The endocrine system, along with the nervous system, functions in the regulation of body activities. The nervous system acts through electrical impulses and neurotransmitters to cause muscle contraction and glandular secretion. The effect is of short duration, measured in seconds, and localized. The endocrine system acts through chemical messengers called hormones that influence growth, development, and metabolic activities. The action of the endocrine system is measured in minutes, hours, or weeks and is more generalized than the action of the nervous system.

There are two major categories of glands in the body - exocrine and endocrine.

Exocrine Glands

Exocrine glands 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

The endocrine glands do not have ducts to carry their product to a surface. They are called ductless glands. The word endocrine is derived from the Greek terms "endo," meaning within, and "krine," meaning to separate or secrete. 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.

Other Endocrine Glands

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 gonadotropin, 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.

Referrence:Endocrine System(Chinese Version)

Endocrine System (KidsHealth-Teens-Your Body-Body Basics Library http://kidshealth.org/index.html)

Although we rarely think about them, the glands in the endocrine system and the hormones they secrete influence practically every cell, organ, and function of our bodies. Hormones are the body's chemical messengers - they transfer information and instructions from one set of cells to another. They regulate our mood, growth and development, tissue function, metabolism, and sexual function, and in women they support pregnancy and other reproductive processes.

Why Is the Endocrine System Important for Living? The endocrine system's effects on the body are wide-reaching. Its message-sending hormones influence the operations of all of the body's tissues and organ systems. Because of its centralized role in controlling body functions, the endocrine system is sometimes compared to the nervous system. But in general, the endocrine system operates in a less rapid but longer-lasting manner than the nervous system. The endocrine system regulates slower processes, such as metabolism and cell growth. The nervous system's functions, on the other hand, are more immediate, such as the control of breathing and body movement.

Although hormones circulate throughout the bloodstream, each kind of hormone affects only its target cells that are genetically programmed to receive and respond to its message. Hormone levels vary and can be influenced by factors such as stress, infection, and changes in the balance of fluid and minerals in blood. When the endocrine system doesn't function properly, a variety of disorders can arise.

The major glands that make up the human endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body, and the reproductive glands - the ovaries and testes. The pancreas is also part of this hormone-secreting system, even though it's also associated with the digestive system because it produces and secretes digestive enzymes into the intestine. Although the endocrine glands are the body's main hormone producers, some nonendocrine organs - such as the brain, heart, lungs, kidneys, liver, and skin - also produce and release hormones.

Why Is the Endocrine System Important for Living?

The hypothalamus, a collection of specialized cells that are located in the lower central part of the brain, is the primary link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the hormone secretions of the pituitary gland.

Although it's no bigger than a pea, the pituitary gland, which is located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It's often called the "master gland" because it makes hormones that control several other endocrine glands. The production and secretion of pituitary hormones can be influenced by factors such as emotions and seasonal changes. The influence of these factors occurs when the hypothalamus relays information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.

The tiny pituitary is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands. Among the hormones it produces are growth hormone, which stimulates the growth of bone and other body tissues and plays a role in the body's handling of carbohydrates, fats, protein, and minerals. The anterior lobe also produces prolactin, which activates milk production in women who are breast-feeding, thyrotropin, which stimulates the thyroid gland to produce thyroid hormones, and corticotropin, which stimulates the adrenal gland to produce cortisol and other hormones.

The pituitary also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain, and hormones that signal the ovaries and testes to make sex hormones and that control ovulation and the menstrual cycle in women. The posterior lobe of the pituitary releases antidiuretic hormone, which helps control body water balance through its effect on the kidneys and urine output and oxytocin (pronounced: ok-sih-toe-sin), which triggers the contractions in the uterus that occur when a woman is in labor.

The thyroid, which is located in the lower neck, is shaped like a bow tie or butterfly. The thyroid produces hormones called thyroxine (pronounced: thigh-rock-seen) and triiodothyronine (pronounced: try-eye-uh-doo-thigh-ruh-neen), which control the rate at which cells burn body fuels (from food) to produce energy. As the level of thyroid hormones increases in the bloodstream, so does the speed at which chemical reactions occur in your body. Thyroid hormones also play a key role in bone growth and the development of the brain and nervous system in children. The release of thyroid hormones is controlled by the pituitary gland.

Attached to the thyroid are four tiny glands that function together called the parathyroids. They release parathyroid hormone that regulates the level of calcium in the blood with the help of calcitonin, which is produced in the thyroid.

The body has two triangular adrenal glands, one on top of each kidney. The adrenal glands have two parts, each of which produces a set of hormones and has a different function. The outer part, the adrenal cortex, produces hormones called corticosteroids that influence or regulate salt and water balance in the body, the body's response to stress, metabolism, the immune system, and sexual function. The inner part, the adrenal medulla, produces catecholamines (pronounced: kat-uh-koh-luh-meens), such as epinephrine. Also called adrenaline, epinephrine increases blood pressure and heart rate when the body experiences stress.

