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　　Hormones in the Body

　　Up to the beginning of the twentieth century, the nervous system was thought to control

all communication within the body and the resulting integration of behavior. Scientists had

determined that nerves ran, essentially, on electrical impulses. These impulses were thought

to be the engine for thought, emotion, movement, and internal processes such as digestion.

However, experiments by William Bayliss and Ernest Starling on the chemical secretin, which

is produced in the small intestine when food enters the stomach, eventually challenged that

view. From the small intestine, secretin travels through the bloodstream to the pancreas.

There, it stimulates the release of digestive chemicals. In this fashion, the intestinal

cells that produce secretin ultimately regulate the production of different chemicals in a

different organ, the pancreas.

　　Such a coordination of processes had been thought to require control by the nervous

system; Bayliss and Starling showed that it could occur through chemicals alone. This

discovery spurred Starling to coin the term hormone to refer to secretin, taking it from the

Greek word hormon, meaning “to excite” or “to set in motion.” A hormone is a chemical

produced by one tissue to make things happen elsewhere.

　　As more hormones were discovered, they were categorized, primarily according to the

process by which they operated on the body. Some glands (which make up the endocrine system)

secrete hormones directly into the bloodstream. Such glands include the thyroid and the

pituitary. The exocrine system consists of organs and glands that produce substances that

are used outside the bloodstream, primarily for digestion. The pancreas is one such organ,

although it secretes some chemicals into the blood and thus is also part of the endocrine system.

　　Much has been learned about hormones since their discovery. Some play such key roles in

regulating bodily processes or behavior that their absence would cause immediate death. The

most abundant hormones have effects that are less obviously urgent but can be more far-

reaching and difficult to track: They modify moods and affect human behavior, even some

behavior we normally think of as voluntary. Hormonal systems are very intricate. Even minute

amounts of the right chemicals can suppress appetite, calm aggression, and change the

attitude of a parent toward a child. Certain hormones accelerate the development of the

body, regulating growth and form; others may even define an individual’s personality

characteristics. The quantities and proportions of hormones produced change with age, so

scientists have given a great deal of study to shifts in the endocrine system over time in

the hopes of alleviating ailments associated with aging.

　　In fact, some hormone therapies are already very common. A combination of estrogen and

progesterone has been prescribed for decades to women who want to reduce mood swings, sudden

changes in body temperature, and other discomforts caused by lower natural levels of those

hormones as they enter middle age. Known as hormone replacement therapy (HRT), the treatment

was also believed to prevent weakening of the bones. At least one study has linked HRT with

a heightened risk of heart disease and certain types of cancer. HRT may also increase the

likelihood that blood clots—dangerous because they could travel through the bloodstream and

block major blood vessels—will form. Some proponents of HRT have tempered their enthusiasm

in the face of this new evidence, recommending it only to patients whose symptoms interfere

with their abilities to live normal lives.

　　Human growth hormone may also be given to patients who are secreting abnormally low

amounts on their own. Because of the complicated effects growth hormone has on the body,

such treatments are generally restricted to children who would be pathologically small in

stature without it. Growth hormone affects not just physical size but also the digestion of

food and the aging process. Researchers and family physicians tend to agree that it is

foolhardy to dispense it in cases in which the risks are not clearly outweighed by the

benefits.

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