17.5 Physiology of the Endocrine System

The endocrine system communicates with other parts of the body by secreting hormones from the endocrine glands. Hormones are transported primarily via the bloodstream throughout the body. Each hormone binds to receptors on specific target cells, creating a desired response.^[1]

For example, when a person is exposed to a dangerous situation, the “fight-or-flight response” (fīt-ŏr-flīt rĭ-spŏns) occurs. The fight-or-flight response is triggered by the release of epinephrine and norepinephrine hormones from the adrenal gland. These hormones act on target organs like the heart (to beat faster), the lungs (to breathe more efficiently), and blood vessels to the skeletal muscles (to provide more oxygenated blood with glucose for energy). These targeted actions are intended to help the individual to fight or run away from the dangerous situation.^[2]

Although hormones travel throughout the body, they only affect cells with receptors for that particular hormone, called target cells. After the hormone binds to the receptor, a chain of events is initiated that leads to the target cell’s response. Hormones play a critical role in the regulation of physiological processes such as metabolism, growth and development of body tissues, fluid, electrolyte balance, sleep, and reproduction.^[3]