Sperm leaves the body through a two-phase process: first, sperm mixes with fluid to form semen near the base of the penis, then rhythmic muscle contractions push that semen out through the tip. The whole sequence takes only a few seconds, but it involves a surprisingly long journey and multiple organs working in coordination.
The Path Sperm Travels Before Ejaculation
Sperm cells are produced in the testicles, but they don’t go straight from there to the outside world. After being made, they move into a tightly coiled tube behind each testicle called the epididymis, where they mature and are stored until needed. When ejaculation begins, sperm travel through a long muscular tube (one on each side) that loops up into the abdomen and back down behind the bladder. These tubes squeeze sperm toward the prostate gland, where they merge into a single channel called the urethra, the same tube urine passes through. From there, semen exits through the tip of the penis.
That full route, from storage to exit, means sperm passes through four distinct structures: the epididymis, the vas deferens (the long tubes), the ejaculatory duct, and the urethra.
What Happens During the Two Phases
Ejaculation isn’t one single event. It happens in two distinct phases, called emission and expulsion.
During the emission phase, the tubes that store and transport sperm contract to push sperm toward the prostate. At the same time, the seminal vesicles and prostate gland release their fluids, which mix with sperm to create semen. This pooling of fluid happens at the base of the penis, and it’s what produces the sensation that ejaculation is about to happen, sometimes described as the “point of no return.”
During the expulsion phase, muscles at the base of the penis contract rhythmically, about once every 0.8 seconds, to force semen out in several spurts. The primary muscle responsible for this pumping action wraps around the base of the penis and acts like a compressor, propelling semen from deep inside the body to the opening at the tip. These rhythmic contractions are also closely tied to the sensation of orgasm.
What Semen Is Actually Made Of
Most people assume semen is mostly sperm, but sperm cells make up only a tiny fraction of the total volume. About 60% of semen comes from the seminal vesicles, two small glands behind the bladder that produce a thick, energy-rich fluid containing sugars that fuel sperm. Most of the remaining volume comes from the prostate gland, which adds a thinner, slightly acidic fluid that helps sperm survive. The sperm cells themselves, along with a small contribution from glands near the urethra, account for a very small portion of what comes out.
A typical ejaculation produces at least 1.4 milliliters of semen, roughly a quarter of a teaspoon, according to World Health Organization reference values. The actual amount varies widely from person to person and even from one ejaculation to the next depending on hydration, time since last ejaculation, and arousal level.
What Keeps Semen Going the Right Direction
The body has a built-in valve system to make sure semen exits forward through the penis rather than backward into the bladder. A ring of muscle at the opening of the bladder, called the bladder neck muscle, tightens during ejaculation to seal off the bladder. This forces all the semen in the urethra to move in only one direction: out.
When this muscle doesn’t tighten properly, a condition called retrograde ejaculation occurs. Semen flows backward into the bladder instead of out through the penis. A person with retrograde ejaculation may still feel an orgasm but produce little or no semen. It’s not harmful (the semen simply passes out later with urine) but it can affect fertility. Common causes include certain medications, nerve damage from diabetes, and surgery on the prostate or bladder.
Pre-ejaculate: What Comes Out Before
Before ejaculation, a small amount of clear fluid often appears at the tip of the penis during arousal. This is pre-ejaculate, sometimes called pre-cum, and it serves a different purpose than semen. It’s produced mainly by the Cowper’s gland, a small gland located below the prostate, and its job is to lubricate the urethra and neutralize any leftover acidity from urine, creating a more hospitable path for sperm to follow.
Pre-ejaculate isn’t supposed to contain sperm, but studies have found that more than 40% of men have sperm present in this fluid. Some men consistently have sperm in their pre-ejaculate, while others never do. The sperm count is much lower than in a full ejaculation, but it can still be enough to cause pregnancy. This is why the withdrawal method is unreliable as contraception.
How the Nervous System Coordinates It All
The entire process is controlled by the autonomic nervous system, the part of your nervous system that handles things you don’t consciously control, like heart rate and digestion. During arousal, signals can originate in the brain (from visual or mental stimulation) or from physical touch to the genitals. These signals trigger the release of chemical messengers that increase blood flow to the penis and prepare the reproductive tract.
Different branches of this system handle different parts of the response. The parasympathetic branch, which generally handles “rest and relax” functions, plays a key role in erection by promoting blood flow into the penis. The sympathetic branch, which handles “fight or flight” responses, drives the emission phase, triggering the contractions that move sperm and fluid into position. The expulsion phase involves both involuntary nerve signals and rhythmic contractions of skeletal muscles that you can partially feel but not fully control. This layered coordination is why ejaculation feels automatic once it starts.