Endocrine System: Facts, Functions - Term Paper Example

 Endocrine System: Facts, Functions Abstract. The endocrine system works in tandem with the brain in the control and coordination of bodily functions. It consists of the endocrine glands, hormones and receptors in the target cells. The major endocrine glands are the pituitary gland, the pineal gland, the thyroid gland, the parathyroid gland, the adrenal gland, the pancreas, the gonads (ovaries and testes), the thymus and the hypothalamus. Hormones are chemical messenger secreted by the endocrine glands. Hormones target receptors in particular cells or organs and effect changes in cell activity. Imbalances in hormone levels cause endocrine disorders. The Endocrine System. The control and coordination of all bodily processes is done by the nervous system in tandem with the endocrine system. The two systems are so closely associated that they are collectively called the neuroendocrine system. Neural control centers in the hypothalamus of the brain control the endocrine system. The nervous system uses electrical impulses to exercise control over the body, while the endocrine system uses chemical messengers called hormones to communicate and link between the nervous system and bodily functions. The major functions of the endocrine system are response to stress and injury, energy metabolism, water and electrolyte balance, reproduction, birth and lactation. The endocrine system is concerned with continuous, long-term effects, which are slow and widespread. The three major components of the endocrine system are the endocrine glands, the hormones and the receptors in the target cells. (Adam 2001). Glands are aggregations of epithelial cells embedded within connective tissue and surrounded by rich vascular networks. The two major categories of glands are exocrine and endocrine. Those glands which excrete substances for elimination are called exocrine glands (derived from the Greek ‘exo’ for outside and ‘krine’ for secrete). These glands have ducts or tubes which carry their secretions to the surface of the skin or into body cavities. Other glands which secrete substances for further use by the body are called endocrine glands (‘endo’ for inside). Endocrine glands are ductless glands which release their secretions directly into the surrounding tissue and blood. The endocrine glands are not physically connected but are distributed throughout the body. Some of them also have non-endocrine functions (Eg. The pancreas also secrete digestive enzymes) and are called mixed glands. The main endocrine glands are the pituitary, the pineal, the thyroid, the parathyroid, the adrenal, the pancreas, the gonads (ovaries and testes), the thymus and the hypothalamus. (Health Encyclopedia 2007). The word hormone is derived from the Greek word ‘hormodezein,’ meaning to arouse or set in motion. Hormones are chemical messengers secreted by the endocrine glands which travel through the cardiovascular system to various parts of the body. Hormones are mainly composed of amino acids or proteins, while a smaller number are steroids. Each hormone affects only specific tissue cells or glands, which are called the target cells. The target cells have receptor sites that respond to a particular hormone. If the hormone fits the receptor site, the hormone attaches or bonds to the receptor and there will be an effect through the modification of cell activity. Protein hormones bind with receptors on the cell membrane and their effect is rapid. Steroid hormones bind with receptors inside the cell and their effect is slow. As hormones are very potent substances, miniscule amounts have deep effects on the metabolic process. Most hormones are controlled by a negative feedback system, in which homeostasis is maintained by the glands which act like thermostats: The gland is sensitive to the concentration of the substance it regulates. When the concentration decreases, the gland stimulates production of the hormone; when the concentration increases, the gland stops production of the hormone. Thus, the ideal, normal level is maintained. (Seer’s Training web site). The pituitary gland, or the hypophysis, is located at the base of the brain, behind the nose. It is a small, oval gland, about 1 centimeter in diameter. It rests in the sella turcica, a depression in the sphenoid bone. It is suspended from the hypothalamus by a slender stalk called the infundibulum. The pituitary is divided into a distinct anterior lobe, or adenohypophysis, and a posterior lobe, or neurohypophysis. The activity of the adenohypophysis is controlled by the releasing hormones of the hypothalamus, while the neurohypophysis is controlled by nerve stimulation. As the pituitary hormones control and regulate the secretion of the other endocrine glands, it is called the ‘master gland’ of the endocrine system. (Seer’s Training web site). The adenohypophysis secretes six hormones (1) The Follicle Stimulating Hormone stimulates the ovaries and testes for the development