Ejaculation is the emission of semen, a mixture of fluids and sperm, from the male reproductive tract. Generally the result of physical or mental sexual stimulation, ejaculation accompanies almost all orgasms, though some men achieve orgasm without ejaculation either through purposeful training or through physical abnormalities in the reproductive tract. While the prerequisite to ejaculation is sexual arousal and erection of the penis, these alone are not sufficient to produce an emission—instead, a sequence of events leads up to ejaculation.
During the period of arousal that precedes ejaculation, glands of the male reproductive system produce pre-ejaculatory fluid that prepares the urethra for the movement of sperm. This fluid, largely secreted by an organ situated close to the base of the penis and called the bulbourethral gland, coats the urethra so that sperm are not damaged by their high-speed trajectory during ejaculation. Lauralee Sherwood, in her book “Human Physiology,” notes that there’s also some speculation that bulbourethral gland fluid helps buffer the semen against pH changes experienced upon being deposited in the female vagina, which may enhance survival.
The first phase of ejaculation is called seminal emission, and involves the movement of sperm from the epididymis—the organ in which they mature—to the urethra, notes a 2001 article published in “Medical Aspects of Human Sexuality.” Because the male urethra is shared by both the reproductive system and the urinary system, an important prerequisite to ejaculation is ensuring that the ejaculate can’t progress backwards into the bladder, but instead must exit the urethra to the outside. During seminal emission, the bladder neck closes off, preventing retrograde seminal movement. Most men have some control over the process of seminal emission, and can delay it a bit as desired. Once it occurs, however, ejaculation is eminent.
Once semen has been deposited in the urethra, the second phase of ejaculation involves its expulsion. This is accomplished through the rhythmic muscular contractions of the pelvic region that characterize orgasm, notes Dr. Elliot. Further, the seminal vesicles and prostate gland—also fluid-producing glands responsible for secreting components of the liquid portion of semen—contract, squeezing their seminal contributions into the urethra for ejaculation along with sperm. While contractions of the pelvic floor muscles are not directional and can’t force sperm toward the opening of the penis to the outside, prior closure of the bladder neck ensures that ejaculation occurs through the penile opening, called the urinary meatus.