Semen is mostly fluid produced by glands in the reproductive tract, with sperm making up a surprisingly small portion of the total volume. A typical ejaculate is about 2 to 5 milliliters, and the liquid component (called seminal plasma) accounts for roughly 95% of it. The rest is sperm cells and cellular material. Each gland along the reproductive tract adds its own mix of sugars, minerals, enzymes, and proteins designed to keep sperm alive and moving.
Where the Fluid Actually Comes From
Four structures each contribute a different share of the total volume. The seminal vesicles, two small glands behind the bladder, produce the largest portion: 50 to 65% of the ejaculate, or about 1.5 to 2 milliliters. The prostate gland adds another 20 to 30%, roughly 0.6 to 0.9 milliliters. The testes and epididymis (a coiled tube where sperm mature) contribute about 5%, and the bulbourethral glands (also called Cowper’s glands) add less than 5%.
These fluids don’t mix until ejaculation. They combine in the urethra in a specific sequence, which is why the first fraction of an ejaculate has a different chemical profile than the last.
What Each Gland Adds
Seminal Vesicles
The bulk of semen comes from the seminal vesicles, and their main contribution is fructose, a simple sugar that serves as fuel for sperm. They also produce prostaglandins, signaling molecules that may help sperm travel through the female reproductive tract by stimulating gentle muscle contractions. On top of that, the seminal vesicles secrete bicarbonate, which acts as a buffer to neutralize the naturally acidic environment of the vagina, and proteins that cause semen to thicken into a gel immediately after ejaculation.
Prostate Gland
The prostate’s contribution is thinner and more watery, and it’s packed with enzymes, citric acid, and zinc. Zinc concentrations in prostatic fluid are roughly 75 times higher than in blood, and citric acid levels are about 1,000 times higher. The prostate also produces a protein called PSA (prostate-specific antigen), the same molecule measured in prostate cancer screening. In semen, PSA’s actual job is to break down the gel that forms after ejaculation, gradually liquefying it over 5 to 30 minutes so sperm can swim freely.
Bulbourethral Glands
These pea-sized glands near the base of the penis produce pre-ejaculate, the clear, slippery fluid that appears during arousal before orgasm. It contains mucus and enzymes but no sperm of its own. However, research has found that about 41% of men have sperm present in their pre-ejaculate, likely left over in the urethra from a previous ejaculation. In most of those cases, the sperm were still motile, which is why the withdrawal method is unreliable for contraception.
Testes and Epididymis
The testes produce the sperm cells themselves, along with testosterone. As sperm pass through the epididymis over about two weeks, they gain the ability to swim and pick up additional compounds including carnitine (which helps cells produce energy), lipids, and steroids.
Sperm Content
A normal ejaculate contains at least 39 million sperm in total, according to World Health Organization reference values. Despite that number, sperm cells are microscopically small and take up very little volume. By weight and volume, semen is overwhelmingly liquid. The sperm are suspended in the seminal plasma the way dust floats in air.
Minerals and Nutrients in Semen
Seminal plasma contains measurable amounts of potassium, calcium, and zinc, along with smaller quantities of magnesium, sodium, and chloride. Zinc is the most studied of these because of its role in sperm health; it helps stabilize sperm DNA and supports the membrane around each sperm cell. Calcium and potassium play roles in triggering the burst of swimming activity sperm need to reach an egg.
Semen also contains small amounts of vitamin C, vitamin B12, and various amino acids. The caloric content is minimal. A full ejaculate contains roughly 5 to 25 calories, mostly from fructose.
Why Color and Texture Vary
Normal semen is whitish-gray and has a thick, gel-like texture that becomes more watery within about 30 minutes. That shift from thick to thin is the liquefaction process driven by PSA and related enzymes from the prostate.
Color changes are common and usually harmless. Semen may appear more yellow with age, after a long period without ejaculating, or after eating foods like turmeric, garlic, or asparagus. Certain medications and B vitamins can give it a yellow-orange tint. Clear or watery semen often means a lower sperm concentration, which can happen with frequent ejaculation.
A yellow-green color, on the other hand, can signal an infection. STIs like chlamydia, gonorrhea, or genital herpes sometimes cause this change. A condition called pyospermia, where excess white blood cells accumulate in semen, can also turn it yellow and damage sperm in the process.
How Diet and Habits Affect Composition
What you eat and how you live do influence semen quality and volume, though the effects are moderate. Research on men planning for pregnancy found that regular consumption of milk and eggs was associated with healthier semen volume. Smoking and alcohol use were both linked to reduced semen quality across multiple measures.
Hydration matters for volume. Dehydration concentrates the seminal plasma, which can make ejaculate thicker and lower in volume. Sleep duration also showed a relationship with semen quality, as did sedentary behavior and diets high in processed meat and sugary drinks. The pH of healthy semen falls between 7.2 and 7.8, slightly alkaline, and lifestyle factors don’t appear to push it outside that range in healthy men.