The endocrine system is the collection of glands that produce hormones that regulate metabolism, growth and development, tissue function, sexual function, reproduction, sleep, and mood, among other things.
The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In invertebrates, the hypothalamus is the neural control center for all endocrine systems. In humans, the major endocrine glands are the thyroid gland and the adrenal glands. The study of the endocrine system and its disorders is known as endocrinology.
Structure
Major endocrine systems
The human endocrine system consists of several systems that operate via feedback loops. Several important feedback systems are mediated via the hypothalamus and pituitary.[rx]
- TRH – TSH – T3/T4
- GnRH – LH/FSH – sex hormones
- CRH – ACTH – cortisol
- Renin – angiotensin – aldosterone
- leptin vs. insulin
Glands
Endocrine glands are glands of the endocrine system that secrete their products, hormones, directly into interstitial spaces and then absorbed into the blood rather than through a duct. The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus, and adrenal glands. The hypothalamus and pituitary gland are neuroendocrine organs.
The hypothalamus and the anterior pituitary are two out of the three endocrine glands that are important in cell signaling. They are both part of the HPA axis which is known to play a role in cell signaling in the nervous system.
Hypothalamus: The hypothalamus is a key regulator of the autonomic nervous system. The endocrine system has three sets of endocrine outputs[rx] which include the magnocellular system, the parvocellular system, and autonomic intervention. The magnocellular is involved in the expression of oxytocin or vasopressin. The parvocellular is involved in controlling the secretion of hormones from the anterior pituitary.
Anterior Pituitary: The main role of the anterior pituitary gland is to produce and secret tropic hormones.[rx] Some examples of tropic hormones secreted by the anterior pituitary gland include TSH, ACTH, GH, LH, and FSH.
Cells
There are many types of cells that make up the endocrine system and these cells typically make up larger tissues and organs that function within and outside of the endocrine system.
- Hypothalamus
- Anterior pituitary gland
- Pineal gland
- Posterior pituitary gland
- The posterior pituitary gland is a section of the pituitary gland. This organ secretes hormones such as antidiuretic hormone (ADH) and oxytocin. ADH functions to help the body to retain water; this is important in maintaining a homeostatic balance between blood solutions and water. Oxytocin functions to induce uterine contractions, stimulate lactation, and allows for ejaculation.[5][6]
- Thyroid gland
- follicular cells of the thyroid gland produce and secrete T3 and T4 in response to elevated levels of TRH, produced by the hypothalamus, and subsequently elevated levels of TSH, produced by the anterior pituitary gland, which further regulates the metabolic activity and rate of all cells, including cell growth and tissue differentiation.
- Parathyroid gland
- Epithelial cells of the parathyroid glands are richly supplied with blood from the inferior and superior thyroid arteries and secrete parathyroid hormone (PTH). PTH acts on bone, the kidneys, and the GI tract to increase calcium reabsorption and phosphate excretion. In addition, PTH stimulates the conversion of Vitamin D to its most active variant, 1,25-dihydroxyvitamin D3, which further stimulates calcium absorption in the GI tract.[rx]
- Adrenal glands
- Adrenal cortex
- Adrenal medulla
- Pancreas
- Alpha cells
- The alpha cells of the pancreas secrete hormones to maintain homeostatic blood sugar. Insulin is produced and excreted to lower blood sugar to normal levels. Glucagon, another hormone produced by alpha cells, is secreted in response to low blood sugar levels; glucagon stimulates glycogen stores in the liver to release sugar into the bloodstream to raise blood sugar to normal levels.[7]
- Beta cells
- Delta cells
- F Cells
- Alpha cells
- Ovaries
- Granulosa cells
- Testis
- Leydig cells
The endocrine system is a system of ductless glands that secrete hormones—chemical messengers that are carried for long distances.
Key Points
The endocrine system is a system of ductless glands that secrete hormones directly into the circulatory system to be carried long distances to other target organs that regulate key body and organ functions.
The major endocrine glands include the pituitary, pineal, ovaries, testes, thyroid, hypothalamus, and adrenal glands.
