Important Point: Chemical messengers from the endocrine system help regulate body activities. Their effect is of longer duration and is more generalized than that of the nervous system.The endocrine system works in concert with the nervous system to control and coordinate the functions of the other organ systems.
Topics in This Chapter
The endocrine system involves the secretion and release of chemical messengers known as hormones throughout the body via the bloodstream. The organs that make up the endocrine system are called the endocrine glands. Hormones contribute to the control of organ function, growth and development, reproduction and sexual characteristics, how the body uses and stores energy (metabolism), and the volume of fluids and levels of salt and sugar in the blood. The endocrine system works in parallel with the nervous system to control growth and maturation along with homeostasis.
Important Point: Chemical messengers from the endocrine system help regulate body activities. Their effect is of longer duration and is more generalized than that of the nervous system.
Before beginning a discussion of the human endocrine system, it is important to list and define pertinent terms:
• Adrenal—secretes androgens (male hormones) and aldosterone, which helps maintain the body’s salt and potassium balances. Also epinephrine (adrenaline) and norepinepherin (noradrenaline), which are involved in “fight or flight’ responses.
• Antagonistic hormones—hormones that have an opposite effect on the body.
• Endocrine glands—ductless glands that are located throughout the body.
• Exocrine glands—secrete nonhormonal chemicals into ducts, which transport the chemicals to a specific location inside and outside the body.
• Feedback mechanism—the last step in a series of events controls the first step.
• Glands—group of cells that produce and secretes, or gives off, chemicals.
• Homeostasis—stable internal environment.
• Hormone—is a chemical signal, made in one place and delivered to another that regulates the body’s activities.
• Hypothalamus—secretes “supervisory” hormone to regulate the pituitary gland.
• Negative feedback—release of an initial hormone stimulates release or production of other hormones or substances that subsequently inhibits further release of the initial hormone.
• Ovaries—secrete “female” hormones estrogen, which develops and maintains female characteristics, and progesterone, which prepares the uterus for pregnancy.
• Pancreas—secretes insulin, the hormones controlling the use of sugar in the body, and other hormones involved with sugar metabolism.
• Parathyroid—regulates the use of calcium and phosphorus.
• Pineal—secretes melatonin, a hormone involved with daily biological rhythms.
• Pituitary—the master gland, secretes hormones that influence many other glands and organs, affecting growth and reproduction.
• Positive feedback—release of an initial hormone stimulates release or production of other hormones or substances, which stimulates further release of the initial hormone.
• Prostaglandins—group of hormone-like lipids that also regulate cell activities.
• Receptors—proteins that are located both inside the cytoplasm and on the surface of a target cell.
• Steroid hormones—body synthesizes from cholesterol.
• Thymus—stimulates T-cell development of the immune system.
• Thyroid—regulates metabolism and blood calcium levels.
Important Point: Neurons are the nerve cells that transmit impulses. Supporting cells are called neuroglia.
There are 2 major categories of glands in the body—exocrine and endocrine:
• Exocrine glands—(Greek: exo, meaning “outside, beyond,” and krine, meaning “to separate or secrete”) have ducts that carry their secretory product to a surface. These glands include the sweat, sebaceous, and mammary glands, and the glands that secrete digestive enzymes.
• Endocrine glands—(Greek: endo, meaning “within”) consists of a group of organs, often referred to as glands of internal secretion. The endocrine glands do not have ducts to carry their product to a surface. The secretory products of endocrine glands are called hormones and are secreted directly into the blood and then carried throughout the body where they influence only those cells that have receptor sites for that hormone.
Important Point: Endocrine glands secrete hormones directly into the blood, which transports the hormones through the body.
Chemically, hormones may be classified as either proteins or steroids (i.e., lipids derived form cholesterol). All of the hormones in the human body, except the sex hormones and those from the adrenal cortex, are proteins or protein derivatives.
The so-called action hormones are carried by the blood throughout the entire body, yet they affect only certain cells. The specific cells that respond to a given hormone have receptor sites for that hormone. This is sort of a lock and key mechanism. If the key fits the lock, then the door will open. If a hormone fits the receptor site, then there will be an effect. If a hormone and a receptor site do not match, then there is no reaction. All the cells that have receptor sites for a given hormone make up the target tissue for that hormone. In some cases, the target tissue is localized in a single gland or organ. In other case, the target tissue is diffuse and scattered throughout the body so that many areas are affected. Hormones bring about their characteristic effects on target cells by modifying cellular activity.
