From Encyclopedia of Sex and Sexuality
Male ejaculation occurs when fluid is expelled through the opening at the tip of the penis during the contractions experienced at the moment of sexual orgasm. The fluid is called semen (less accurately, “sperm”) and the semen produced during one ejaculation is referred to as the ejaculate. Each ejaculate normally contains hundreds of millions of tiny sperm (reproductive) cells.
The process of ejaculation requires the coordination of several functions. Emission (secretion) of semen occurs into the urethra, the muscular tube through which urine flows. This happens during foreplay, when a man is sexually aroused, and during the penis’s thrusting before orgasm. Just before orgasm the neck of the bladder opening closes to prevent the flow of semen back into the bladder. During orgasm, contractions of the urethra and the pelvic musculature result in a forceful ejaculation of semen through the meatus—the opening at the tip of the penis.
Human ejaculate has an average volume of 3 milliliters, though it ranges from 1.5 to 6 milliliters. It is composed of sperm and seminal fluid. The volume of sperm contained in it is insignificant—less than 1 percent of the volume of the total ejaculate. Indeed, postvasectomy men report no change in the volume of their ejaculate.
Semen comes from two glands called the seminal vesicles. Its composition has been studied extensively and contains many components including electrolytes, carbohydrates (fructose), nitrogenous compounds, prostaglandin, enzymes, amino acids, zinc, and cholesterol.
After emission of semen into the back part of the urethra has occurred and the phenomenon of orgasm begins, it is difficult for men to voluntarily contain ejaculation. A refractory period occurs after ejaculation, during which further stimulation will not lead to another orgasm and ejaculation. The duration of this period varies directly with age. It can be as short as a few minutes for an adolescent or a man in his early twenties or several weeks in an elderly male.
Clinical conditions can cause a lack of emission or retrograde ejaculation into the bladder. A total lack of ejaculation during orgasm can occur in men with insulin-dependent diabetes, following spinal cord injury, or after abdominal radical surgery for colon and testicular cancer. Men with multiple sclerosis, psychological disorders, or tuberculosis of the vas deferens (the tube that connects the testes to the urethra) also may suffer from a total lack of ejaculation. However, semen for artificial insemination can be obtained from men suffering from lack of ejaculation by a medical procedure called electroejaculation.
 Retrograde Ejaculation
Ejaculatory dysfunction is responsible for 2 percent of all cases of male infertility. The most common form is retrograde ejaculation. Retrograde ejaculation is suspected when little or no semen is seen to flow from the penis’s opening during orgasm. The diagnosis can be confirmed by the presence of sperm in an analysis of a postejaculatory urine sample. A total lack of emission (anejaculation) is demonstrated by a lack of sperm in postejaculatory urine, provided no obstruction of the vas deferens is noted and the testes are manufacturing sperm. Failure of the bladder neck to close during orgasm results in either partial or total retrograde ejaculation.
Any medical or surgical condition that interferes with the nerves, anatomy, or function of the bladder neck can result in retrograde ejaculation. Over 60 percent of men who are sexually active following transurethral removal of the prostate report no ejaculation during orgasm. In that situation, most of the ejaculate is retrograde and is eliminated by voiding after orgasm.
The phenomenon of retrograde ejaculation may also occur in men with diabetes, multiple sclerosis, and following surgical interruption of the sympathetic nerves. Medications such as alpha-Methyldopa (Aldomet) and phenoxybenzamine (Dibenzyline) may cause a chemical nerve interruption and retrograde ejaculation. Surgical removal of the colon or rectum and deseminated testicular cancer may result in retrograde ejaculation. Abdominal aortic aneurysm repair may also cause both retrograde ejaculation and erectile dysfunction.
Even with no treatment some men with retrograde ejaculation may spontaneously recover normal ejaculation. Although there is no surgical treatment to correct retrograde ejaculation, a number of medications have been used with some benefit. In men who do not respond to medication, an alternative method is available involving sperm retrieval from the bladder for use in artificial insemination.