Key Terms
hormone: A molecule released by a cell or a gland in one part of the body that sends out messages affecting cells in other parts of the organism.
endocrine system: The system of ductless glands that secretes hormones directly into the circulatory system.
The Endocrine System
The endocrine system is a system of ductless glands that secretes hormones directly into the circulatory system to be carried long distances to other target organs regulating key body and organ functions. For example, the pineal gland, located at the base of the brain, secretes the hormone melatonin, responsible for regulating sleep patterns.
Endocrine glands are typically well vascularized and the cells comprising the tissue are typically rich in intracellular vacuoles or granules that store hormones prior to release. Endocrine signaling is typically slow to initiate but is prolonged in response; this provides a counterpoint to the more rapid and short-lived nervous system signals.
The endocrine system is in contrast to the exocrine system, which features ducted glands that secrete substances onto an epithelial surface; for example, a sweat gland. Additionally, the endocrine system is differentiated from shorter distance signaling such as autocrine (a cell affecting itself), juxtacrine (a cell affecting its direct neighbors), and paracrine (a cell affecting other nearby cells) signaling.
Key Endocrine Glands
The major endocrine glands include the pituitary, pineal, ovaries, testes, thyroid, hypothalamus and adrenal glands, additionally other tissues such as the kidney and liver also display secondary adrenal functions.
Endocrine glands of the head and neck: The endocrine systems found in the head and neck include the hypothalamus, pineal, pituitary, and thyroid glands.
Hormones Produced by the Major Hormone-Producing (i.e., Endocrine) Glands and Their Primary Functions
| Endocrine Gland | Hormone | Primary Hormone Function |
|---|---|---|
| Hypothalamus | Corticotropin-releasing hormone (CRH) | Stimulates the pituitary to release adrenocorticotropic hormone (ACTH) |
| Gonadotropin-releasing hormone (GnRH) | Stimulates the pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH) | |
| Thyrotropin-releasing hormone (TRH) | Stimulates the pituitary to release thyroid-stimulating hormone (TSH) | |
| Growth hormone-releasing hormone (GHRH) | Stimulates the release of growth hormone (GH) from the pituitary | |
| Somatostatin | Inhibits the release of GH from the pituitary | |
| Dopamine | Inhibits the release of prolactin from the pituitary | |
| Anterior pituitary gland | ACTH | Stimulates the release of hormones from the adrenal cortex |
| LH | In women, stimulates the production of sex hormones (i.e., estrogens) in the ovaries as well as during ovulation; in men, stimulates testosterone production in the testes | |
| FSH | In women, stimulates follicle development; in men, stimulates sperm production | |
| TSH | Stimulates the release of thyroid hormone | |
| GH | Promotes the body’s growth and development | |
| Prolactin | Controls milk production (i.e., lactation) | |
| Posterior pituitary gland1 | Vasopressin | Helps control the body’s water and electrolyte levels |
| Oxytocin | Promotes uterine contraction during labor and activates milk ejection in nursing women | |
| Adrenal cortex | Cortisol | Helps control carbohydrate, protein, and lipid metabolism; protects against stress |
| Aldosterone | Helps control the body’s water and electrolyte regulation | |
| Testes | Testosterone | Stimulates development of the male reproductive organs, sperm production, and protein anabolism |
| Ovaries | Estrogen (produced by the follicle) | Stimulates development of the female reproductive organs |
| Progesterone (produced by the corpus luteum) | Prepares uterus for pregnancy and mammary glands for lactation | |
| Thyroid gland | Thyroid hormone (i.e., thyroxine [T4] and triiodothyronine [T3]) | Controls metabolic processes in all cells |
| Calcitonin | Helps control calcium metabolism (i.e., lowers calcium levels in the blood) | |
| Parathyroid gland | Parathyroid hormone (PTH) | Helps control calcium metabolism (i.e., increases calcium levels in the blood) |
| Pancreas | Insulin | Helps control carbohydrate metabolism (i.e., lowers blood sugar levels) |
| Glucagon | Helps control carbohydrate metabolism (i.e., increases blood sugar levels) | |
Comparing the Nervous and Endocrine Systems
The nervous system and endocrine system both use chemical messengers to signal cells, but each has a different transmission speed.
