The Endocrine System Overview
Introduction
The endocrine system is a collection of endocrine glands and cells that act as a control and communication system for the body. The main job of the endocrine system is to maintain homeostasis, which is a stable state of internal conditions optimal for survival. Endocrine organs do this by secreting chemicals called hormones directly into the bloodstream, which carries them to target organs or cells to initiate biochemical and physiological changes.
The primary endocrine organs in the body include the hypothalamus, pituitary gland, pineal gland, thyroid gland, parathyroid glands, thymus, pancreas, thymus, as well as the ovaries in females and the testes in males.
Hypothalamus
The hypothalamus is a structure found in the diencephalon of the brain and is considered part of the endocrine system and nervous system. The main role of the hypothalamus is to control all other endocrine glands. The hypothalamus does this by secreting hormones that regulate the release of other hormones in various endocrine glands. In addition, the hypothalamus is anatomically and functionally linked to the pituitary gland, which hangs from a thin stalk just below it. The hypothalamus-pituitary complex takes stimuli from the nervous system and translates them into hormones which initiate the specific response. The hypothalamus secretes two main types of hormones: releasing hormones such as growth-hormone-releasing hormone (GHRH), which stimulate the secretion of pituitary hormones, and inhibiting hormones such as growth-hormone-inhibiting hormone (GHIH), which stop the secretion of pituitary hormones.
Hormones | Main Effect |
Corticotropin-releasing hormone (CRH) | Stimulates anterior pituitary (AP) to release adrenocorticotropic hormone (ACTH) |
Gonadotropin-releasing hormone (GnRH) | Stimulates AP to release follicle stimulating hormone (FSH) and luteinizing hormone (LH) |
Growth-hormone-releasing hormone (GHRH) | Stimulates anterior pituitary to release growth hormone |
Growth-hormone-inhibiting hormone (GHIH) | Stops anterior pituitary from releasing growth hormone (GH) |
Prolactin-releasing hormone (PRH) | Stimulates anterior pituitary to release prolactin |
Prolactin-inhibiting hormone (PIH) | Stops anterior pituitary from releasing prolactin |
Thyrotropin-releasing hormone (TRH) | Stimulates anterior pituitary to release thyroid-stimulating hormone (TSH) and prolactin |
Pituitary Gland
The pituitary gland is a bean-shaped organ connected to the hypothalamus by a thin stalk called the infundibulum. It forms part of the hypothalamus-pituitary complex that controls numerous other endocrine glands, which is why the pituitary is often called the master gland. The pituitary gland consists of two lobes: an anterior lobe called the adenohypophysis and a posterior lobe called the neurohypophysis. The neurohypophysis secretes a class of hormones called neurohormones that includes oxytocin, which are produced in the hypothalamus but stored in the posterior pituitary. The adenohypophysis secretes a class of hormones called tropic hormones or tropins that includes human growth hormone (HGH), which control the release of other hormones.
Posterior Pituitary Hormones | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
neurohormones | A group of hormones produced by neurosecretory cells in the hypothalamus | ||
antidiuretic hormone (ADH) | Stimulates kidneys to retain water and constrict blood vessels | SIADH | diabetes insipidus |
oxytocin | Stimulates uterine contractions in labor; letdown of milk during breastfeeding | benign prostatic hyperplasia in males | difficult labor; insufficient lactation |
Anterior Pituitary Hormones | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
tropic hormones / tropins | A group of hormones that control the release of other hormones in the body | ||
adrenocorticotropic hormone (ACTH) | Stimulates production of cortisol by adrenal glands | Cushing’s syndrome | Addison’s disease |
follicle stimulating hormone (FSH) | Regulates reproductive processes such as puberty and gonad function | enlarged ovaries in females | lack of sexual development; infertility |
human growth hormone (HGH) | Stimulates body growth and development | gigantism; acromegaly | pituitary dwarfism |
luteinizing hormone (LH) | Regulates gonad function and the menstrual cycle in females | decreased sex steroid production by gonads | lack of sexual development; infertility |
melanocyte-stimulating hormone (MSH) | Stimulates the production of the pigment melanin in melanocytes | skin darkening | skin lightening |
prolactin | Stimulates milk production and breast development in females | excessive lactation | insufficient lactation |
thyroid-stimulating hormone (TSH) | Stimulates thyroid gland and regulates thyroid function | hypothyroidism, Hashimoto’s thyroiditis | hyperthyroidism; Grave’s disease |
Anterior Pituitary
Posterior Pituitary
Pineal Gland
The pineal gland is a small, cone-shaped gland located between the two lobes of the thalamus in the brain. The main role of the pineal gland is to regulate the body’s circadian rhythm or sleep-wake cycle. The pineal gland does this by releasing the hormone melatonin. Melatonin levels vary with the amount of light in the environment. When light levels are rise, less melatonin is released, promoting wakefulness. When light levels fall, more melatonin is released, causing sleepiness.
