The prostate gland is the largest of the male reproductive glands and serves a vital function in the male reproductive system. It secretes a thin, milky-white liquid called prostatic fluid that is rich in zinc, citric acid, choline, and various proteins and hormones. Prostatic fluid contributes about 30% of the volume of seminal fluid. The various components of the prostatic fluid aid the sperm by providing a protective medium for them to move in as they make their way through the vagina. Without this protective medium, most sperm would die soon after ejaculation.
One of the chemicals in prostatic fluid is prostate specific antigen (PSA). PSA is an enzyme (a catalyst that causes biochemical reactions). During ejaculation, the muscular portion of the prostate contracts and pushes the prostatic fluid through tiny ducts into the urethra, where it mixes with other elements of seminal fluid. The muscular contractions of the prostate help to expel semen during ejaculation.
What Is Semen?
Semen has two main components: sperm and seminal plasma. Seminal plasma is a complex fluid that nurtures and protects the sperm and helps to ensure they pass unharmed through the vagina and into the uterus.
Semen is produced through a complex series of steps and by a number of organs, ducts, and glands:
- Testes. Sperm and testosterone are made in the testes, which are composed of tightly coiled structures called seminiferous tubules. Certain cells inside the testes act like nurseries, nurturing the immature sperm until they reach maturity and are released into the seminiferous tubules.
- Epididymis. Located on top of the testes, the epididymis is a tightly coiled tube where sperm are stored for up to 2 weeks while they mature, develop motility, and become capable of fertilization.
- Vas deferens. The vas deferens is a long curving tube that transports sperm from the epididymis to the seminal vesicle. It acts as a storage site for sperm until ejaculation. The entire process of sperm maturation takes about 74 days.
- Ejaculatory ducts. The vas deferens and the ducts of the seminal vesicles join together to form the ejaculatory ducts. They descend through the prostate gland and into the urethra.
- Seminal vesicles. The seminal vesicles supply a viscous, alkaline secretion that forms about 60% of the semen. The fluid from the seminal vesicles is rich in nutrients to provide energy for sperm metabolism and to increase sperm motility. It also contains a substance that causes the semen to clot after ejaculation, thought to be useful for keeping the semen at the neck of the uterus.
- Prostate gland. The prostate gland discharges prostatic fluid into the urethra during ejaculation. The prostate's muscular contractions help to propel the seminal fluid through the urethra and out of the penis. Once inside a woman's vagina, the PSA in the prostatic fluid liquifies the semen that has been clotted by the seminal fluid, so that the sperm can swim off to fertilize the ovum (egg).
- Bulbourethral (Cowper's) glands. The bulbourethral glands, sometimes known as the Cowper's glands, are two pea-sized glands found just under the prostate. They produce the thick, clear, pre-ejaculate fluid that acts as a lubricant and may clear out the urethra in preparation for ejaculation.
- Urethra. The urethra discharges both urine and semen. During male orgasm and ejaculation, the muscular sphincter at the neck of the bladder closes, preventing urine from entering the urethra.
What Happens During Ejaculation?
During male orgasm and ejaculation, sperm travel upwards through the vas deferens and then down into the upper portion of the prostate. The sperm and fluid from the seminal vesicles then mix with prostatic fluid and fluid from the bulbourethral glands to form the semen. The ejaculate, now containing sperm and fluids from the seminal vesicles, prostate, and bulbourethral glands, flows from the ejaculatory ducts into the urethra. From the urethra it passes out through the end of the penis.
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