Key Points
The nervous system can respond quickly to stimuli, through the use of action potentials and neurotransmitters.
Responses to nervous system stimulation are typically quick but short-lived.
The endocrine system responds to stimulation by secreting hormones into the circulatory system that travel to the target tissue.
Responses to endocrine system stimulation are typically slow but long-lasting.
Key Terms
hormone: A molecule released by a cell or a gland in one part of the body that sends out messages affecting cells in other parts of the organism.
neurotransmitters: Endogenous chemicals that transmit signals from a neuron to a target cell across a synapse.
The body must maintain a constant internal environment, through a process termed homeostasis, while also being able to respond and adapt to external events. The nervous and endocrine systems both work to bring about this adaptation, but their response patterns are different. The nervous system and the endocrine system use chemical messengers to signal cells, but the speed at which these messages are transmitted and the length of their effects differs.
Nervous System
The nervous system responds rapidly to stimuli by sending electrical action potentials along neurons, which in turn transmit these action potentials to their target cells using neurotransmitters, the chemical messenger of the nervous system. The response to stimuli by the nervous system is near-instantaneous, although the effects are often short-lived. An example is the recoil mechanism of an arm when touching something hot.
Endocrine System
The endocrine system relies on hormones to elicit responses from target cells. These hormones are synthesized in specialized glands at a distance from their target and travel through the bloodstream or inter-cellular fluid. Upon reaching their target, hormones can induce cellular responses at a protein or genetic level.
This process takes significantly longer than that of the nervous system, as endocrine hormones must first be synthesized, transported to their target cell, and enter or signal the cell. However, although hormones act more slowly than a nervous impulse, their effects are typically longer lasting.
Additionally, the target cells can respond to minute quantities of hormones and are sensitive to subtle changes in hormone concentration. For example, the growth hormones secreted by the pituitary gland are responsible for sustained growth during childhood.
The hypothalamic hormones are released into blood vessels that connect the hypothalamus and the pituitary gland (i.e., the hypothalamic-hypophyseal portal system). Because they generally promote or inhibit the release of hormones from the pituitary gland, hypothalamic hormones are commonly called releasing or inhibiting hormones. The major releasing and inhibiting hormones include the following
-
Corticotropin-releasing hormone (CRH) – which is part of the hormone system regulating carbohydrate, protein, and fat metabolism as well as sodium and water balance in the body
-
Gonadotropin-releasing hormone (GnRH), which helps control sexual and reproductive functions, including pregnancy and lactation (i.e., milk production)
-
Thyrotropin-releasing hormone (TRH), which is part of the hormone system controlling the metabolic processes of all cells and which contributes to the hormonal regulation of lactation
-
Growth hormone-releasing hormone (GHRH), which is an essential component of the system promoting the organism’s growth
-
Somatostatin, which also affects bone and muscle growth but has the opposite effect as that of GHRH
-
Dopamine, a substance that functions primarily as a neurotransmitter but also has some hormonal effects, such as repressing lactation until it is needed after childbirth.
THE MAIN HORMONE-PRODUCING GLANDS ARE:
- Hypothalamus: The hypothalamus is responsible for body temperature, hunger, moods, and the release of hormones from other glands; and also controls thirst, sleep, and sex drive.
- Pituitary: Considered the “master control gland,” the pituitary gland controls other glands and makes the hormones that trigger growth.
- Parathyroid: This gland controls the amount of calcium in the body.
- Pancreas: This gland produces insulin that helps control blood sugar levels
- Thyroid: The thyroid produces hormones associated with calorie burning and heart rate.
- Adrenal: Adrenal glands produce the hormones that control sex drive and cortisol, the stress hormone.
- Pineal: This gland produces melatonin which affects sleep.
- Ovaries: Only in women, the ovaries secrete estrogen, testosterone, and progesterone, the female sex hormones.
- Testes: Only in men, the testes produce the male sex hormone, testosterone, and produce sperm.
References