Endocrine system

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Last Medical Review: March 31, 2020
Medically Reviewed by Dr. Elisabeth Vincent Hamelin
The Endocrine System
(March 31, 2020)
Anatomy of the Endocrine System
(March 31, 2020)

The body’s hormone system consists of several glands that form signaling substances, called hormones. They are transported by the blood and reach the entire body. Hormones are specialized and affect only certain cells, so called target cells. These then react in a predetermined manner.

Hormone-forming glands

The hormone system, together with the nervous system, has overall tasks in the body. The two systems do not work independently, but cooperate in an intricate way. The nervous system has a superior function and is also faster than the hormone system.

Hormones control much of what happens in the body, among other things

  • metabolism
  • growth
  • salt and water balance
  • sexual maturity and sexual drive
  • reproduction.

The body’s most important glands

The hormones are formed by single cells or cells that are collected into glands. The hormone-forming glands are also called endocrine organs.

The body’s most important hormone-forming glands and cells are

  • pituitary
  • thyroid
  • parathyroid
  • adrenal glands
  • Langerhans islands in the pancreas
  • ovaries and testes
  • cells in the gastrointestinal tract
  • cells in the kidneys.

The glands secrete the hormones directly into the blood.

The pituitary gland controls other glands

The pituitary gland is located on the underside of the brain in a small pit in the skull’s bone. Through a small stalk, the pituitary gland is directly connected to the brain, which controls the hormone production in the gland. The pituitary gland is very small, much like a pea. It weighs less than one gram, yet is the body’s most important hormone-forming gland.

The pituitary gland produces several different hormones, which in turn stimulate other glands in the body to form hormones. In the pituitary gland hormones such as

  • stimulates the thyroid gland
  • stimulates the adrenal cortex
  • stimulates the ovaries and testicles
  • stimulates growth
  • stimulates the milk glands
  • concentrates the urine
  • helps with childbirth and breastfeeding.

Hormones from the thyroid activate the cells

The thyroid gland is the body’s largest hormone-forming gland. It lies on the front of the trachea just below the larynx, reaching partially around the larynx. It is made up of two halves, each containing a variety of small blisters. The walls of the bladder consist of cells that form hormones. The newly formed hormones can be stored in the vesicles before they are excreted in the blood.

The thyroid gland produces important hormones that control the body’s metabolism and activity in the cells. Pituitary hormones stimulate the thyroid gland to increase thyroid hormone production. When enough thyroid hormones are formed, they inhibit the pituitary gland. Then the pituitary gland produces less amount of hormone that stimulates the thyroid gland. This is an example of so-called feedback, which is described further down in the text. In the thyroid gland, the hormone calcitonin is also formed, which affects the circulation of calcium in the body.

The parathyroid glands regulate the amount of calcium and phosphate

There are a total of four small parathyroid glands, located on the back of the thyroid gland. Each gland is about the same size as a grain of rice. The parathyroid glands form so-called parathormone, which regulates the amount of calcium and phosphate in the body.

Several different hormones from the adrenal glands

The adrenal glands are small glands located on the top of each kidney. They are embedded in a fat capsule that surrounds the kidneys. Each adrenal gland consists of an outer bark and an inner marrow, which work completely separate from each other.

In the adrenal cortex several different hormones are formed which control the metabolism of sugar and salt in the body One of the hormones is aldosterone. In the adrenal cortex, sex hormones are also formed. It is the hormone from the pituitary gland that stimulates the adrenal cortex to form the hormones. Cortisol is also formed in the adrenal cortex, which regulates, among other things, the metabolism of sugar, fat and protein in the body. Cortisol also inhibits inflammation.

In the adrenal marrow, are formed and are primarily excreted in stress situations to activate the body in different ways. Blood pressure and heart rate rise, the trachea dilates, blood sugar increases and pupils grow larger. All these reactions make the body ready to handle the stress situation. You usually say that you get an adrenaline rush, and thanks to this you can cope with an extra stressful situation.

The adrenal gland is both a hormone-forming gland and part of the sympathetic nervous system.

The pancreas regulates blood sugar

In the pancreas are small groups of hormone-forming cells, called the islands of Langerhans. They make up about one percent of the entire weight of the pancreas. The islands of Langerhans form the hormones insulin and glucagon, which regulate blood sugar levels.

The rest of the pancreas has important tasks in digestion because it forms pancreas, which contains various substances that help break down the food.

Ovaries and testes form sex hormones

The ovaries and testicles are important for reproduction because they form eggs and sperm, respectively, but they also function as glands because they form sex hormones.
Female and male sex hormones, among other things, stimulate the development of the genitals and affect sexuality.

Hormone-forming cells in the gastrointestinal tract

In the gastrointestinal tract are scattered hormone-forming cells. They form, among other things, the hormones gastrin, secretin and cholecystokinin, which increase the production of various secretions from the gastrointestinal tract.

Hormone-forming cells in the kidneys

In the kidneys are small groups of hormone-forming cells. They form the hormones renin which helps to raise blood pressure and blood flow, and erythropoietin which increases the number of red blood cells.

Hormones

Hormones act as chemical messengers and can therefore affect cells far away from the site where the hormone is formed. There are many different hormones with special tasks.

Each hormone affects only certain cells. Sometimes a hormone is compared to a key that only fits in a particular lock. Some hormones affect almost all cells in the body, while others affect only certain cells.

