Male Reproductive System
Reproduction is essential for any species to sustain its population. In the simplest sense, the most important function of every living organism is reproduction. Organs of the male and female reproductive systems play a central role in sexual reproduction by creating, nourishing, and housing sex cells called gametes . The human male reproductive system consists of gonads called testes, a series of ducts (epididymis, vas deferens , ejaculatory duct, urethra) that serve to transport spermatozoa to the female reproductive tract, and accessory sex glands (seminal vesicles, prostate, and bulbourethral glands).
The testes (singular, testis) are paired structures that originally develop in the abdomen and descend into the scrotum, a sac of skin and connective tissue positioned outside the pelvic cavity. This scrotal location is important for maintaining a testicular temperature, approximately 1.5 to 2.5 degrees Celsius (34.7 to 36.5 degrees Fahrenheit) below body temperature, required for spermatogenesis (sperm production). Testes also serve important endocrine functions as the source of male sex steroids called androgens. The most abundant androgen is testosterone.
Surrounding the tubules are clusters of interstitial cells, which synthesize testosterone and secrete it into the bloodstream. Testosterone is present in infant boys, although synthesis increases dramatically at puberty around age thirteen. This increase stimulates the onset of spermatogenesis and development of accessory sex glands. All male reproductive organs require testosterone for functions such as protein synthesis, fluid secretion , cell growth, and cell division. Androgens also play important roles in the male sexual response and stimulate secondary sex characteristics such as skeletal development, facial hair growth, deepening of the voice, increased metabolism , and enlargement of the testes, scrotum, and penis.
Spermatozoa leave each testis through small tubes called efferent ductules. Fluid pressure from secretions in the testis and ciliated cells in the efferent ductules help move spermatozoa into the epididymis. Testicular spermatozoa are immature because they cannot swim and lack the ability to penetrate an egg. Sperm maturation occurs in the epididymis. Located adjacent to the testis, the epididymis contains a single, highly coiled tubule nearly 6 meters (19.6 feet) long. Sperm transport through the epididymis takes approximately twenty days. As sperm transit the epididymis, they are bathed in a specialized fluid rich in proteins, ions , and a number of other molecules. Complex interactions between spermatozoa and epididymal fluid contribute to sperm maturation. The epididymis is also a site for sperm storage and for the protection of sperm against chemical injury.
Sperm Formation and Ejaculation
From the epididymis, spermatozoa enter a muscular tube called the vas deferens (approximately 45 centimeters [17.7 inches] long). The vas deferens contracts during the release of sperm—a process called ejaculation—to move spermatozoa out of the epididymis and into the ejaculatory duct, where sperm are mixed with secretions from the seminal vesicles. The ejaculatory duct enters the urethra as it passes through the prostate gland. In males, the urethra serves a dual purpose transporting sperm to the penis and urine from the urinary bladder.