Biology of the Male Reproductive System

Introduction
The external structures of the male reproductive system include the penis and scrotum. The internal structures include the vas deferens, testes (testicles), urethra, prostate gland, and seminal vesicles. Sperm, which carries the man's genes, is made in the testes and stored in the seminal vesicles. During ejaculation, sperm is transported along with a fluid called semen through the urethra.

Effects of Aging
It is not clear whether aging itself or the disorders associated with aging cause the gradual changes that occur in men's sexual functioning. The frequency, duration, and rigidity of erections gradually decline throughout adulthood. Levels of the male sex hormone (testosterone) tend to decrease, reducing sex drive (libido). Blood flow to the penis decreases. Other changes include decreases in penile sensitivity and ejaculatory volume, reduced forewarning of ejaculation, orgasm without ejaculation, more rapid detumescence, and a longer refractory period.
Function
• The penis becomes erect through a complex interaction of physiologic and psychologic factors.
• Contractions during ejaculation impel semen into the urethra and out of the penis.
During sexual activity, the penis becomes erect, enabling penetration during sexual intercourse. An erection results from a complex interaction of neurologic, vascular, hormonal, and psychologic actions. Pleasurable stimuli cause the brain to send nerve signals through the spinal cord to the penis. The arteries supplying blood to the corpora cavernosa and corpus spongiosum respond by dilating. The widened arteries dramatically increase blood flow to these erectile areas. At the same time, muscles around the veins that normally drain blood from the penis tighten, slowing the outflow of blood and elevating blood pressure in the penis. This combination of increased inflow and decreased outflow is what causes the penis to become engorged with blood and increase in length, diameter, and stiffness.
At the climax of sexual excitement (orgasm), ejaculation usually occurs, caused when friction on the glans penis and other stimuli send signals to the brain and spinal cord. Nerves stimulate muscle contractions along the seminal vesicles, prostate, and the ducts of the epididymis and vas deferens. These contractions force semen into the urethra. Contraction of the muscles around the urethra further propels the semen through and out of the penis. The neck (base) of the bladder also constricts to keep semen from flowing backward into the bladder.
Once ejaculation takes place, or the stimulation stops, the arteries constrict and the veins relax, reducing blood inflow, increasing blood outflow, and causing the penis to become limp (detumescence). After detumescence, erection cannot be obtained for a period of time (refractory period), commonly about 20 minutes in young men.
Puberty
• Puberty may begin as early as age 9 and continue until age 16.
• At puberty, the testes start to produce testosterone.
• Testosterone causes reproductive organs to mature, facial and pubic hair to appear, and the voice to deepen.
Puberty is the stage during which a person reaches full reproductive ability and develops the adult features of their gender. In boys, puberty usually occurs between the ages of 10 and 14 years. However, it is not unusual for puberty to begin as early as age 9 or to continue until age 16.
The pituitary gland, which is located in the brain, initiates puberty. The pituitary gland secretes luteinizing hormone and follicle-stimulating hormone, which stimulate the testes to produce testosterone. Testosterone is responsible for the development of secondary sex characteristics, features that distinguish the sexes but are not part of the reproductive system, such as facial hair growth and voice change.
Testosterone also produces many changes in the male reproductive organs, including
• Elongation and thickening of the penis
• Enlargement of the scrotum, testes, epididymis, and prostate
• Darkening of the skin of the scrotum
• Growth of pubic hair
Sperm usually develops by age 14. Ejaculation first occurs during late puberty.
Structure
• The penis and the urethra are part of the urinary and reproductive systems.
• The scrotum, testes, vas deferens, and prostate gland comprise the rest of the reproductive system.
The penis consists of the root (which is attached to the abdominal wall), the body (the middle portion), and the glans penis (the cone-shaped end). The opening of the urethra or orifice (the channel that transports semen and urine) is located at the tip of the glans penis. The base of the glans penis is called the corona. In uncircumcised males, the foreskin (prepuce) extends from the corona to cover the glans penis.
The body of the penis consists primarily of three cylindrical spaces (sinuses) of erectile tissue. The two larger ones, the corpora cavernosa, occur side by side. The third sinus, the corpus spongiosum, surrounds the urethra and ends as the glans penis. When these spaces fill with blood, the penis becomes large and rigid (erect).
The scrotum is the thick-skinned sac that surrounds and protects the testes. The scrotum also acts as a climate-control system for the testes, because they need to be slightly cooler than body temperature for normal sperm development. The cremaster muscles in the wall of the scrotum relax or contract to allow the testes to hang farther from the body to cool or to be pulled closer to the body for warmth or protection.
The testes are oval bodies that average about 1.5 to 3 inches (4 to 7 centimeters) in length and 2 to 3 teaspoons (20 to 25 milliliters) in volume. Usually the left testis hangs slightly lower than the right one. The testes have two primary functions: producing sperm and producing testosterone (the primary male sex hormone). The epididymis is a coiled tube almost 20 feet (6 meters) long. It collects sperm from the testis and provides the space and environment for sperm to mature. One epididymis lies against each testis.
The vas deferens is a firm duct that transports sperm from the epididymis. One such duct travels from each epididymis to the back of the prostate and joins with the seminal vesicle. In the scrotum, other structures, such as blood vessels and nerves, also travel along with each vas deferens and together form an intertwined structure, the spermatic cord.
The urethra serves a dual function in males. This channel is the part of the urinary tract that transports urine from the bladder and the part of the reproductive system through which semen is ejaculated.
The prostate lies just under the bladder and surrounds the urethra. Walnut-sized in young men, the prostate enlarges with age. When the prostate enlarges too much, it can block urine flow through the urethra. The seminal vesicles, located above the prostate, join with the vas deferens to form the ejaculatory ducts. The prostate and the seminal vesicles produce fluid that nourishes the sperm. This fluid provides most of the volume of semen, the secretion in which the sperm is expelled during ejaculation. Other fluid that makes up a very small amount of the semen comes from the vas deferens and from mucous glands.

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