Endocrinology Made simple- Human Endocrine System Overview

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Human endocrine system, group of ductless glands that regulate body processes by secreting chemical substances called hormones. Hormones act on nearby tissues or are carried in the bloodstream to act on specific target organs and distant tissues. Diseases of the endocrine system can result from the oversecretion or undersecretion of hormones or from the inability of target organs or tissues to respond to hormones effectively.


Modern endocrinology largely originated in the 20th century, however. Its scientific origin is rooted in the studies of French physiologist Claude Bernard (1813–78), who made the key observation that complex organisms such as humans go to great lengths to preserve the constancy of what he called the “milieu intérieur” (internal environment). Later, American physiologist Walter Bradford Cannon (1871–1945) used the term homeostasis to describe this inner constancy.

The endocrine system, in association with the nervous system and the immune system, regulates the body’s internal activities and the body’s interactions with the external environment to preserve the internal environment. This control system permits the prime functions of living organisms—growth, development, and reproduction—to proceed in an orderly, stable fashion; it is exquisitely self-regulating, so that any disruption of the normal internal environment by internal or external events is resisted by powerful countermeasures. When this resistance is overcome, illness ensues.

The nature of endocrine regulation
Endocrine gland secretion is not a haphazard process; it is subject to precise, intricate control so that its effects may be integrated with those of the nervous system and the immune system. The simplest level of control over endocrine gland secretion resides at the endocrine gland itself. The signal for an endocrine gland to secrete more or less of its hormone is related to the concentration of some substance, either a hormone that influences the function of the gland (a tropic hormone), a biochemical product (e.g., glucose), or a biologically important element (e.g., calcium or potassium). Because each endocrine gland has a rich supply of blood, each gland is able to detect small changes in the concentrations of its regulating substances.

Some endocrine glands are controlled by a simple negative feedback mechanism. For example, negative feedback signaling mechanisms in the parathyroid glands (located in the neck) rely on the binding activity of calcium-sensitive receptors that are located on the surface of parathyroid cells. Decreased serum calcium concentrations result in decreased calcium receptor binding activity that stimulates the secretion of parathormone from the parathyroid glands. The increased serum concentration of parathormone stimulates bone resorption (breakdown) to release calcium into the blood and reabsorption of calcium in the kidney to retain calcium in the blood, thereby restoring serum calcium concentrations to normal levels. In contrast, increased serum calcium concentrations result in increased calcium receptor-binding activity and inhibition of parathormone secretion by the parathyroid glands. This allows serum calcium concentrations to decrease to normal levels. Therefore, in people with normal parathyroid glands, serum calcium concentrations are maintained within a very narrow range even in the presence of large changes in calcium intake or excessive losses of calcium from the body.

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