Testosterone (tes-TOS-tĕ-rōn) is an androgen (AN-drŏ-jĕn) (i.e., male sex hormone) produced in the testes. In a male fetus, testosterone is secreted by the testes by the seventh week of development, with peak concentrations reached in the second trimester. This early release of testosterone results in the anatomical differentiation of the male sexual organs. In childhood, testosterone concentrations are low and then increase during puberty (PŪ-bĕrt-ē), the period during which adolescents develop secondary sex characteristics and become capable of reproduction. During puberty, the production of sperm begins.^[1]

Testosterone is released into the systemic circulation and plays an important role in muscle development, bone growth, development of secondary sex characteristics, and maintenance of libido (lĭ-BĒ-dō) (i.e., sex drive) in both males and females. In females, the ovaries secrete small amounts of testosterone, although most is converted to estradiol (a form of estrogen). A small amount of testosterone is also secreted by the adrenal glands in both sexes.^[2]

Spermatogenesis (spur-mă-tō-JĔN-ĕ-sĭs), the production of sperm, occurs in the testes. Spermatogenesis requires a two- to three-degree lower temperature than body temperature, which is why it occurs outside of the body. It begins at puberty, after which time sperm are produced constantly throughout a man’s life.^[3]

One production cycle, from spermatogonia through formed sperm, takes approximately 64 days. Sperm counts, the total number of sperm a man produces, slowly decline after age 35. Azoospermia (ā-zō-ŏ-SPĔR-mē-ă) refers to the absence of viable sperm in the semen.

Sperm (SPURM), the male reproductive cell, are smaller than most cells in the body. In fact, the volume of a sperm cell is 85,000 times less than that of the egg (female reproductive cell). Approximately 100 to 300 million sperm are produced each day, whereas women typically ovulate only one oocyte (egg) per month. As is true for most cells in the body, the structure of sperm cells speaks to their function. Sperm have a distinctive head, midpiece, and tail region.^[4] See Figure 6.3^[5] for an illustration of a sperm.

Ejaculation (i-jak-yŭ-LĀ-shŏn) refers to the ejection of sperm-containing semen from the penis during orgasm (OR-gazm), the climax of sexual stimulation.^[6] A condom (KON-dŏm) is a sheath that may be worn during coitus to prevent pregnancy by collecting ejaculate. Condoms also help prevent the spread of sexually transmitted infections.

Penile erection is caused by vasocongestion of the penile tissues as more arterial blood flows into the penis than is leaving in the veins.^[7] Priapism (PRĪ-ă-pizm) is a disorder in which the penis maintains a prolonged, rigid erection for four hours or longer, in the absence of stimulation. Priapism that is caused by blood not being able to leave the penis is a medical emergency requiring prompt surgery to prevent damage to the penis. If not treated promptly, priapism can progress to permanent erectile dysfunction.^[8] Erectile dysfunction (ĕ-RĔK-tīl dis-FŬNGK-shŏn) (ED) refers to the inability to attain or maintain an erection sufficient to perform sexual intercourse.