The Endocrine System
The body requires information in order to develop properly and maintain physiological balance. In order for the body to develop properly and maintain physiological balance it requires many different types of information. The endocrine system is one way in which these communications happen. By using hormones, organs and tissues alike contribute to the regulation of bodily processes.
Hormones are chemicals that the endocrine glands excrete into intercellular spaces that then diffuse into the bloodstream. There are two types of hormones that affect how the individual hormone reacts to its specific target cell: non-steroid and steroid. Steroid hormones are lipid soluble and pass through the cell membranes, where they bind to receptors within the cells and attach to DNA molecules to activate genes (Alcamo, 2003). Because protein, or non-steroid, hormones are not lipid soluble, they must instigate chemical changes by binding to hormone receptors at the plasma membrane of their target cells. This triggers chemical reactions that activate the second messengers, molecules that continue carrying the information, for the “message” to be processed within the cell (Thibodeau & Patton, 2008). In the second-messenger mechanism, it may help to remember that the non-steroid hormone acts as the first messenger, and the molecules activated by the non-steroid hormone reacting with the receptor at the cell membrane are the second messengers.
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The primary purpose of hormones is to affect changes in various body systems in order to create developmental changes. They are also responsible for maintaining the balance of the body as a whole. For example, the hormone epinephrine (commonly known as adrenaline), produced by the medulla of the adrenal glands, affects the sympathetic nervous system as part of a response to stress (Thibodeau & Patton, 2008). While epinephrine is not currently considered a hormone that is required for living, it is one that promotes homeostasis by providing immediate bodily response to cope and survive in perceived emergency situations.
The endocrine system is composed of organs and ductless glands that facilitate body development and homeostasis (Walker & Wood, 2003).The primary organs of the endocrine system include the pituitary gland, hypothalamus, pineal gland, thyroid gland, parathyroid glands, thymus gland, adrenal glands, sex glands, and the pancreas. Many other organs also function in some capacity as part of the endocrine system (Davis, 2006). They may utilize typical hormones or prostaglandins, which are hormones used by tissues in order to act upon other cells within the tissue (Thibodeau & Patton, 2008).
The pituitary gland has been, and still often is, thought of as the “master gland” because of its role controlling the functions of many of the other endocrine glands. The pituitary gland is actually two parts: the adenohypophysis (the anterior pituitary gland), and the neurohypophysis (the posterior pituitary gland). The adenohypophysis is composed of glandular tissue and releases growth hormone (GH), prolactin, and tropic hormones that control other endocrine glands. GH promotes the breaking down of fat while slowing down the catabolism of glucose, and supports normal growth (Thibodeau & Patton, 2008). Prolactin, also known as lactogenic hormone, promotes breast development and lactation in the pregnant and nursing mother.
The tropic hormones secreted by the adenohypophysis are: thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). TSH stimulates growth of the thyroid gland and release of thyroid hormones. ACTH acts as TSH does with the thyroid, but for the adrenal cortex and glucocorticoids. FSH, in females, affects the growth and maturation of ovarian follicles, ovulation, and stimulates estrogen. In males, FSH stimulates sperm production. LH, like FSH, works on the reproductive organs. In females it acts with FSH to increase estrogen secretion from follicles, help follicles and ovum mature, and cause ovulation. LH also stimulates luteinization and the production of progesterone in women, and is sometimes called the “ovulating hormone” (Thibodeau & Patton, 2008). For men, LH acts to instigate the development and secretion of testosterone.
The posterior portion of the pituitary gland, the neurohypophysis, is made up of nervous tissue. This gland secretes anti-diuretic hormone (ADH) and oxytocin when signaled to do so by the nervous system (Walker & Wood, 2003). ADH helps regulate the electrolyte balance of the body by reducing the amount of fluid urinated -as it increases how much water is reabsorbed into the body from the kidneys. Oxytocin promotes a positive feedback loop, temporarily encouraging the disruption of homeostasis, by stimulating uterine contractions during childbirth. Oxytocin also affects the cells of milk-filled breasts to encourage lactation by causing the breast milk to flow.
The hypothalamus is the main reason the pituitary gland no longer has the same reputation for being the dominant endocrine gland. This is because the hypothalamus, in effect, regulates much of the pituitary gland. On top of producing the hormones the neurohypophysis secretes, the hypothalamus itself secretes hormones that act on the adenohypophysis of the pituitary gland: inhibiting and releasing hormones.
The thyroid gland produces and stores the thyroid hormones thyroxine (T4) and triiodothyronine (T3), as well as secretes calcitonin. T4 and T3 increase the rate cells metabolize energy. Calcitonin promotes calcium balance in the blood by inhibiting the breakdown of bone.
The parathyroid glands are located on the posterior side of the thyroid gland and are responsible for parathyroid hormone (PTH), also known as parathormone (Alcamo, 2003). PTH acts in the opposite regard as calcitonin and increases blood calcium levels by stimulating bone resorption.
