Biology of Ejaculation
Ejaculation is a critical part of male sexual and reproductive function. It refers to the release of sperm and semen from the urethra, which opens at the meatus, located in the glans penis, at the tip of the penis. During a typical sexual encounter, most men ejaculate 3-5 milliliters of seminal fluid, the mixture of semen and sperm. A simple mixture is the biology of ejaculation! This is approximately equivalent to a teaspoon. A tablespoon of seminal fluid contains between 750 million and 2.5 billion sperm.
The male reproductive system relies upon the interplay of hormones, nerves, muscles, and blood vessels. The hormone testosterone is supplied by the testes, which produce millions of sperm daily. The sperm are mixed with fluid produced by the seminal vesicles and prostate gland. This fluid allows the sperm to survive in the outside environment for a brief period after ejaculation.
Ejaculation usually occurs in response to sexual stimulation, although it may occur without prior stimulation, during sleep. Arousal stimulates the sympathetic nervous system, which results in expansion of the two main arteries of the penis. The two chambers on each side of the penis, known as the corpus cavernosum, fill with blood, causing an erection. The resulting pressure on the veins of the penis prevents blood from draining normally.
With arousal, sperm are released from the testes through a muscular duct called the vas deferens. The seminal vesicles and the prostate gland release protein rich fluid that is mixed with sperm and propelled through the ejaculatory ducts to the urethra, at the base of the chamber in the penis known as the corpus spongiosum. As the urethra fills with semen, the sphincter muscles of the bladder neck contract, which prevents sperm from traveling backwards (retrograde) into the bladder. Contraction of the bladder sphincters also prevents leakage of urine during a sexual encounter.
At peak arousal, nerve receptors at the base of the urethra send signals to the spinal cord, which results in a reflex stimulation of nerves traveling from the spinal cord to the muscles at the base of the penis and surrounding the urethra. Rapid contraction of these muscles forces semen through the urethra and out of the penis at climax. After ejaculation, muscular relaxation occurs rapidly. The sperm, when deposited in proximity to female reproductive organs, will fertilize the ovum, which is known as insemination.