By Dr. Ananya Mandal, MD
Semen is a greyish white bodily fluid that is secreted by the gonads of male animals. It carries sperm or the spermatozoa and fructose and other enzymes that help the sperm to survive to facilitate successful fertilization.
The whitish opalescence is due to the large amount of protein that it contains and its slightly turbid appearance is due to the spermatozoa contained within it.
Process of ejaculation
Semen is released during the process of ejaculation and is processed in the seminal vesicle in the pelvis, which is where it is produced.
How does ejaculation occur?
Ejaculation is controlled by the central nervous system and occurs when there is friction on the genitalia and other forms of sexual stimulation. The stimuli lead to impulses that are sent up the spinal cord and into the brain.
Two phases of ejaculation
Ejaculation has two phases:
Phase 1: emission in which the vas deferens (the tubes that store and transport sperm from the testes) contract to squeeze the sperm toward the base of the penis through the prostate gland and into the urethra. The seminal vesicles release their part of the semen that combine with the sperm. The ejaculation is unstoppable at this stage.
Phase 2: ejaculation in which the muscles at the base of the penis and urethra contract. This leads to forcing the semen out of the penis (ejaculation and orgasm) and this phase also has a bladder neck contraction. The bladder neck contracts to prevent the back flow of the semen into the urinary tract. Dry orgasm can occur even without delivery of semen (ejaculation) from the penis. Erection declines normally following ejaculation.
The semen travels through the ejaculatory ducts and mixes with fluids from the seminal vesicles, the prostrate, and the bulbourethral glands.
The seminal vesicles produce a viscous, fructose-rich fluid forming around 65-70% of the semen base.
The white color of the semen is due to secretion from the prostate glands containing enzymes, citric acid, lipids, and acid phosphatase. This forms around 25-30% of the semen base.
At each ejaculation around 200-500 million sperms are released by the testes. This forms about 2-5% of the semen composition.
Apart from these, the bulbourethral glands produce a clear secretion. This helps in mobility of the sperm cells in the vagina and cervix. The glands’ secretion contribute less than 1% to the overall semen composition.
The semen comprises of:
blood group antigens
Semen per ejaculation
Ejaculation is a complex process and the compositions of the final semen come together in the posterior urethra and only become mixed after ejaculation is complete.
The volume of semen released per ejaculate varies. Approximately an average around 3.4 milliliters is ejaculated at one time. It can be as high as 4.99 milliliters or as low as 2.3 milliliters.
If there is a prolonged gap between ejaculations, the number of sperm in the semen increases but there is no overall increase in the semen.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)
Last Updated: Sep 19, 2013