The pineal body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin, a hormone that may help regulate the wake-sleep cycle.

The male gonads, which are located in a guy's scrotum, are the main source of sex hormones in males. A guy's gonads, or testes, secrete hormones called androgens, the most important of which is testosterone. Androgens regulate body changes associated with sexual development, including enlargement of the penis, the height growth spurt that occurs during puberty, and the appearance of other male secondary sex characteristics such as deepening of the voice, growth of facial and pubic hair, and the increase in muscle growth and strength. Working with hormones from the pituitary gland, testosterone also supports the production of sperm by the testes.

The female gonads, the ovaries, are located in the pelvis. They produce eggs and secrete female hormones such as estrogen and progesterone. These hormones control the development of female sexual features such as breast growth, the accumulation of body fat around the hips and thighs, and the height growth spurt that occurs during puberty. These hormones are also involved in pregnancy and the regulation of menstruation.

The pancreas produces (in addition to others) two 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.

Normal Physiology

Once a hormone is secreted, it travels from the endocrine gland through the bloodstream to its target cells. Along the way, special proteins bind to some of the hormones. The proteins act as carriers that control the amount of hormone that is available to interact with and affect the target cells. Also, the target cells have receptors that attract only specific hormones, and each hormone has its own receptor. This means that each hormone will communicate only with specific cells that have receptors for that hormone. When the hormone reaches its target cell, it locks onto the cell's specific receptors and these combinations transmit chemical instructions to the inner workings of the cell.

When hormone levels reach a certain normal or necessary amount, important body mechanisms are needed to maintain that level of hormone in the blood. This regulation may involve the hormone itself or another substance in the blood related to the hormone. For example, if the thyroid gland has secreted normal amounts of thyroid hormones into the blood, the pituitary gland senses this and gears down its release of the pituitary hormone that stimulates the thyroid gland to produce thyroid hormones. This turn-off process is called a "negative feedback" system.

Diseases, Conditions, Disorders, and Dysfunctions 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 child may grow excessively tall and develop the features of gigantism (see below). If it produces too little, normal growth may be slowed or interfered with. Controlling the production of or replacing specific hormones can treat many hormonal disorders in kids and teens, some of which include:

Adrenal insufficiency - This condition, which has multiple causes, is characterized by decreased function of the adrenal cortex. In this disorder, adrenal corticosteroid hormones are underproduced. The symptoms may include weakness, fatigue, abdominal pain, nausea, dehydration, and deepening of skin color. Doctors treat adrenal insufficiency by administering replacement corticosteroid hormones.

Cushing's syndrome - Excessive adrenal secretion of glucocorticoid hormones can lead to Cushing's syndrome. In kids, it most often results from large doses of synthetic corticosteroid drugs (such as prednisone) used to treat other ailments. If the condition is due to a tumor in the pituitary gland that produces excessive amounts of corticotropin and stimulates the adrenals to overproduce corticosteroids, it's known as Cushing's disease. Symptoms take years to develop and include obesity, growth failure, muscle weakness, easy bruising of the skin, acne, high blood pressure, and psychological changes. Depending on the specific cause, doctors may treat the condition with surgery, radiation therapy, chemotherapy, or drugs that block hormone production.

Diabetes insipidus - This is a condition in which inadequate amounts of antidiuretic hormone are produced by the posterior lobe of the pituitary gland. This is associated with decreased ability of the kidney to concentrate the urine, resulting in excessive urination, thirst, and body fluid and blood sodium level imbalances. Diabetes insipidus may be inherited, or it may result from tumors in or injuries to the pituitary or hypothalamus. The condition can be treated by giving the kid or teen a form of antidiuretic hormone intranasally (in the nose), or by pill or injection.

Diabetes mellitus (Type 1) - When the pancreas fails to produce enough insulin, type 1 diabetes occurs. Symptoms include excessive thirst, hunger, urination, and weight loss. The disease can cause multiple long-term complications including kidney problems, pain from nerve damage, blindness, and early coronary heart disease and stroke. Teens with type 1 diabetes need regular injections of insulin to control their blood sugar levels and reduce the risk of developing diabetes complications.

Diabetes mellitus (Type 2) - Unlike type 1 diabetes, in which the body can't produce normal amounts of insulin, in type 2 diabetes the body is unable to respond to insulin normally. As is the case with adults who have type 2 diabetes, kids and teens with the condition tend to be overweight, and it's believed that excess body fat plays a role in the insulin resistance that characterizes the disease. The symptoms and possible complications of type 2 diabetes are basically the same as those of type 1. Some patients can control their blood sugar level with diet modifications, exercise, and oral medications, but many will need to take insulin injections like patients with type 1 diabetes.

Gigantism - Too much growth hormone in kids who are still growing will make their bones and other body parts grow excessively, resulting in gigantism. This rare condition is usually caused by a pituitary tumor and can be treated by removing the tumor.