Hormone | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
Melatonin | Regulates body’s circadian rhythm; regulates secretion of other hormones | seasonal affective disorder (SAD) | sleep problems; mood disorders |
Thyroid Gland
The thyroid is a butterfly-shaped gland located in the throat just below the larynx. The thyroid wraps around the front and sides of the trachea and consists of two lateral lobes connected by a band of tissue called an ismus. The main role of the thyroid is to regulate body metabolism and to reduce blood calcium levels. The thyroid does this by releasing three hormones: triiodothyronine (T3), thyroxine (T4), and calcitonin. Triiodothyronine and thyroxine increase the metabolism of body cells and calcitonin stimulates bone cells to absorb more calcium from the blood, thus decreasing blood calcium levels.
Hormones | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
calcitonin | Stimulates absorption of calcium by bones; lowers blood calcium levels | hypocalcemia | hypercalcemia |
triiodothyronine (T3) | Stimulates body metabolism; more powerful than thyroxine (T4) | hyperthyroidism; Grave’s disease | hypothyroidism, Hashimoto’s thyroiditis |
thyroxine (T4) | Stimulates body metabolism; less powerful than triiodothyronine (T3) |
Parathyroid Glands
The parathyroid consists of four rice-shaped glands embedded in the posterior surface of the thyroid, two glands per lobe. The main role of the parathyroid is to increase blood calcium levels. The parathyroid does this by releasing parathyroid hormone (PTH). PTH stimulates bone cells to release calcium into the blood and causes the kidneys to increase calcium reabsorption. These two effects act to increase blood calcium levels. In this way, parathyroid hormone acts in opposition (antagonistically) to the hormone calcitonin that is released by the thyroid.
Hormone | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
parathyroid hormone (PTH) | Stimulates release of calcium by bones; increases blood calcium levels | hypercalcemia | hypocalcemia |
Thymus
The thymus is a triangular gland located directly under the sternum in the chest area between the lungs. The main role of the thymus is to stimulate the development and maturation of T-cells which function in the body’s immune system. To do this, the thymus releases thymosin. The thymus is relatively large in children and continues to grow until puberty. In adults, the thymus slowly decreases in size and is eventually replaced by fatty tissue.
Hormone | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
Thymosin | Stimulates development of mature T-cells for the immune system | Myasthenia gravis | Reduced immunity |
Pancreas
The pancreas is a long, flat organ located just below the stomach that belongs to both the endocrine system and digestive system. The digestive functions of the pancreas include releasing digestive enzymes into the gastrointestinal tract. The endocrine functions of the pancreas include releasing hormones which regulate blood glucose levels. The functional unit of the pancreas are clusters of glandular cells called the islets of Langerhans, which come in two types. Alpha cells release the hormone glucagon, which stimulates the liver to release glucose into bloodstream, thus increasing blood glucose levels. Beta cells release the hormone insulin, which stimulates body cells to absorb glucose from the bloodstream, thus decreasing blood glucose levels.
Hormones | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
glucagon (beta cells) | Stimulates liver to release glucose into blood; increases blood glucose levels | hyperglycemia; diabetes (Type 1) | - |
insulin (alpha cells) | Stimulates body cells to absorb glucose; decreases blood glucose levels | hypoglycemia; insulin shock | hyperglycemia; diabetes (Type 1) |
Adrenal Glands
The adrenal glands are two cone-shaped organs located on top of each kidney.
There are two major layers of each adrenal gland: an inner layer called the adrenal medulla, and an outer layer called the adrenal cortex.
The main role of the adrenal medulla is to help stimulate the body’s “fight or flight” response to stress. To do this, the medulla releases a class of hormones called catecholamines that includes epinephrine and norepinephrine. Epinephrine and norepinephrine complement the action of the sympathetic nervous system and prepare the body for greater physical performance.
The main role of the adrenal cortex is to help regulate the body’s mineral balance, energy balance, and reproductive functions. To do this, the cortex releases three classes of hormones. The first class of hormones are called glucocorticoids and include cortisol. Cortisol regulates blood glucose levels and acts as an anti-inflammatory compound. The second class of hormones are called mineralocorticoids and include aldosterone. Aldosterone regulates the concentration of electrolytes in the blood, especially sodium and potassium, which in turn affects blood volume and blood pressure. The adrenal cortex also releases a small amount of steroid sex hormones, including androgens, estrogen, and progestin. Sex hormones influence the development of secondary sexual characteristics and regulate reproductive system function in both sexes.