A cell can act as a target cell for many different hormones. This means that the cell is affected by several different hormones, which have different effects on the cell. The same hormone has the same effect every time on the target cell, but different hormones thus have different effects on the target cell. Very small amounts of a hormone are needed for it to be effective.

The blood transports the hormones

The hormones are formed by substances that get into the food. The newly formed hormones are secreted directly into the blood and are transported throughout the body. Sometimes they bind to special transport substances in the blood, which makes it easier for the hormones to reach the body. The hormones then bind to their target cells or enter them. In both cases, the target cell is affected in a specific way, and this leads to different reactions in the body.

The hormones regulate themselves

The hormones regulate themselves in a very complicated interaction. If there is already much of a certain hormone, the production of that hormone decreases. Often, the reduction occurs in several steps by the excess of one hormone inhibiting the formation of another hormone, which has the task of stimulating the formation of the first hormone.

If there is too little of a hormone, the hormone-forming gland senses this and increases its production. Here too, an intermediary is often used in the form of a stimulating hormone whose production first increases. The mechanism is called feedback or feedback. Thanks to this, the interaction between the different hormones is fine-tuned.

The body’s most important hormones

Here are some of the body’s most important hormones:

From the pituitary gland

Growth hormone is formed in the pituitary gland. The hormone stimulates growth in children and adolescents. It also increases metabolism in the body throughout life.

The pituitary gland produces the hormone prolactin, which stimulates the mammary glands. It is necessary for the mammary glands in the woman’s breast to grow during pregnancy and be able to produce milk after childbirth.

Anti-diuretic hormone, ADH, affects the kidneys and causes a decrease in the amount of water excreted in the urine. This causes small amounts of concentrated urine to form. ADH is formed in the part of the brain called the hypothalamus, but is transported from there to the pituitary gland that releases the hormone.

The hormone oxytocin starts the labor and helps the uterus contract after birth. It also helps with breastfeeding as the hormone contracts the smooth muscle around the mammary glands. The hormone at the same time relaxes the ring muscle around the mouths of the exits so that the baby can suck more easily. Oxytocin is formed in the hypothalamus, but is secreted from the pituitary gland.

The hormone TSH stimulates the thyroid to form the hormone thyroxine, which increases the body’s metabolism.

From the thyroid gland

The thyroid hormone thyroxine is available in two forms, T3 and T4, and they increase the body’s metabolism. The hormone is important for the body to grow and develop normally. T4 is a precursor to T3, which is the active hormone.

From the parathyroid glands

Parathormone is formed in the parathyroid glands. The hormone increases the amount of calcium and reduces the amount of phosphate in the blood by affecting the skeleton, intestines and kidneys.

From the adrenal glands

Glucocorticoids, such as cortisol, are formed in the adrenal cortex. They regulate the metabolism of sugar, fat and protein in the body and inhibit inflammation and allergies. Cortisone is an inactive precursor to cortisol.

Mineralocorticoids, such as aldosterone, are also formed in the adrenal cortex. The hormones regulate the conversion of the salts sodium and potassium. Aldosterone causes the body to save on sodium, but gets rid of potassium. Water always comes with sodium. Therefore, less water is excreted, which increases blood pressure.

Adrenaline and norepinephrine are formed in the adrenal gland and activate the body in stress situations. The hormones cause the blood pressure and pulse to rise, the blood sugar increases, the trachea dilates and the pupils grow larger. The body gets ready to cope with a stressful situation.

From the pancreas

Insulin is formed in the islands of Langerhans in the pancreas. The hormone increases the absorption and storage of sugar in the body’s cells. Then the amount of sugar in the blood drops. Insulin also increases cell uptake of broken down proteins and the storage of fat.

Glucagon is also formed in the islands of Langerhan. The hormone increases the blood’s concentration of sugar. Thus, insulin and glucagon have the opposite effect on blood sugar levels.

From the ovaries

Female sex hormones are formed primarily in the ovaries, but also in the adrenal cortex. They affect the development of the woman’s genitals, regulate menstruation and ensure that a pregnancy can be completed. The amount of female sex hormones in the blood varies greatly during the menstrual cycle. Female sex hormones are estrogen and progesterone, but women also form smaller amounts of male sex hormones.

From the testicles

Male sex hormone is formed in the testicles and in the adrenal cortex. The hormone is essential for the development of the male genitalia and also stimulates the growth of both boys and girls. Examples of male sex hormone are testosterone.

From the gastrointestinal tract

Gastrin is formed in the hormone-forming cells of the gastrointestinal tract and increases the production of hydrochloric acid in the stomach, which is needed for digestion.

Secretion increases the production of basic secretion from the liver and pancreas, which is also needed for digestion.

Cholecystokinin stimulates the gallbladder and pancreas. It affects the flow of bile and pancreas to the gastrointestinal tract.

From the kidneys

Renin is excreted from the kidneys when the blood pressure is so low that the kidneys get too little blood. The hormone activates another substance, which stimulates the adrenal cortex to form the hormone aldosterone. The result is that blood pressure and blood flow increase. Renin also stimulates the conversion of other substances that greatly increase blood pressure.

Erythropoietin is also excreted from the kidneys and is needed to form red blood cells. When there is less oxygen in the blood, the kidneys feel it and form more erythropoietin. As a result, more red blood cells are formed that can carry more oxygen.

The pineal gland

Melatonin is formed in the pineal gland of the brain. The hormone is important in order to maintain the daily rhythm. It is formed especially when you are in the dark and the hormone tells the body that it is time to sleep.

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