Semen

• ALSO KNOWN AS: Ejaculate, seminal fluid
• ANATOMY OR SYSTEM AFFECTED: Genitals, glands, reproductive system

DEFINITION: The fluid ejaculated by the male at orgasm, consisting of sperm and secretions from accessory glands.

Structure and Functions

Semen is released from the male reproductive system during ejaculation. It is a milky white, sticky fluid, approximately 2 to 5 milliliters in volume, with an alkaline pH (7.2–7.8). It consists of spermatozoa (sperm) produced in the testes plus secretions of accessory glands: paired seminal vesicles that lie on the posterior surface of the bladder, the singular prostate gland that surrounds the urethra, and paired bulbourethral glands that lie just inferior to the prostate.

Sperm comprises only 5 percent of the semen, with 20 to 150 million sperm per milliliter. Seminal fluid contributes about 60 percent of the volume and contains high levels of fructose used by the sperm as an energy source. Seminal fluid also contains prostaglandins that stimulate smooth muscle contraction. This helps propel the semen through the urethra for release and aids in the movement of sperm upward in the female reproductive tract. Two proteins, semenogelin I and II, inhibit sperm motility and protect the semen in the male urethra. The semenogelins cause the semen to form a temporary coagulate when deposited in the vagina. After fifteen to thirty minutes, the coagulum dissolves.

The prostate contributes about 30 percent of semen volume as a milky, slightly acidic fluid. It is high in citrate, used as a nutrient by the sperm, plus several enzymes, including a protease called prostate-specific antigen (PSA). PSA hydrolyzes the semenogelins and helps to activate the motility of sperm. Prostatic fluid also contains seminal plasmin, an antibiotic active against Escherichia coli (E. coli), and several other bacteria that can cause urinary tract infections. The bulbourethral glands produce a thick, alkaline mucus that contributes only about 5 percent of semen volume. It is secreted in advance of ejaculation to neutralize any residual acidity from urine in the urethra. It also acts as a lubricant on the head of the penis to aid in vaginal penetration.

Disorders and Diseases

Disorders relating to semen involve pathologies of the individual glands. Infertility occurs in approximately 15 percent of males when there is a sperm count less than 20 million per milliliter, low sperm motility, or high levels of abnormal sperm. Decreased counts may be a result of testicular tumors, trauma, radiation, or infection with the mumps virus. Infertility can also occur with autoimmunity when antibodies that bind to sperm are produced. Testicular cancer usually requires surgical removal of the testis, called radical inguinal orchiectomy. Semen can also transmit sexually transmitted infections, including human immunodeficiency virus (HIV).

Prostate hypertrophy, a noncancerous enlargement of the prostate, occurs frequently in men over the age of fifty. It causes partial urethral obstruction, resulting in urinary hesitation, frequent urination, and possible painful urination. It may be treated with medication or corrected surgically. Prostate cancer occurs in one of every eight men in their lifetime. Treatments include surgery (radical prostatectomy), radiation (brachytherapy), hormonal therapy, chemotherapy, and immunotherapy. Serum PSA increases in prostate cancer, and monitoring of PSA levels is important.

Bibliography

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"Enlarged Prostate (Benign Prostatic Hyperplasia) ." National Institute of Diabetes and Digestive and Kidney Disease, June 2024, www.niddk.nih.gov/health-information/urologic-diseases/prostate-problems/prostate-enlargement-benign-prostatic-hyperplasia. Accessed 28 Aug. 2025.

Lilja, Hans, et al. “Prostate-Specific Antigen and Prostate Cancer: Prediction, Detection, and Monitoring.” Nature Reviews Cancer, vol. 8, no. 4, Apr. 2008, pp. 268–78.

Marieb, Elaine N., and Katja Hoehn. Human Anatomy and Physiology. 11th ed., Pearson, 2019.

Nieschlag, Eberhard. Andrology: Male Reproductive Health and Dysfunction . 4th ed., Springer, 2023.

Poiani, Aldo. “Complexity of Seminal Fluid: A Review.” Behavioral Ecology and Sociobiology, vol. 60, no. 3, July 2006, pp. 289–310.

"Semen Analysis." Lab Tests Online, 15 May 2024, labtestsonline.org.uk/tests/semen-analysis. Accessed 28 Aug. 2025.

"Spermatoceles." Urology Care Foundation, Mar. 2024, www.urologyhealth.org/urology-a-z/s/spermatoceles. Accessed 28 Aug. 2025.