Adrenal Medulla Hormones | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
catecholamines | A group of hormones that help the body respond to stress or fright | ||
epinephrine (adrenalin) | Stimulates “fight or flight” response; prepares body for stress | symptoms of stress | - |
norepinephrine (noradrenalin) | |||
Adrenal Cortex Hormones | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
glucocorticoids | A group of hormones involved in metabolizing carbohydrates, proteins, and fats | ||
cortisol (cortisone) | Regulates blood glucose levels; helps metabolize carbohydrates, proteins &fats | Cushing’s syndrome | Addison’s disease |
mineralocorticoids | A group of hormones involved in regulating fluid and electrolyte levels | ||
aldosterone | Regulates electrolyte levels; influences blood pressure and blood volume | water retention | dehydration |
steroid sex hormones | A group of hormones responsible for reproductive and secondary sex characteristics | ||
androgens | Stimulates development of secondary sexual characteristics; regulates reproductive system function | premature sexual development | lack of sexual development |
estrogen | |||
progestin | Promotes conditions needed for pregnancy in females | low blood pressure, swelling | infertility |
The Ovaries
The ovaries are two oval-shaped glands located on either side of the uterus in females. Ovaries are the female gonads and produce mature female sex cells called ova. As endocrine organs, the main role of the ovaries is to produce steroid sex hormones. Sex hormones, which include estrogen and progesterone, regulate the female reproductive system and stimulate secondary sex characteristics such as breast development and menstruation.
Hormone | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
estrogen | Stimulates development of female sex characteristics; regulates reproductive system | premature sexual development | fatigue; loss of sex drive, depression |
progesterone | Regulates female menstrual cycle; prepares and maintains body for pregnancy | adrenal hyperplasia | irregular menstruation; early labor |
The Testes
The testes are two oval-shaped glands located inside the scrotal sac in males. The testes are the male gonads and produce mature male sex cells called sperm or spermatocytes. As endocrine organs, the main role of the testes is to produce steroid sex hormones. Sex hormones, which include testosterone and androgens, regulate the male reproductive system and stimulate secondary sex characteristics such as muscle development and hair growth.
Hormone | Main Effect | Hypersecretion Disorders | Hyposecretion Disorders |
testosterone | Stimulates development of male sex characteristics; regulates reproductive system | premature sexual development | fatigue; loss of sex drive, depression |
androgens |
WHAT ARE HORMONES?
Introduction to the Endocrine System
The endocrine system is a collection of endocrine glands and cells that acts as a control and communication system for the body. The main job of the endocrine system is to maintain homeostasis, which is a stable state of internal conditions optimal for survival.
To achieve homeostasis, endocrine glands secrete hormones, which are chemicals transported by the bloodstream throughout the body that act on target cells to produce a specific response. Hormones regulate numerous body functions (listed below) and come in two types: steroid hormones and nonsteroid or amino acid hormones.
Body Functions Regulated by Hormones | ||||
Metabolism and Energy Balance | Water and Electrolyte Balance | Body Defenses | Growth and Development | Reproduction |
Steroid Hormones
Steroid hormones include cortisol, testosterone, and estrogen.
Steroid hormones are lipid-based since they are derived from a cholesterol precursor and are therefore structurally similar to it. Thus, steroid hormones are complex carbon-ring structures.
Steroid hormones act by the primary messenger pathway. Since they are lipid-based, they can easily diffuse across the cell membrane of their target cell. Once inside, steroid hormones bind with an intracellular receptor in the cytoplasm to form a hormone-receptor complex. The complex passes through the nuclear membrane and binds to the cell’s DNA where it regulates the expression of certain genes. This action causes an increase or decrease in the production of corresponding proteins, such as cellular enzymes, which alters the activity or function of the target cell.
Nonsteroid Hormones
Oxytocin, melatonin, and epinephrine are nonsteroid hormones.
Nonsteroid hormones are not lipid based as they are derived from amino acids. Nonsteroid hormones come in 3 types. Protein hormones, such as insulin, are long chains of amino acids. Peptide hormones, such as oxytocin, are short chains of amino acids. Amine hormones, like melatonin and epinephrine, are a single modified amino acid.
Nonsteroid hormones act by the secondary messenger pathway. Since they are not derived from lipids, they are not fat soluble and cannot diffuse across the cell membranes of their target cell. Nonsteroid hormones bind with an external receptor on the cell membrane of the target cell. This triggers the release of a secondary messenger molecule, which activates cellular enzymes, which alters the activity or function of the target cell.
Feedback Loops
Many body processes are regulated by mechanisms called feedback loops. A feedback loop is a circuit of cause-and-effect where the output of the system is used as input for future changes. In other words, the system “feeds back” into itself. A home thermostat is an example of a feedback loop. The temperature in the home is set by the thermostat and is continually monitored by a sensor inside the unit. When the temperature of the home drops below the point at which the thermostat is set, the thermostat turns on the furnace. As the temperature in the house rises, the thermostat senses the change and turns the furnace off when the temperature reaches the set point. There are two main types of feedback loops in the body: negative feedback loops and positive feedback loops.
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