The growth spurt is kicked off by the hypothalamus gland, a pea-sized gland in the base of the brain (Aleshire 1982, 39) and part of the body's endocrine system. (It also regulates eating, drinking, and sexual desire.) The endocrine system consists of the ductless glands (secrete hormones directly into the bloodstream) and the structures in the central nervous system which regulate the release of hormones. The hypothalamus is the endocrine system's alarm clock with regard to puberty. It matures in late childhood, bypassing the snooze button to activate the pituitary gland (also located at the base of the brain).

The anterior lobe of the pituitary gland is the master gland in the body,because it regulates the feedback loop (Cobb 1998, 90). The hypothalamus gland secretes a hormone called GnRH which tells the anterior pituitary to manufacture gonadotrophic hormones which signal the gonads to produce androgens (masculine hormones) and estrogens (hormones that cause feminine traits). One important pituitary hormone (and one that has made the news in athletic circles) is the human growth hormone or HGH. The pituitary gland, then, kicks off the growth spurt which begins around 11 and a half years old for girls and around 14 and a half for boys. This period of phenomenal growth lasts only a short time. (Many parents tell of their adolescents growing four to six inches in height during a summer.) The growth spurt usually shuts down by the 17th birthday for girls and the 19th birthday for boys. By then they have reached 98 percent of adult height. Other glands which come into play in the feedback loop are the:

Thyroid—Located at the front of the throat, it is directly related to metabolism.

Parathyroids—On either side of the thyroid glands, these glands control absorption of calcium, which directly relates to the development of bones and clotting of blood.

Adrenal glands—Located above each kidney, they generally provide unusual strength in times of stress or athletic activity.

Gonads—Testes in males, located in the scrotum, produces testosterone, which stimulates the beginning of the secondary sexual characteristics. In females, the ovaries are located in lower abdomen and produce estrogen, which stimulates the emergence of the secondary sex characteristics. In females, progesterone aides in pregnancy and controls menstrual cycle. Estradoil is another female parallel to testosterone.

During the growth spurt changes occur in body fat. Subcutaneous fat is the fat below the skin as opposed to intramuscular fat, which is the fat inside the muscles (marbling in the meat you buy at the grocery). In preparation for childbearing (and much to the dismay of most), girls develop more subcutaneous fat, while boys add more muscle mass.

Another external change common to teenagers is acne. This occurs when the skin's sebaceous glands are active and produce oils at a rate faster than the skin's pores are able to open. The ducts of these glands become plugged and infected, resulting in acne. Acne is a problem for many youth, and it can affect their self-esteem and emotional development in a number of ways (Boshers 1997, 84).

One more system is changing that may further help in understanding the 21st-century teenager. The counterpart to the endocrine system is the exocrine system. Recall that the endocrine system secretes inward, into the bloodstream. The exocrine system secretes from glands to bloodstream through ducts and eventually through the skin. The merocrine glands are sweat glands that occur on all skin areas other than armpit, genital, and anal regions. Sweat from these glands is generally without odor. The apocrine glands are the sweat glands found in the armpits, genital, and anal regions. When the apocrine glands mature during puberty, the odor of sweat changes significantly. A preteen baseball team that comes into your kitchen for water before resuming the game smells like wet carpet. The sweat that comes from the same team a few years later would produce an ammonia-like body odor that would not be permitted in your kitchen!

A final set of exocrine glands that bears mention is the sebaceous gland, oil-producing glands in face, neck, and upper back and affecting complexion. They cannot be controlled, but if they become clogged, they can cause acne.