The endocrine system produces hormones that function to control and regulate many different body processes. The endocrine system coordinates with the nervous system to control the functions of the other organ systems. Cells of the endocrine system produce molecular signals called hormones. These cells may compose endocrine glands, may be tissues or may be located in organs or tissues that have functions in addition to hormone production. Hormones circulate throughout the body and stimulate a response in cells that have receptors able to bind with them. The changes brought about in the receiving cells affect the functioning of the organ system to which they belong. Many of the hormones are secreted in response to signals from the nervous system, thus the two systems act in concert to effect changes in the body.
Maintaining balance within the body requires the coordination of many different systems and organs. One mechanism of communication between neighboring cells, and between cells and tissues in distant parts of the body, occurs through the release of chemicals called hormones. Hormones are released into body fluids, usually blood, which carries them to their target cells where they elicit a response. The cells that secrete hormones are often located in specific organs, called endocrine glands, and the cells, tissues, and organs that secrete hormones make up the endocrine system. Examples of endocrine organs include the pancreas, which produces the hormones insulin and glucagon to regulate blood-glucose levels, the adrenal glands, which produce hormones such as epinephrine and norepinephrine that regulate responses to stress, and the thyroid gland, which produces thyroid hormones that regulate metabolic rates. In addition, many organs in other body systems such as the reproductive system also secrete hormones and are therefore also part of endocrine system.
The level of hormones present in the blood is directly related to the magnitude of the responses, or actions, performed by the target cells. For this reason, the body has evolved mechanisms to tightly regulate the level of hormone production for many hormones. The release of hormones by endocrine cells is usually related to the amount of stimulus the receive. A stimulus is any variable that can change the level of hormone secreted by an endocrine cell; common stimuli include nutrients, neurotransmitters, and even other hormones. Most stimuli have a positive effect on endocrine cells, that is, with increasing amounts of stimuli increasing amounts of hormone are secreted. Some stimuli are negative regulators of endocrine cells, however. When increased negative stimuli are present, hormone secretion decreases. Many endocrine cells integrate inputs from multiple stimuli to regulate their level of hormone secretion.
How Hormones Work
Hormones cause changes in target cells by binding to specific cell-surface or intracellular hormone receptors, molecules embedded in the cell membrane or floating in the cytoplasm with a binding site that matches a binding site on the hormone molecule. In this way, even though hormones circulate throughout the body and come into contact with many different cell types, they only affect cells that possess the necessary receptors. Receptors for a specific hormone may be found on or in many different cells or may be limited to a small number of specialized cells. For example, thyroid hormones act on many different tissue types, stimulating metabolic activity throughout the body. Different types of cells can also respond to hormones in different ways. For example, immune cell function is decreased when the hormone cortisol acts upon them, but cortisol causes liver cells to do something completely different, release glucose into the blood. Cells can have many receptors for the same hormone but often also possess receptors for different types of hormones.
- endocrine gland
- an organ containing endocrine cells, cells that secrete hormones
- a chemical secreted by an endocrine gland into body fluids (usually blood) that affects the function of target cells
- hormone receptor
- a molecule located on the surface of a cell, or inside it, that binds to a hormone; binding of a hormone to its receptor causes changes in the cell with the receptor
- a signal that increases or decreases the level of hormone secreted by an endocrine cell
- a cell that expresses a hormone receptor, and changes its functions after the hormone binds to the receptor