Semen is the fluid that leaves the penis during ejaculation. It carries sperm cells, but sperm make up only about 10% of the total volume. The other 90% is a mix of fluids produced by several glands in the reproductive tract, each contributing specific nutrients and protective compounds that keep sperm alive and functional.
Semen vs. Sperm
People often use “semen” and “sperm” interchangeably, but they’re not the same thing. Sperm are individual reproductive cells, each carrying half the genetic material needed to create an embryo. Semen is the complete package: sperm cells suspended in a nutrient-rich fluid called seminal plasma. The two components only combine at the moment of ejaculation, meaning semen technically doesn’t exist inside the body. Before that point, sperm are stored separately from the fluids that will eventually carry them.
Where Semen Comes From
Three sets of glands contribute fluid to semen, each with a distinct role.
The seminal vesicles produce roughly 60% of the total volume. Their secretion is thick and contains fructose (a sugar that serves as fuel for sperm), prostaglandins that help sperm move, and proteins that cause semen to temporarily thicken after ejaculation.
The prostate gland supplies most of the remaining fluid. Its secretion is thin, milky, and slightly alkaline, which helps sperm swim more effectively. It also contains citric acid, zinc, and enzymes that gradually break down the initial clot so semen becomes liquid again within minutes.
The bulbourethral glands, two small structures near the base of the penis, contribute only a tiny volume. They release a slippery, alkaline fluid before ejaculation that neutralizes leftover urine acidity in the urethra and helps reduce acidity inside the vagina, creating a more hospitable environment for sperm.
What Semen Contains
Beyond sperm cells, seminal plasma is a surprisingly complex mixture. It contains fructose for energy, citric acid, zinc, various salts, proteins, lipids, and enzymes. Zinc in particular plays a role in stabilizing sperm DNA and supporting overall sperm quality. The fluid also carries compounds like carnitine and glycerophosphocholine from the epididymis (the coiled tube where sperm mature before ejaculation).
Healthy semen has a slightly alkaline pH, typically between 7.2 and 7.8. That alkalinity is intentional: it protects sperm from the naturally acidic environment of the vagina. A pH below 7.2 can sometimes signal a blockage in the seminal vesicles, while a pH above 8.0 may point to an infection.
Normal Appearance and Volume
Fresh semen is usually whitish-gray and slightly thick. It clots almost immediately after ejaculation, then liquefies within 5 to 30 minutes as prostate enzymes break it down. This liquefaction is a normal part of the process and allows sperm to swim freely.
Color can vary. A slight yellow tint becomes more common with age and can also result from certain medications or mild dehydration. Green or strongly yellow semen may indicate an infection or jaundice. Red or pink semen sometimes appears after eating beet-heavy meals, but it can also mean blood is present, potentially from an infection, injury, or prior medical procedure. Occasional changes in color are usually harmless, but persistent discoloration is worth investigating.
Fertility Reference Ranges
The World Health Organization published its most recent semen analysis guidelines in 2021, establishing baseline thresholds below which fertility may be reduced. These aren’t pass/fail numbers. They represent the fifth percentile of men whose partners conceived within a year, meaning most fertile men score well above these minimums.
- Total sperm count: at least 39 million per ejaculate
- Total motility: at least 42% of sperm moving
- Normal morphology: at least 4% of sperm with typical shape
That 4% morphology figure often surprises people, but it’s normal for the vast majority of sperm to have some structural irregularity. Fertility depends on having enough well-formed, actively swimming sperm to reach and fertilize an egg, not on every single cell being perfect.
Pre-ejaculate and Pregnancy Risk
Pre-ejaculate (the clear fluid released during arousal, before orgasm) comes primarily from the bulbourethral glands and is not the same as semen. However, it can contain sperm. In one study of 27 men, 41% produced pre-ejaculate samples that contained sperm cells, and in most of those cases, a reasonable proportion of the sperm were motile. The counts were low compared to a full ejaculation, but not zero. This is why the withdrawal method has a relatively high real-world failure rate for preventing pregnancy.
How Long Sperm Survive After Ejaculation
Once inside the female reproductive tract, sperm can remain viable for about 3 to 5 days. That window is why pregnancy is possible even if intercourse happens several days before ovulation. Outside the body, sperm die much faster. On dry surfaces, they typically lose motility within minutes to an hour as the seminal fluid dries out. In warm, moist conditions they may last slightly longer, but survival outside the body is dramatically shorter than inside it.