Important Point: Cells in a target tissue have receptor sites for specific hormones.
Protein hormones react with receptors on the surface of the cell, and the sequence of events that results in hormone action is relatively rapid. Steroid hormones typically react with receptor sites inside a cell. Because this method of action actually involves synthesis of proteins, it is relatively slow.
Hormones are very potent substances, which means that very small amounts of a hormone may have profound effects on metabolic processes. Because of their potency, hormone secretion must be regulated within very narrow limits in orders to maintain homeostasis in the body.
Many hormones are controlled by some form of a negative feedback mechanism. In this type of system, a gland is sensitive to the concentration of a substance that it regulates. A negative feedback system causes a reversal of increases and decreases in body conditions in order to maintain a state of stability or homeostasis. Some endocrine glands secrete hormones in response to other hormones. The hormones that cause secretion of other hormones are called tropic hormones. A hormone from gland A causes gland B to secrete its hormone. A third method of regulating hormone secretion is by direct nervous stimulation. A nerve stimulus causes gland A to secrete its hormone.
Important Point: Many hormones are regulated by a negative feedback mechanism; some are controlled by other hormones and others are affected by direct nerve stimulation.
As mentioned, the endocrine system is made up of the endocrine glands that secrete hormones. Although there are 8 major endocrine glands scattered throughout the body, they are still considered to be a single system because they have similar functions, similar mechanisms of influence, and many important interrelationships.
Although not listed below as 1 of the 8 major endocrine glands, the hypothalamus is a collection of specialized cells located in the lower central part of the brain and is the main link between the endocrine and nervous system. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.
Some glands have nonendocrine regions that have functions other than hormone secretion. For example, the pancreas has a major exocrine portion that secretes digestive enzymes and an endocrine portion that secretes hormones. The ovaries and testes secrete hormones and also produce the ova and sperm. Some organs, such as the stomach, intestines, and heart, produce hormones, but their primary function is not hormone secretion.
The major glands that make up the human endocrine include the:
• Pituitary
• Pineal
• Thyroid
• Parathyroid
• Adrenal
• Pancreas
• Gonads
• Other Endocrine Glands
The pituitary gland or hypophysis (often called the “master gland”) is a small gland about 1 centimeter in diameter or the size of a pea. It is nearly surrounded by bone as it rests in the sella turcica, a depression in the sphenoid bone. The gland is connected to the hypothalamus of the brain by a slender stalk called the infundibulum. The production and secretion of pituitary hormones can be influenced by factors such as emotions and changes in the seasons. There are 2 distinct regions in the gland: the anterior lobe and the posterior lobe. The activity of the anterior lobe is controlled by releasing hormones from the hypothalamus. The anterior lobe is controlled by nerve stimulation.
1. Growth hormone is a protein that stimulates the growth of bones, muscles, and other organs by promoting protein synthesis and the handling of nutrients and minerals. This hormone drastically affects the appearance of an individual because it influences height. If there is too little growth hormone in a child, that person may become a pituitary dwarf of normal proportions but small stature. An excess of the hormone in a child results in an exaggerated bone growth, and the individual becomes exceptionally tall or a giant.
2. Thyroid-stimulating hormone, or thyrotropin, stimulates the glandular cells of the thyroid to secrete thyroid hormone. When there is a hypersecretion of thyroid-stimulating hormone, the thyroid gland enlarges and secretes too much thyroid hormone.
3. Adrenocorticotropic hormone reacts with receptor sites in the cortex of the adrenal gland to stimulate the secretion of cortical hormones, particularly cortisol.
4. Gonadotropic hormones react with receptor sites in the gonads, or ovaries and testes, to regulate the development, growth, and function of these organs.
5. Prolactin hormone promotes the development of glandular tissue in the female breast during pregnancy and stimulates (activates) milk production after the birth of the infant.
1. Antidiuretic hormone promotes the reabsorption of water by the kidney tubules, with the result that less water is lost as urine. This mechanism conserves water for the body. Insufficient amounts of antidiuretic hormone cause excessive water loss in the urine.
2. Oxytocin causes contraction of the smooth muscle in the wall of the uterus. It also stimulates the ejection milk from the lactating breast.