The thymus gland releases hormones for the development and differentiation of T cells, and is most productive in children as it is responsible for the development of the immune system. The hormones currently known to work for the thymus gland are: thymosin, thymopoietin, thymulin, thymic humoral factor, interleukins, and interferon (Eroschenko, 2008).
The pineal gland regulates the internal clock through melatonin. Melatonin inhibits the adenohypophysis in releasing gonadotropins (Alcamo, 2003). Its secretion is controlled by the amount of light the eyes register.
The adrenal glands are located on top of each kidney. They consist of the adrenal medulla, the core of the adrenal gland, and the outer portion of the adrenal gland known as the adrenal cortex. The adrenal medulla is the part that secretes epinephrine and norepinephrine in order to support the body in stressful situations.
The adrenal cortex is made up of three layers: the outer layer (zona glomerulosa), the middle, or second, layer (zona fasciculate), and the zona reticularis -the third, innermost layer (Alcamo, 2003). The hormones these layers release are corticoids. More specifically, the corticoids each layer secretes are the mineralocorticoids, glucocorticoids, and sex hormones, respectively. Mineralcorticoids help maintain electrolyte balance. Glucocorticoids aid in the process of gluconeogenesis, which is a process whereby liver cells convert amino and fatty acids into glucose to increase blood glucose levels. The sex hormones secreted by the zona reticularis are androgens.
The testes are the sex glands in the male and the ovaries in the female. The testes secrete the hormone testosterone from their interstitial spaces, or the spaces between the cells. Testosterone is a steroid hormone that regulates the secondary sex characteristics of males (Alcamo, 2003). Ovaries consist of two structures that have endocrine functions. The corpus luteum secretes primarily progesterone, but also secretes some estrogen, while the ovarian follicles secrete estrogen. Estrogen has many roles the female’s system, and contributes to maturation of the female reproductive organs, maintaining pregnancy, preparing the breasts for lactation, and developing the secondary sex characteristics (Walker & Wood, 2003). Progesterone essentially serves to prepare the uterus for pregnancy.
The pancreas acts as both an endocrine and exocrine gland. Its endocrine functions are extremely important to regulating blood glucose and ensuring cells receive sufficient energy. The pancreas has islet cell clusters, known as islets of Langerhans, which contain alpha, beta, and delta cells (Alcamo, 2003). The alpha cells secrete glucagon, which acts to increase blood glucose levels. The beta cells secrete the hormone insulin. Insulin behaves antagonistically to glucagon, so it acts to reduce glucose levels in the blood. The role of somatostatin, the hormone released by the delta cells, is still unclear, although it’s considered to affect the secretion of both insulin and glucagon (Alcamo).
Other organs and tissues not immediately associated as endocrine organs also use hormones to regulate their functions. Among these organs are the heart and the kidneys. The heart produces atrial natriuretic hormone (ANH) to help regulate sodium in the blood, while kidneys have endocrine cells that release the hormone erythropoietin, helping bone marrow produce red blood cells (Davis, 2006).
Research continues to teach new things about the endocrine system. Recently, in 2007, researchers at Columbia University Medical Center determined that the skeleton also performs endocrine functions by regulating blood sugar (Ballantyne). Until research reveals new information, it can be hard to define how much is still unknown about the endocrine system. Additionally, there are unanswered question concerning what is currently known – such as “what is the purpose of androgens in males?” and “what does somatostatin really do?
The structure of the endocrine system is unique in that most of its organs aren’t actually physically attached to each other. Another interesting attribute of this system is that many other body systems also contribute to endocrine functions. This allows for the organs to communicate more quickly in localized systems, such as in the case of endocrine functions in the digestive system. The endocrine system serves to communicate with a broader group of cells, more gradually and more long-term, than the other primary communication system of the body: the nervous system (Walker & Wood, 2003). The chemical transmission of information this system provides aids the body in balancing its activities throughout the other body systems, thereby promoting homeostasis.
References
Alcamo, I.E. (2003). Anatomy coloring workbook (2nd ed.). New York City, NY: Princeton Review Publishing, L.L.C.
Ballantyne, C. (2007). Is bone-fat chitchat the key to weight loss? Scientific American. Retrieved from http://www.scientificamerican.com.
Davis, G. (2006). Hormonal control and the endocrine system: achieving homeostasis. Nurse Prescribing, 4(11), 446-453. http://search.ebscohost.com.
Eroschenko, V.P. (2008) diFiore’s atlas of histology with functional correlations (11th ed.). Baltimore, MD: Lippincott Williams & Wilkins.
Thibodeau, G.A. & Patton, K.T. (2008). Structure and function of the body (13th ed.). St. Louis, MO: Mosby, Inc.
Walker, P. & Wood, E. (2003). The endocrine system, Farmington Hills, MI: Lucent Books.
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