The main function of endocrine glands is to secrete hormones directly into the bloodstream. Hormones are chemical substances that affect the activity of another part of the body (target site). In essence, hormones serve as messengers, controlling and coordinating activities throughout the body. (See also Endocrine Glands.)
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, affects only the thyroid gland. In contrast, thyroid hormone, produced in the thyroid gland, 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.
Most hormones are derived from proteins or their building blocks (called amino acids). Others are steroids, which are fatty substances derived from cholesterol.
Major Hormones
Where Hormone Is Produced | Hormone | Function |
|---|---|---|
Adipose (fat) tissue | Leptin | Regulates energy balance by controlling appetite |
Resistin | Blocks the effects of insulin on muscle | |
Adrenal glands | Aldosterone | Helps regulate salt and water balance by causing the kidneys to retain salt and water and excrete potassium |
Cortisol | Has widespread effects throughout the body Especially has anti-inflammatory action Maintains blood sugar level, blood pressure, and muscle strength Helps control salt and water balance | |
Dehydroepiandrosterone (DHEA) | Used in the production of androgens (male sex hormones) and estrogens (female sex hormones) Has effects on bone, mood, and the immune system | |
Epinephrine and norepinephrine | Stimulate the heart, lungs, blood vessels, and nervous system | |
Digestive tract | Cholecystokinin | Controls gallbladder contractions that cause bile to enter the intestine Stimulates release of digestive enzymes from the pancreas |
Ghrelin | Controls growth hormone release from the pituitary gland Causes sensation of hunger | |
Glucagon-like peptide | Increases insulin release from the pancreas | |
Vasoactive intestinal polypeptide | Controls smooth muscle cell activity, epithelial cell secretions, and blood flow in the gastrointestinal tract | |
Hypothalamus | Corticotropin-releasing hormone | |
Gonadotropin-releasing hormone | Stimulates release of luteinizing hormone and follicle-stimulating hormone | |
Growth hormone–releasing hormone | Stimulates release of growth hormone | |
Somatostatin | Inhibits release of growth hormone, thyroid-stimulating hormone, and insulin | |
Thyrotropin-releasing hormone | Stimulates the release of thyroid-stimulating hormone and prolactin | |
Kidneys | Erythropoietin | Stimulates red blood cell production |
Renin | Controls sodium, potassium, and water levels that affect blood pressure | |
Ovaries | Estrogen | Controls the development of female sex characteristics and the reproductive system |
Progesterone | Prepares the lining of the uterus for implantation of a fertilized egg and readies the mammary glands to secrete milk | |
Pancreas | Glucagon | Raises the blood sugar level |
Insulin | Lowers the blood sugar level Affects the processing (metabolism) of sugar, protein, and fat throughout the body | |
Parathyroid glands | Parathyroid hormone | Controls bone formation, blood calcium level, and the excretion of calcium and phosphorus by the kidneys |
Pituitary gland | Corticotropin (also called adrenocorticotropic hormone [ACTH]) | Controls the production and secretion of hormones by the adrenal glands |
Growth hormone | Controls growth and development Promotes protein production | |
Luteinizing hormone and follicle-stimulating hormone | Control reproductive functions, including the production of sperm and semen in men and egg maturation and menstrual cycles in women Control male and female sexual characteristics (including hair distribution, muscle formation, skin texture and thickness, pitch of voice, and perhaps even personality traits) | |
Oxytocin | Causes muscles of the uterus to contract during childbirth and after delivery and stimulates contractions of milk ducts in the breast, which move milk to the nipple | |
Prolactin | Starts and maintains milk production in the ductal glands of the breast (mammary glands) | |
Thyroid-stimulating hormone | Stimulates the production and secretion of hormones by the thyroid gland | |
Vasopressin (antidiuretic hormone) | Causes kidneys to retain water to prevent dehydration and, along with aldosterone, helps control blood pressure | |
Placenta | Chorionic gonadotropin | Stimulates ovaries to continue to release progesterone during early pregnancy |
Estrogen and progesterone | Keep uterus receptive to fetus and placenta during pregnancy | |
Testes | Testosterone | Controls the development of male sex characteristics and the reproductive system |
Thyroid gland | Calcitonin | Tends to decrease blood calcium levels and helps regulate calcium balance |
Thyroid hormones | Regulate the rate at which the body functions (metabolic rate) |
Endocrine Controls
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 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, 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.
