-cum or semen?
Yes, both pre-cum and semen can transmit HIV and STDs like gonorrhea, nongonococcal urethritis (NGU) and cytomegalovirus (CMV). "Safer sex is imperative to prevent the spread of these diseases," says Dr. Lowe.

Unprotected oral, anal and vaginal sex all carry risks for HIV and STD transmission. In the case of HIV, unprotected anal sex is the highest risk sex act two men can perform. We do know that unprotected oral sex is much less risky than unprotected anal sex -- particularly if you don't swallow cum or take a load in your mouth. However, it's important to remember that low risk is not no risk. Condom use is the best way to protect yourself from HIV transmission during sex.

If you have HIV, your semen may contain high levels of the virus, even if you're taking antiretroviral medications and have a low or undetectable amount of virus in your blood. This means that patients with no detectable blood viral load can still transmit HIV to their sexual partners.

In a study published in the New England Journal of Medicine in 1998, HIV-positive men with no detectable levels of HIV in their blood still had HIV DNA (provirus) present in cells within their semen. The researchers found that the provirus was able to replicate HIV when antiretroviral medication dosages fell. In addition, the provirus can be transmitted to sexual partners, who will then become infected with HIV.

Information for this article was provided by Franklin Lowe, M.D., M.P.H., F.A.C.S., associate director of Urology at St. Luke's/Roosevelt Hospital Center in New York, NY.