The pineal gland, also called pineal body, is a small cone-shaped structure (in the middle of the brain) that extends posteriorly from the third ventricle of the brain. The pineal gland consists of portions of neurons, neuroglial cells, and specialized secretory cells called pinealocytes. The pinealocytes synthesize the hormone melatonin and secrete it directly into the cerebrospinal fluid, which takes it into the blood. Melatonin affects reproductive development and daily physiologic cycles (it may help regulate sleep at night and waking in the morning).
The thyroid gland, located in the front part of the lower neck, consists of 2 lobes, 1 on each side of the trachea, just below the larynx or voice box; it is shaped like a bow tie or butterfly. The 2 lobes are connected by a narrow band of tissue called the isthmus. Internally, the gland consists of follicles, which produce thyroxine and triiodothyronine hormones. These hormones contain iodine and control the rate at which cells burn fuels from food to produce energy.
About 95% of the active thyroid hormone is thyroxine, and most of the remaining 5% is triiodothyronine. Both of these require iodine for their synthesis. Thyroid hormone secretion is regulated by a negative feedback mechanism that involves the amount of circulating hormone, hypothalamus, and adenohypophysis.
If there is an iodine deficiency, the thyroid cannot make sufficient hormone. This stimulates the anterior pituitary to secrete thyroid-stimulating hormone, which causes the thyroid gland to increase in size in a vain attempt to produce more hormones. But it cannot produce more hormones because it does not have the necessary raw material, iodine. This type of thyroid enlargement is called simple goiter or iodine deficiency goiter.
Calcitonin is secreted by the parafollicular cells of the thyroid gland. This hormone opposes the action of the parathyroid glands by reducing the calcium level in the blood. If blood calcium becomes too high, calcitonin is secreted until calcium ion levels decrease to normal.
Attached to the thyroid are 4 tiny glands (embedded in the connective tissue capsule on the posterior surface of the thyroid) that function together, called the parathyroids. They release parathyroid hormone, which regulates the level of calcium in the blood with the help of calcitonin, which, as mentioned, is produced in the thyroid.
Hypoparathyroidism, or insufficient secretion of parathyroid hormone, leads to increased nerve excitability. The low blood calcium levels trigger spontaneous and continuous nerve impulses, which then stimulate muscle contraction.
The adrenal, or suprarenal, gland is paired with 1 gland located near the upper portion of each kidney. Each gland is divided into an outer cortex and an inner medulla. The cortex and medulla of the adrenal gland, like the anterior and posterior lobs of the pituitary, develop from different embryonic tissues and secrete different hormones. The adrenal cortex is essential to life, but the medulla may be removed with no life-threatening effects.
The hypothalamus influences both portions of the adrenal gland but by different mechanisms. The adrenal cortex is regulated by negative feedback involving the hypothalamus and adrenocorticotropic hormone; the medulla is regulated by nerve impulses from the hypothalamus.
The adrenal cortex consists of 3 different regions, with each region producing a different group of hormones. Mineralocorticoids are secreted by the outermost region of the adrenal cortex. The principal mineralocorticoid is aldosterone, which acts to conserve sodium ions and water in the body. Glucocorticoids are secreted by the middle region of the adrenal cortex. The principal glucocorticoid is cortisol. The third group of steroids secreted by the adrenal cortex is the gonadocorticoids, or sex hormones. These are secreted by the innermost region. Male hormones (androgens) and female hormones (estrogens) are secreted in minimal amounts in both sexes by the adrenal cortex, but their effect is usually masked by the hormones from the testes and ovaries. In females, the masculinization effect of androgen secretion may become evident after menopause, when estrogen levels from the ovaries decrease.
Important Point: Chemically, all the cortical hormones are steroid.
The adrenal medulla develops from neural tissue and secretes 2 hormones, epinephrine (also called adrenaline) and norepinephrine. These 2 hormones are secreted in response to stimulation by sympathetic nerve, particularly during stressful situations. A lack of hormones from the adrenal medulla produces no significant effects. Hypersecretion, usually from a tumor, causes prolonged or continual sympathetic responses.
The pancreas is also part of the body’s hormone-secreting system, even though it is also associated with the digestive system because it produces and secretes digestive enzymes. The pancreas is a long, soft organ that lies transversely along the posterior abdominal wall, posterior to the stomach, and extends from the region of the duodenum to the spleen. In addition to others, the pancreas produces 2 important hormones, insulin and glucagon. They work together to maintain a steady level of glucose, or sugar, in the blood and to keep the body supplied with fuel to produce and maintain stores of energy.
