Hormones – endocrine system
„The endocrine system may be compared to an orchestra, in which when an instrument is out of tune, a perfect ensemble is impossible . . . . The leader of the glandular orchestra is the pituitary. It is not only the conductor of the orchestra, but it itself plays several important instruments in the ensemble, as well as acting as a link or mediator between soma and psyche. It is constantly monitoring the internal environment, and it is ideally situated to function in response to psychic stimuli.”
William Boyd (1971)
Regulation and integration of the myriad functions carried out by the body are accomplished by the cooperative efforts of the nervous system and endocrine system. The endocrine glands, via their chemical messengers, the hormones, act to maintain a constant internal body environment, in spite of marked changes occurring in the external environment and within the organism. When, however, a deficient or excessive quantity of hormones is released by these glands, the internal environment of the body may become markedly disrupted.
Naturally occurring hormones or their synthetic analogues are employed therapeutically when a hormone deficiency exists; this is an example of replacement therapy. Conversely, when excessive amounts of hormones are released, drugs may be employed to reduce hormonal synthesis or release, or to antagonize their physiological effects. In 1891 extracts prepared from sheep thyroid were shown to be effective in treating human patients with decreased thyroid gland function. The modern concept of hormones was finally crystallized in 1902 by Bayliss and Starling, when they demonstrated that a chemical secretion released from the upper segment of the small intestines, termed secretin, was able to activate the secretion of pancreatic juice. In 1905 Starling coined the word “hormone” (from a Greek word meaning setting into motion) to refer to secretin. Based on our present concept of endocrine chemicals, this word is inappropriate, but it has nevertheless gained international acceptance.
Efforts in the twentieth century have been directed toward the isolation and identification of the hormones released from endocrine glands, and, in more recent years, scientists have been concerned with determining the mechanism of hormonal action at a molecular level. We may group the physiological functions of hormones into three categories: regulation of metabolism; enhancement of growth and development; and, effects on the nervous system.
The pituitary gland or hypophysis is a relatively small structure located at the base of the brain and connected to the hypothalamus by the hypophyseal stalk. Anatomically and functionally, the hypophysis is divided into two separate and independent portions: the adenohypophysis or anterior pituitary gland and the neurohypophysis or posterior pituitary gland. Separating these two parts is the intermediate lobe. The anterior pituitary gland produces at least six hormones, whose release is activated by specific releasing factors arising from the hypothalamus. The adenohypophyseal hormones regulate the growth of body tissues. The posterior pituitary gland releases two hormones, antidiuretic hormone (vasopressin) and oxytocin. The release of hormones by a number of endocrine glands is controlled by the hypothalamus and the anterior pituitary gland.