CHAPTER 145
Biology of the Endocrine System
The endocrine system consists of a group of glands and organs that regulate and control various body functions by producing and secreting hormones. The glands of the endocrine system do not have ducts but rather release their hormones directly into theë bloodstream. While the individual organs that comprise the endocrine system have different and often unrelated functions, they are traditionally grouped together, although this organization may not have much significance for nondoctors. Doctors who specialize in disorders of the endocrine system are known as endocrinologists. Many endocrinologists further subspecialize in the functions and disorders of specific glands.
Endocrine Glands
The major glands of the endocrine system, each of which produces one or more specific hormones, are the hypothalamus, the pituitary gland, the thyroid gland, the parathyroid glands, the islets of the pancreas, the adrenal glands, the testes in men, and the ovaries in women. During pregnancy, the placenta also acts as an endocrine gland in addition to its other functions.
Not all organs that secrete hormones or hormonelike substances are considered part of the endocrine system. For example, the kidneys produce the hormone renin to help control blood pressure and the hormone erythropoietin to stimulate the bone marrow to produce red blood cells. In addition, the gastrointestinal system produces a variety of hormones that control digestion, affect insulin secretion from the pancreas, and alter behaviors, such as those associated with hunger. Fat (adipose) tissue also produces hormones that regulate metabolism and appetite. Additionally, the term “gland” does not mean that the organ is part of the endocrine system. For example, sweat glands, glands in mucus membranes, and mammary glands secrete substances other than hormones.
Endocrine Function
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.
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 sexual characteristics. Hormones also influence the way the body uses and stores energy and control the volume of fluid and the levels of salts and sugar in the blood. Very small amounts of hormones can trigger very large responses in the body.
Major Endocrine Glands
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 proteins. Others are steroids, which are fatty substances derived from cholesterol.
MAJOR HORMONES
| WHERE HORMONE IS PRODUCED | HORMONE | FUNCTION |
| Pituitary gland | Antidiuretic hormone (vasopressin) | Causes kidneys to retain water and, along with aldosterone, helps control blood pressure |
| Corticotropin (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, egg maturation, and menstrual cycles Control male and female sexual characteristics (including hair distribution, muscle formation, skin texture and thickness, voice, and perhaps even personality traits) | |
| Oxytocin | Causes muscles of the uterus and milk ducts in the breast to contract | |
| 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 | |
| Parathyroid glands | Parathyroid hormone | Controls bone formation and the excretion of calcium and phosphorus |
| Thyroid gland | Thyroid hormone | Regulates the rate at which the body functions (metabolic rate) |
| Calcitonin | In people, has unclear function but in other species, regulates calcium balance | |
| Adrenal glands | Aldosterone | Helps regulate salt and water balance by retaining salt and water and excreting 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) | Has effects on bone, mood, and the immune system | |
| Epinephrine and norepinephrine | Stimulate the heart, lungs, blood vessels, and nervous system | |
| 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 | |
| Kidneys | Erythropoietin | Stimulates red blood cell production |
| Renin | Controls 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 | |
| Testes | Testosterone | Controls the development of male sex characteristics and the reproductive system |
| Digestive tract | Cholecystokinin | Controls gallbladder contractions that cause bile to enter the intestine Stimulates release of digestive enzymes from the pancreas |
| Glucagon-like peptide | Increases insulin release from the pancreas | |
| Ghrelin | Controls growth hormone release from the pituitary gland Causes sensation of hunger | |
| Adipose (fat) tissue | Resistin | Blocks the effects of insulin on muscle |
| Leptin | Controls appetite | |
| 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 |
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, sometimes called the master gland, in turn controls the functions of many other endocrine glands (see page 983). 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.
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.
Endocrine Disorders
Endocrine disorders involve either too much or too little hormone secretion. Disorders may result from a problem in the gland itself or because the hypothalamic-pituitary axis provides too much or too little stimulation. Depending on the type of cell they originate in, tumors can produce excess hormones or squeeze out normal glandular tissue, decreasing hormone production. Sometimes the body’s immune system (see page 1124) attacks an endocrine gland, decreasing hormone production.
Doctors usually measure levels of hormones in the blood to tell how an endocrine gland is functioning. Sometimes blood levels alone do not give enough information about endocrine gland function, so doctors measure hormone levels after giving a stimulus (such as a sugar-containing drink, a drug, or a hormone that can trigger hormone release) or after having the patient take an action (such as fasting).
Effects of Aging
Levels of most hormones decrease with aging, but some hormones remain at levels typical of those in younger adults, and some even increase. Even when hormone levels do not decline, endocrine function generally declines with age because hormone receptors become less sensitive. Although such decreased function suggests that hormone replacement therapy might be beneficial in older people, such therapy generally does not appear to reverse aging or prolong life and, in some cases (such as estrogen replacement in older women), is potentially harmful. However, ongoing research is examining the beneficial effects of providing some hormones to older people.