Upon reaching a target site, a hormone binds to a receptor, much like a key fits into a lock. Once the hormone locks into its receptor, it transmits a message that causes the target site to take a specific action. Hormone receptors may be within the nucleus or on the surface of the cell.
Ultimately, hormones control the function of entire organs, affecting such diverse processes as growth and development, reproduction, and nutrient metabolisms. Hormones also influence the way the body uses and stores energy and control the volume of fluid and the levels of salts and sugar (glucose) in the blood. Very small amounts of hormones can trigger very large responses in the body.
Although hormones circulate throughout the body, each type of hormone influences only certain organs and tissues. Some hormones affect only one or two organs, whereas others have influence throughout the body. For example, thyroid-stimulating hormone, produced in the pituitary gland Overview of the Pituitary Gland The pituitary is a pea-sized gland that is housed within a bony structure (sella turcica) at the base of the brain. The sella turcica protects the pituitary but allows very little room for expansion. read more , affects only the thyroid gland. In contrast, thyroid hormone, produced in the thyroid gland Overview of the Thyroid Gland The thyroid is a small gland, measuring about 2 inches (5 centimeters) across, that lies just under the skin below the Adam’s apple in the neck. The two halves (lobes) of the gland are connected. read more , affects cells throughout the body and is involved in such important functions as regulating growth of cells, controlling the heart rate, and affecting the speed at which calories are burned. Insulin , secreted by the islet cells of the pancreas, affects the processing (metabolism) of glucose, protein, and fat throughout the body.
To control endocrine functions, the secretion of each hormone must be regulated within precise limits. The body is normally able to sense whether more or less of a given hormone is needed.
Many endocrine glands Endocrine Glands The endocrine system consists of a group of glands and organs that regulate and control various body functions by producing and secreting hormones. Hormones are chemical substances that affect. read more are controlled by the interplay of hormonal signals between the hypothalamus, located in the brain, and the pituitary gland, which sits at the base of the brain. This interplay is referred to as the hypothalamic-pituitary axis. The hypothalamus secretes several hormones that control the pituitary gland.
The pituitary gland Pituitary: The Master Gland , sometimes called the master gland, in turn controls the functions of many other endocrine glands. The pituitary controls the rate at which it secretes hormones through a feedback loop in which the blood levels of other endocrine hormones signal the pituitary to slow down or speed up. So, for example, the pituitary gland senses when blood levels of thyroid hormone are low and releases thyroid stimulating hormone, which tells the thyroid gland to make more hormones. If the thyroid hormone level gets too high, the pituitary senses that and decreases the amount of thyroid-stimulating hormone, which then decreases the amount of thyroid hormone produced. This back-and-forth adjustment (feedback) keeps hormone levels in proper balance.
Many other factors can control endocrine function. For example, a baby sucking on its mother’s nipple stimulates her pituitary gland to secrete prolactin and oxytocin , hormones that stimulate breast milk production and flow. Rising blood sugar levels stimulate the islet cells of the pancreas to produce insulin . Part of the nervous system stimulates the adrenal gland to produce epinephrine .