Growth hormone deficiency - When the pituitary gland fails to produce normal amounts of growth hormone, a kid's growth in height is impaired. Hypoglycemia (low blood sugar) may occur in children with growth hormone deficiency, particularly in infants and young kids with the condition.

Hyperthyroidism - This is a condition in which the levels of thyroid hormones in the blood are excessively high. Symptoms may include weight loss, nervousness, tremor, excessive sweating, increased heart rate and blood pressure, protruding eyes, and a swelling in the neck from an enlarged thyroid gland (goiter). In kids and teens, the condition is usually caused by Graves' disease, an autoimmune disorder in which specific antibodies produced by the child's immune system stimulate the thyroid gland to become overactive. The disease may be controlled with medications or through surgery or radiation treatments.

Hypothyroidism - This is a condition in which the levels of thyroid hormones in the blood are abnormally low. Thyroid hormone deficiency slows body processes and may lead to fatigue, a slow heart rate, dry skin, weight gain, constipation, and slowing of growth and delayed puberty. Hashimoto's thyroiditis, which results from an autoimmune process that damages the thyroid and blocks thyroid hormone production, is the most common cause of hypothyroidism in kids and teens. Infants can also be born with an absent or underdeveloped thyroid gland, resulting in hypothyroidism. The condition can be treated with thyroid hormone pills.

Precocious puberty - Puberty may occur at an abnormally young age in some kids if the pituitary gland makes and releases too much of the hormones that stimulate the gonads to produce sex hormones too soon. An injectable medication is available that can block the secretion of these pituitary hormones (known as gonadotropins) and stop the progression of sexual development in most of these kids.

Glossary
Adrenal cortex: The outer part of the adrenal gland.
Adrenal glands: Glands that sit atop the kidneys and secrete hormones, including adrenaline (from the adrenal medulla), cortisol, aldosterone, and adrenal androgens (from the adrenal cortex).
Adrenal medulla: The inner part of the adrenal gland.
Androgens: Hormones that stimulate pubic and underarm hair growth in both males and females, but that are produced in much greater quantities (especially testosterone from the testes) and are more important in stimulating and maintaining secondary sexual characteristics in males.
Antidiuretic hormone: A hormone released from the pituitary gland that controls water balance by regulating the production of urine by the kidneys.
Calcitonin: A hormone produced by the thyroid that affects levels of calcium in the blood.
Corticosteroids: Hormones produced by the adrenal cortex that regulate sexual function, salt and water balance, the body's response to stress, metabolism, and immune system function.
Corticotropin: A hormone released by the pituitary gland that stimulates the adrenal glands' production of hormones.
Endocrine gland: An organ containing a group of cells that produce and secrete hormones into the bloodstream.
Endorphins: Hormones that act on the nervous system to reduce sensitivity to pain.
Epinephrine: The principal blood pressure and heart rate raising hormone secreted by the adrenal medulla; also called adrenaline.
Estrogen: A hormone produced primarily by the ovaries in girls that stimulates breast development, menstruation, and other female secondary sexual changes.
Exocrine gland: A gland (like a salivary gland or the digestive enzyme-producing part of the pancreas) that releases a secretion external to or at the surface of an organ, often through a canal or duct.
Glucagon: A hormone that works with insulin to regulate glucose levels in the blood.
Gonadotropin: Any hormone that stimulates the gonads.
Growth hormone: A hormone secreted by the anterior lobe of the pituitary gland that regulates growth.
Hormones: Chemical messengers secreted by endocrine glands that transfer information and instructions from one set of cells to other cells.
Hypothalamus: A centrally located structure of the brain that regulates the pituitary gland.
Insulin: A hormone that works with glucagon to regulate glucose levels in the blood and supply fuel to the body's cells for the production of energy.
Melatonin: A hormone that may help regulate the sleep-wake cycle and other body rhythms.
Ovaries: Female reproductive organs (gonads) that release eggs and female sex hormones.
Oxytocin: A hormone released by the pituitary gland that stimulates contractions in the uterus during labor.
Pancreas: A gland that produces insulin and glucagon.
Parathyroids: Four glands attached to the thyroid that function together, they regulate the levels of calcium in the blood with the help of calcitonin (which is produced in the thyroid).
Pineal body: A gland in the brain that secretes melatonin.
Progesterone: A female hormone involved in the menstrual cycle.
Prolactin: A hormone that stimulates milk production in women who are breast-feeding.
Testes: Male reproductive glands (gonads) in the scrotum that produce sperm and the hormone testosterone.
Testosterone: A hormone, produced primarily in the testes, that stimulates the development of secondary sexual characteristics and supports the production of sperm in males.
Thyroid: A gland that produces thyroid hormones (thyroxine and triiodothyronine), which play an important role in the control of body metabolism.
Thyroxine: A hormone produced by the thyroid gland.
Triiodothyronine: A hormone produced by the thyroid gland.