The endocrine system is made up of certain special glands and the hormones produced by those glands in the body. The hormones are important for instructing specific cells to produce particular actions in response to a stimulus or imbalance in the body. The glands are responsible to recognize the need for hormone secretion.
The hormones travel in the bloodstream to the cells where they act, and bind to the cells to initiate their effects.
These processes are tightly regulated by the endocrine system to maintain metabolic balance in the body. They are essential to keeping body functions normal. Such processes include cellular metabolism, sexual development and reproduction, homeostasis of sugar and other nutrients, and regulation of the heart rate, blood pressure, sleep cycles and digestion.
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Components of the Endocrine System
The following are the chief glands of the endocrine system, which are involved in the secretion of the related hormones:
- Hypothalamus: also called the master gland of the endocrine system, it releases the following:
- thyrotropin-releasing hormone (TRH)
- growth hormone-releasing hormone (GHRH)
- growth hormone inhibiting hormone (GHIH)
- gonadotropin-releasing hormone (GnRH)
- corticotropin-releasing hormone (CRH)
- antidiuretic hormone (ADH)
- Posterior and anterior pituitary gland: though the pituitary gland appears to be a single gland, it is made up of two different units, which secrete quite different hormones regulating diverse physiologic processes:
- thyroid stimulating hormone (TSH)
- adrenocorticotropic hormone (ACTH)
- follicle stimulating hormone (FSH)
- luteinizing hormone (LH)
- human growth hormone (HGH)
Oxytocin and ADH are produced by the hypothalamic cells. The axons of these neurosecretory cells extend downwards to form the posterior pituitary gland, where these hormones are stored and released when required.
- Pineal gland: found in the brain, it secretes hormones like melatonin which regulate the circadian rhythm in response to light.
- Thyroid gland: this gland in the neck is essential for the proper function of almost all body tissues, releasing:
- triiodothyronine (T3)
- thyroxine (T4)
- Parathyroid glands: these are concerned with calcium metabolism and are embedded in the thyroid gland, secreting parathyroid hormone (PTH)
- Adrenal glands: these are situated like tiny caps on the kidneys, and secrete the stress hormones called glucocorticoids, the salt-water balance regulators called mineralocorticoids, and the male sex hormones, called the androgens. All these are from the adrenal cortex. As well, the adrenal produces the classic fight-fright-flight catecholamine hormones called epinephrine and norepinephrine, from the inner medulla.
- Pancreas: this is a large exocrine gland secreting pancreatic juice to digest our food, but contains numerous scattered cell clumps called the islets of Langerhans. These are made up of endocrine alpha and beta cells that secrete the glucose-regulating hormones glucagon and insulin respectively.
- Digestive system: cells in the digestive tract secrete various hormones which modulate digestive functions, such as cholecystokinin (CCK), secretin, gastrin and ghrelin.
- Gonads: these are the primary sexual organs, which secrete testosterone, estrogens, and progesterone.
- Thymus: this gland in the neck releases the thymosins which mature the T-lymphocytes for immune function.
- Adipose tissue: fat cells throughout the body produce the hormone leptin. Low leptin levels encourage the body to enter ‘starvation mode’. Here it attempts to conserve energy by slowing down or avoiding all unnecessary activity, and to increase its intake of food by feelings of hunger.
Hormone, Receptor and Cell Interaction
An endocrine hormone is released by a specific gland and travels throughout the body in the bloodstream to reach its target cell, where it will exert a certain effect. Each hormone recognizes their target cells from the many other cells in the body by means of the receptors that exist on the cell, which they are able to bind to.
The receptor then initiates a series of chemical reactions within the cell to produce the intended effect of the hormone. For example, many endocrine hormones may stimulate the release of a chemical that induces or prevents the production of a certain gene.
After the action of the hormone, the release of the hormone from the endocrine gland must be regulated by a negative feedback loop to control the process and prevent the continuous and excessive activation of receptors.
Various processes in the endocrine system help to regulate the secretion of hormones and the resulting actions. This is essential for the body to maintain control over the action of the hormones. In other words, regulation is required to allow a hormone to initiate the intended reaction when needed and bring it to an end once the action has been completed.
For example, when the body is stressed the hypothalamus gland begins to secrete the corticotropin-releasing hormone (CRH) into the blood. This travels to the anterior pituitary gland to stimulate the release of adrenocorticotrophic hormone (ACTH). ACTH then travels in the blood to the adrenal glands, to stimulate the adrenal cortex cells which secrete the hormone cortisol.
Cortisol is responsible for stimulating the liver and skeletal muscles to increase the metabolism of glycogen (a storage form of glucose) so as to increase the blood glucose levels. The aim is to provide more energy as a response to the initial stimulus of stress.
When the body has adapted or reacted sufficiently to the stress and the body no longer requires more energy, the secretion of cortisol is cut off by a negative feedback loop. That is, the higher concentrations of the cortisol hormone in the blood ‘tell’ the hypothalamus that the action has been completed.
As a result, the hypothalamus ceases to secrete the corticotropin-releasing hormone and the production of cortisol is reduced.
Chemicals can also regulate the release of hormones, such as when an endocrine gland responds to a change in concentration of a chemical in the body.
For example, the parathyroid hormone is responsible for regulating the levels of calcium in the body. When the calcium levels in the blood drop below a certain threshold, the parathyroid gland begins to secrete more parathyroid hormone, which helps to increase the concentration of calcium in the bloodstream.
Once the levels of calcium rise to sufficient levels to fulfil the normal cellular function in the body, the production of the hormone reduces accordingly.
The nervous system can also affect the release of hormones in the body.
For example, during the process of childbirth, the head of the fetus pushes against the cervix. The stimulation of the nerves in the cervix activates the release of the hormone oxytocin from the pituitary gland to increase the frequency and intensity of uterine contractions, and to release more oxytocin.
Unlike hormonal and chemical regulation, this is a positive feedback loop, where the reaction causes an increase in the initial stimulus and further response.
The hormone only ceases to be produced when the baby is born and the pressure on the cervix is relieved.