Hormones

Hormones are chemical messengers found in both animals and plants. In animals, hormones are produced by glands and travel through the blood to certain target tissues. There they act as chemical regulators. Hormones influence reproduction, growth, and overall bodily balance, among other things.

Hormones are important to both plants and animals, but especially to animals. Hormones regulate key bodily functions like body growth, sexual maturity, reproduction, and the maintenance of a stable, or balanced, internal environment. Some hormones have a temporary effect, such as those that regulate the body's blood sugar level. Others cause permanent changes, such as those that make a person grow tall and mature sexually. Still others are present only in certain situations, such as those that prepare a body for stressful situations. Whatever their effect, hormones help an organism to adapt to its environment in the best manner possible. The word hormone comes from the Greek hormaein meaning to excite or to set into motion, and this describes what hormones do—they have a stimulating effect.

In vertebrate animals (animals with a backbone), hormones are produced by certain glands, tissues, or organs. They travel through the circulatory system (a network that carries blood throughout an animal's body) to target cells. Hormones do not produce an effect until they reach these specifically receptive cells. The target cells are programmed to react when stimulated by a certain hormone. Only the target cells in the target organ are able to produce the desired effect, since they have receptors that recognize and bind to the hormone.

THE ENDOCRINE SYSTEM

It is estimated that vertebrates have at least fifty different hormones, and many are produced by what is called the endocrine system. Some of the major glands in the human endocrine system are the pituitary gland and the pineal gland at the base of the brain; the thyroid and parathyroid in the throat; and the adrenal, gonads, thymus, and pancreas in the trunk or lower half of the body. Each of these endocrine glands releases its own particular hormone into the bloodstream and each produces the desired effect when it reaches the appropriate target cells. Thus, some hormones

stimulate the growth of muscle and bone, others begin the secretion of milk, and still others influence blood pressure. All are usually produced in tiny quantities, yet all have profound and major effects on the body.

HORMONES IN HUMANS

For example, female sex hormones, like the group called estrogens, are produced by the pituitary gland as well as the ovaries. These hormones produce the dramatic physical changes that take place when a young girl starts to become a young woman. The estrogens trigger the development of what are called secondary sexual characteristics, like breasts. Later, if a woman becomes pregnant, these and other hormones prepare her body to carry and nourish a developing fetus. The male sex hormone, testosterone, is made by the testes and produces the typical male secondary sex characteristics during puberty.

Another well-known hormone is adrenaline. This produces what is known as the "fight-or-flight" response. When an individual senses danger, the body automatically enters a state of readiness to either fight for survival or to take measures to avoid a conflict. This powerful hormone works with great suddenness and a person can actually feel its effects. The heart rate and blood pressure quicken, the skin goes pale, the body's blood sugar level rises, and a person's strength increases. All of these and other physiological reactions take place immediately and without the person's conscious will. These reactions give people a better chance to act immediately and possibly survive a threat.

In humans and animals, hormones are made of either proteins or steroids (which are a type of lipid or organic compound that includes fats, oils, and waxes). The adrenal gland and the gonads (male testes and female ovaries) produce steroid hormones, while the rest of the endocrine system makes hormones that are protein-based and, therefore, are made out of amino acids (the building blocks of proteins). Abnormally functioning endocrine glands can result in too much or too little hormones. For example, a lack of human growth hormone from the pituitary gland can result in dwarfism, while too much can produce giants who suffer from a condition known as acromegaly.

HORMONES IN OTHER ORGANISMS

While other living things also have hormones, their hormones are nowhere near as dominating an influence as they are for animals. Other organisms do not have as elaborate a system of transport and reception as do vertebrates. However, hormones are still very important to invertebrates (animals without a backbone). Invertebrates use hormones mainly in their growth and development. For example, insects that molt, or periodically shed, their skin produce a hormone that allows this to happen at the right time. Metamorphosis (the complete bodily change that takes place in an insect, such as when a caterpillar changes into a butterfly) is controlled by hormones. When an octopus changes its color during stress, it is a hormone that causes this dramatic reaction.

ERNEST HENRY STARLING

English physiologist Ernest Starling (1866–1927) helped create endocrinology, a major branch of medicine and physiology that studies the glands of the body. Starling not only discovered the digestive hormone secretin, but he also suggested the name "hormone" to describe the body's chemical messengers.

Ernest Starling was born in London, England. His father worked in Bombay, India, for the British government and was able to come home only once every three years. The young Starling was educated at King's College School in London and then enrolled at Guy's Hospital Medical College in London. While there, he took advantage of the opportunity to do research in Heidelberg, Germany, and study with the eminent German physiologist, Wilhelm Kuhne (1837–1900). After obtaining his medical degree in 1889, he was appointed demonstrator of physiology at Guy's, and he eventually became head of the department of physiology there. By the time he left there in 1899 to become Professor of Physiology at University College, he had made a name for himself studying the conditions that cause fluids to leave blood vessels and enter the tissues. In fact, in 1896 he demonstrated a phenomenon that came to be known as "Starling equilibrium."

Starling is best known, however, for his work with the English physiologist William Maddock Bayliss (1860–1924), who became his brother-in-law in 1893 when he married Starling's sister. Together they began a study of the secretion of digestive juices by the pancreas (a gland). The normal pancreas releases several different juices into the duodenum (the top part of the small intestine) to assist digestion. After they had cut all the nerves to the pancreas, they found that the organ continued to release its juices. This proved that it was not under nervous control (that is, not controlled directly by the brain), and so they concluded in 1902 that it must have received a chemical rather than an electrical message. This meant that the message must have been sent to the pancreas through the blood when food entered the duodenum.

They soon found that the small intestine secretes a substance, or a chemical messenger, into the blood that they named "secretin." Further research showed that the secretin was released under the influence of stomach acid. This was the first time that a certain chemical had been proven to act as a stimulus for an organ that was located somewhere else in the body. Starling and Bayliss eventually came to call any chemical that transmits a message from one part of the body to another part a "hormone." This word was taken from the Greek root meaning "to excite." Although hormones had actually been known before the discovery of secretin in 1902, it was Starling who first clearly defined the concept in 1905 and who detailed the role that such substances play in the body. It was thought that Starling and Bayliss were strong candidates for a Nobel Prize, but World War I (1914–18) intervened, and no awards were given for those years. As for recognition from his own country, Starling had been such an outspoken critic of the way his country had managed the war effort that he was given no awards in his lifetime.

For plants, hormones allow them to react to the changing conditions of their environment. Some hormones promote cell division, others stimulate or slow growth, and others cause a plant's fruit to ripen. Plants do not have specialized structures for hormones as animals do. In fact, a plant can even be affected by the hormone of its neighbor. This sometimes occurs when a plant releases the ripening hormone called ethylene into the atmosphere. Thus, a fruit like an apple continues to produce this ripening hormone even after it is picked, and will therefore speed up the ripening of any other fruit nearby.

[See alsoEndocrine System; Reproduction System ]