How the Endocrine System Works
When released by a specific gland, a hormone travels through your bloodstream to get to a specific cell in your body, which is called a target cell. Hormones recognize their target cells by specific receptors. When a hormone arrives at a target cell, the hormone binds to its receptors. The receptors then stimulate a series of chemical reactions inside the cell to produce the hormone’s intended effect, such as the release of a chemical or the “turning on or off” of a gene. After its task has been completed, the hormone’s production needs to be regulated in order to prevent its continuous action upon the cells.
Your endocrine system uses a few different processes to accomplish hormone regulation. When a hormone’s response is no longer needed, the most common type of regulation is to turn off the continual release of the hormone. Your body’s response to stress is an example of this type of regulation.
First, stress causes your hypothalamus to secrete corticotropin-releasing hormone. Then, corticotropin-releasing hormone travels from your hypothalamus to your anterior pituitary gland where it stimulates the release of adrenocorticotrophic hormone (ACTH). Next, ACTH travels from your pituitary gland to your adrenal glands, where it stimulates the cortex (outer layer) to secrete the hormone cortisol. Finally, cortisol stimulates your liver and skeletal muscles to increase metabolism and blood glucose levels in order to provide energy for your body’s responses to stress. When the response is sufficient, the increased cortisol levels in your blood inhibit the release of more corticotropin-releasing hormone and ACTH to turn off the response.
Chemical control of hormone release occurs when one of your endocrine system’s glands senses a drop in the amount of a chemical that your body needs, and reacts by producing more of it. When the hormone has stimulated a sufficient amount of the chemical’s production, the higher levels of the chemical stop the gland from secreting more hormones.
One example of chemical hormone regulation is when your parathyroid hormone regulates your body’s levels of calcium, an important element for proper cellular function. Low calcium levels stimulate your parathyroid gland to secrete parathyroid hormone, which stimulates your body to increase the amount of calcium in your blood. As blood calcium levels rise, the secretion of parathyroid hormone decreases.
Nerves in the body can also control the release of hormones. An example of neural control of hormone release occurs during childbirth when the head of a fetus pushes against the cervix. The stretching and pressure detected by the nerves in the cervix cause the release of the hormone oxytocin from the mother's posterior pituitary gland. This hormone causes the uterus to contract more, resulting in the further release of oxytocin and more contractions. In this case, the ultimate birth of a child turns off the loop, as the stretching signals from the cervix are lost. When physicians induce labor, they use a synthetic form of oxytocin and cause the uterus to contract.