of eggs and follicles and the production of sperm (2) The Luteinizing Hormone stimulates ovulation and the secretion of estrogen and progesterone in females and stimulates the interstitial cells of the testes to secrete testosterone in males. (3) The Thyroid Stimulating Hormone stimulates the growth of the thyroid gland and the secretion of the thyroid hormones (4) Adrenocorticotropic Hormone regulates responses to stress and stimulates the adrenal cortex to secrete corticosteroids. (5) Prolactin stimulates milk synthesis after childbirth. In males, it increases sensitivity to the Luteinizing Hormone, thus increasing testosterone production. (6) Growth Hormone, or Somatotropin, promotes tissue growth through cell division and differentiation, increases protein synthesis, stimulates lipid metabolism, bone and muscle growth in children, maintains electrolyte balance and increases osteoblastic activity in adults. The neurohypophysis stores and releases (1) Oxytocin, which stimulates labor contractions, lactation and emotional bonding (2) Antidiuretic Hormone (ADH), which targets the kidneys to increase water retention and reduce urine and functions as a neurotransmitter. (Rodriquez). Disorders of the pituitary gland are (1) Acromegaly, which is ‘enlargement’ in Greek. Excessive production of Growth Hormone causes excessive growth, leading to swelling of hands and feet, arthritis, enlargement of organs, perspiration, fatigue, debility and other symptoms. When it occurs in childhood, it causes ‘gigantism.’ (2) Diabetes Insipidus results from the insufficient production of ADH, which causes excessive thirst and urination. (3) Hypopituitarism, is the result of the partial or complete loss of function of the adenohypophysis and affects the glands it regulates. (University of Maryland Medical Center web site). The Pineal Gland, or the epiphysis cerebri, is cone-shaped (derived from the Latin ‘pinea,’ or pine-shaped) and is located deep in the third ventricle of the brain. It is made up of neurons, neuroglial cells and specialized secretory cells called the pinealocytes. These cells synthesize and secrete the hormone serotonin by day and convert it into melatonin by night. Melatonin plays a role in sleep and helps to set the body’s twenty-four hour clock. It also plays a role in the onset of puberty and sexual development. Serotonin functions as a neurotransmitter and plays a role in depression. (Seer’s Training web site). Lack of sunlight results in low levels of serotonin, causing a type of depression called Seasonal Affective Disorder. Melatonin may be used to correct jet lag, as it alters our biological clock. The Thyroid Gland is one of the largest endocrine glands in the body. It is a butterfly-shaped, vascular, two inch gland which wraps around the front and sides of the trachea at the base of the throat, just below the larynx. It is divided into two lobes, connected by a narrow band of tissue called the isthmus. It contains a large number of blood vessels and is deep red in color. (Health Encyclopedia). The thyroid is composed of hollow, spherical structures made of two types of cells: follicular and parafollicular cells. The follicular cells secrete iodine containing hormones, called thyroxine T4 and triodothyronine T3, which increase the body’s metabolic rate and oxygen consumption, increase heart rate and contraction strength, increase respiratory rate, stimulate appetite and breakdown of carbohydrates, lipids and proteins. The parafollicular cells secrete calcitonin, which decreases blood calcium concentration, promotes calcium deposition and bone formation in children. (Rodriquez). Imbalance in the thyroid hormone causes (1) Hashimoto’s Thyroiditis, which is the inflammation of the thyroid. It leads to Goitre: enlargement of the thyroid, with a bulging neck. It also causes other endocrine disorders, autoimmune diseases, fatigue, muscle weakness and weight gain as it leads to hypothyroidism. (2) Hypothyroidism is due to insufficient thyroid hormones. It can lead to myxedema, characterized by dry, thickened skin and coarse facial features, constipation, weight gain, muscle cramps and other symptoms. Insufficient thyroxine in childhood can lead to dwarfism and cretinism, where the child has dry skin, scanty hair and suffers from mental retardation. (3) Hyperthyroidism is due to the excess production of thyroxine by an overactive thyroid, caused by a thyroid tumor or the autoimmune Graves Disease. It is characterized by weight loss with increased appetite, shortness of breath, nervousness and anxiety, rapid hearts beat, weakened muscles, heat intolerance and sleep difficulties. (Health Encyclopedia). The Parathyroid Gland consists of four small masses of epithelial tissue embedded in the connective tissue capsule on the posterior surface of the Thyroid Gland. (Seer’s Training web site). These secrete the Parathyroid Hormone, or Parathormone. It increases blood calcium concentration by promoting the synthesis of calcitriol, increases