The gonads, the primary reproductive organs, are the testes in the male and the ovaries in the female. These organs are responsible for producing the sperm and ova, but they also secrete hormones and are considered to be endocrine glands.
Male sex hormones, as a group, are called androgens. The principal androgen is testosterone, which is secreted by the testes. A small amount is also produced by the adrenal cortex. Production of testosterone begins during fetal development, continues for a short time after birth, nearly ceases during childhood, and then resumes at puberty. This steroid hormone is responsible for:
• The growth and development of the male reproductive structures
• Increased skeletal and muscular growth
• Enlargement of the larynx accompanied by voice changes
• Growth and distribution of body hair
• Increased male sexual drive
Testosterone secretion is regulated by a negative feedback system that involves releasing hormones from the hypothalamus and gonadotropins form the anterior pituitary.
Two groups of female sex hormones are produced in the ovaries: the estrogens and progesterone. These steroid hormones contribute to the development and function of the female reproductive organs and sex characteristics. At the onset of puberty, estrogens promote:
• The development of the breasts
• Distribution of fat evidenced in the hips, legs, and breast
• Maturation of reproductive organs such as the uterus and vagina
Progesterone causes the uterine lining to thicken in preparation for pregnancy. Together, progesterone and estrogens are responsible for the changes that occur in the uterus during the female menstrual cycle.
In addition to the major endocrine glands, other organs have some hormonal activity as part of their function. These include the thymus, stomach, small intestines, heart, and placenta.
• Thymosin, produced by the thymus gland, plays an important role in the development of the body’s immune system.
• The lining of the stomach, the gastric mucosa, produces a hormone called gastrin in response to the presence of food in the stomach. This hormone stimulates the production of hydrochloric acid and the enzyme pepsin, which are used in the digestion of food.
• The mucosa of the small intestine secretes the hormones secretin and cholecystokinin. Secreting stimulates the pancreas to produce a bicarbonate-rich fluid that neutralizes the stomach acid. Cholecystokinin stimulates contraction of the gallbladder, which releases bile. It also stimulates the pancreas to secrete digestive enzyme.
• The heart also acts as an endocrine organ in addition to its major role of pumping blood. Special cells in the wall of the upper chambers of the heart, called atria, produce a hormone called atrial natriiuretic hormone, or atriopeptin.
• The placenta develops in the pregnant female as a source of nourishment and gas exchange for the developing fetus. It also serves as a temporary endocrine gland. One of the hormones it secretes is human chorionic gonadotropins, which signals the mother’s ovaries to secrete hormones to maintain the uterine lining so that it does not degenerate and slough off in menstruation.
Too much or too little of any hormone can be harmful to the body. For example, if the pituitary gland produces too much growth hormone, a teen may grow excessively tall. If it produces too little, a teen may be unusually short. Doctors can often treat problems with the endocrine system by controlling the production of hormones or replacing certain hormones with medication. Some endocrine problems that affect people include:
• Addison’s disease (also called adrenal insufficiency)—This condition is an endocrine or hormonal disorder that occurs in all age groups and afflicts men and women equally. The disease is characterized by weight loss, muscle weakness, fatigue, low blood pressure, and sometimes darkening of the skin in both exposed and non-exposed parts of the body. Addison’s disease occurs when the adrenal glands do not produce enough of the hormone cortisol and, in some case, the hormone aldosterone. Doctors treat Addison’s disease with medications to replace corticosteroid hormones.
• Diabetes—This condition is marked by high levels of glucose resulting from defects in insulin production, insulin action, or both. Diabetes can lead to serious complications and premature death, but people with diabetes can take steps to control the disease and lower the risk of complications. Approximately 21 million Americans have diabetes—7% of the U.S. population. Of these, 6.2 million do not know they have the disease. Each year, about 1.5 million people age 20 or older are diagnosed with diabetes. The number of people diagnosed with diabetes has risen from 1.5 million in 1958 to 14.6 million in 2005, an increase of epidemic proportions (NIH 2005).
• Type 1 diabetes—When the pancreas fails to produce enough insulin, type 1 diabetes occurs. Type 1 accounts for 5%–10% of all diagnosed cases of diabetes.
• Type 2 diabetes—Unlike type 1 diabetes, in which the body can’t produce normal amounts of insulin, in type 2 diabetes the body can’t respond to insulin normally. Type 2 diabetes accounts for 90%–95% of all diagnosed cases of diabetes.
• Pancreatitis—Pancreatitis is an inflammation of the pancreas. Normally, digestive enzymes do not become active until they reach the small intestine, where they begin digesting food. But if these enzymes become active inside the pancreas, they start “digesting” the pancreas itself.
• Acute pancreatitis can be a severe, life-threatening illness with many complications. About 80,000 cases occur in the U.S. each year; some 20% of them are severe. Acute pancreatitis usually begins with pain in the upper abdomen that may last for a few days. The pain may be severe and may become constant—just in the abdomen—or it may reach to the back and other areas. It may be sudden and intense or begin as a mild pain that gets worse when food is eaten. Someone with acute pancreatitis often looks and feels very sick. Acute pancreatitis occurs more often in men and women. Acute pancreatitis is usually caused by gallstones or by drinking too much alcohol, but these aren’t the only causes. If alcohol use and gallstones are ruled out, other possible causes of pancreatitis should be carefully examined so that appropriate treatment—if available—can begin (NIH 2004).
• Chronic pancreatitis occurs when digestive enzymes attack and destroy the pancreas and nearby tissues, causing scarring and pain. The usual cause of chronic pancreatitis is many years of alcohol abuse but the chronic form may also be triggered by only one acute attack, especially if the pancreatic ducts are damaged. The damaged ducts cause the pancreas to become inflamed, tissue to be destroyed, and scar tissue to develop (NIH 2004).
Important Point: While common, alcoholism is not the only cause of chronic pancreatitis.
• Hyperparathyroidism—If the parathyroid glands secrete too much hormone, as happens in primary hyperparathyroidism, the balance is disrupted: blood calcium rises. This condition of excessive calcium in the blood, called hypercalcemia, is what usually signals the doctor that something may be wrong with the parathyroid glands. In 85% of people with primary hyperparathyroidism, a benign tumor called an adenoma has formed on one of the parathyroid glands, causing it to become overactive. Benign tumors are noncancerous. In most other cases, the excess hormone comes from 2 or more enlarged parathyroid glands, a condition called hyperplasia. This excess parathyroid hormone (PTH) triggers the release of too much calcium into the bloodstream. The bones lose calcium, and too much calcium may be absorbed from food. The levels of calcium may increase in the urine, causing kidney stones. PTH also lowers blood phosphorous levels by increasing excretion of phosphorus in the urine. Doctors often treat this problem with medication. Very rarely, hyperparathyroidism is caused by cancer of a parathyroid gland (NIH 2006).
• Thyroid disease—The thyroid secretes the hormone thyroxine which speeds up metabolism and helps manage growth and development. When the thyroid produces too much hormone, the body uses energy faster than it should. This condition is called hyperthyroidism. When the thyroid doesn’t produce enough hormones, the body uses energy slower than it should. This condition is called hypothyroidism. There are many different reasons why either of these conditions might develop. However, women are 5 to 8 times more likely than men to have thyroid problems.
1. The________system involves the secretion and release of hormones.
2. The________helps maintain the body’s salt and potassium balances.
3. Stable internal environment:________.
4. Secretes melatonin:________.
5. ________glands have ducts that carry their secretory products to the surface.
6. Hormones that cause secretion of other hormones are called________hormones.
7. ________causes contraction of the smooth muscle in the wall of the uterus.
8. Ninety-five percent of the active thyroid hormone is________.
9. Secretes epinephrine:________.
10. The pancreas produces insulin and________.
11. The________are the primary reproductive organs.
12. ________diabetes occurs when the body (pancreas) does not produce normal amounts of insulin.
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NIH. 2004. Pancreatitis. Bethesda, MD: National Institutes of Health.
NIH. 2005. National Diabetes Statistics Fact Sheet: General Information and National Estimates on Diabetes in the United States. Bethesda, MD: U.S. Department of Health and Human Services.
NIH. 2006. Hyperparathyroidism. Bethesda, MD: U.S. Department of Health and Human Services.
SEER. 2000. Hormones. Accessed August 19, 2006, at www.training.seer.cancer.gov.
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Note: Much of this chapter is based on information taken from SEER’s Training Web site, 2000.