absorption of calcium and bone resorption and inhibits calcium urinary excretion. (Rodriquez). Imbalance in hormone secretion causes (1) Hypoparathyroidism which leads to low levels of calcium, causing Tetany, an increased excitability of the nerves. (2) Hyperparathyroidism, or excessive hormone secretion, causes excessive urination, fatigue, confusion, depression, nausea and muscle weakness. (University of Maryland Medical Center web site). The Adrenal, or Suprarenal Gland, is a pair, one of which is found near the upper portion of each kidney, like a cap. Each gland is made up of an outer cortex, which comprises 80% of the gland and is grayish-yellow in color, due to the presence of stored fats. The adrenal cortex is essential for life. The inner medulla develops from neural tissue, contains many blood vessels and is reddish-brown in color. The adrenal cortex has three layers of tissue. (Health Encyclopedia) (1) The outermost zona glomerulosa secretes mineralocorticoids, such as aldosterone, which control electrolyte balance. (2) The middle zona fasciculata secretes glucocorticoids, mainly cortisol, which stimulates fat and protein catabolism, gluconeogenesis and the release of glucose and fatty acids into the blood. It also has an anti-inflammatory effect (3) The innermost zona reticularis secretes the sex steroids, androgen, which is converted into testosterone and estrogen, which is important after menopause. The adrenal medulla secretes Catecholamines (epinephrine and norepinephrine) which increase heart rate, blood pressure, pulmonary air flow and blood flow to the skeletal muscles, decreases digestion and urine formation and stimulates gluconeogenesis. (Rodriquez). Levels of oxygen and glucose in the blood are increased and blood circulation is speeded up. Reflexes and body movements quicken and the body is prepared for short-term emergencies. Imbalances include (1) Addison ’s disease, where insufficient amounts of corticosteroids disrupt numerous bodily functions. There is fatigue, nausea, dehydration, muscle weakness, decreased appetite and diarrhea. It can lead to abnormal heart rhythms, kidney failure and unconsciousness. (2) Cushing’s Syndrome, where the overproduction of steroids can result in obesity and muscle weakness. This is usually caused by an adrenal tumor. The Pancreas is a soft, pink, triangular shaped gland, about six inches long, which lies transversely along the posterior abdominal wall. The Pancreas contain about two million islets, or small groups of endocrine cells, called the Islets of Langerhans, scattered among the exocrine cells which secrete pancreatic enzymes. (Health Encyclopedia). (1) The Alpha cells in the pancreatic islets secrete the hormone glucagon, which stimulates glycogenolysis and fat catabolism and promotes absorption of amino acids. (2) The Beta cells secrete the hormone insulin, which stimulates glucose and amino acid uptake, effects nutrient storage and antagonizes glucagon. (3) The Delta cells secrete somatostatin which modulates the secretions of the alpha and beta cells. (Rodriquez). Imbalances in the pancreatic hormones cause Diabetes Mellitus. This occurs when the pancreas do not produce sufficient insulin, causing a build up of sugar in the blood. Glucose cannot enter the cells, which are starved. Common symptoms include frequent urination, excessive thirst, weight loss, irritability and mood changes. Hypoglycemia (low blood sugar), Hyperglycemia (high blood sugar) and Ketoacidosis (diabetic coma) are complications of diabetes. (University of Maryland Medical Center web site). The Gonads consist of the ovaries in the female and the testes in the male. The two almond-shaped ovaries are located on either side of the pelvis, one at the end of each fallopian tube. They are attached to the uterus by the ovarian ligament. The ovaries secrete estrogen, which is responsible for the development of secondary sex characteristics, such as breast enlargement, hair growth in the underarms and genital area and the accumulation of fat in the hips and thighs. Estrogen also acts along with the second ovarian hormone, progesterone, to stimulate the growth of the uterine lining to receive the fertilized egg. They regulate the menstrual cycle, sustain pregnancy and prepare the mammary glands for action. The testes are two, small, egg-shaped structures suspended in the scrotum, a loose sac of skin that hangs outside the pelvic cavity between the upper thighs. The testes secrete the hormone testosterone, which stimulates the development of the male reproductive structures and the growth of body hair, increases the male sex drive and skeletal and muscular growth and the enlargement of the larynx. It also causes voice changes and maturation of the sperm. Imbalances in the sex hormones lead to the lack of sexual development. (Health Encyclopedia). The Thymus is a soft, flattened, pinkish-grey mass of lymphoid tissue located in the upper chest under the breastbone. It grows until puberty and then shrinks, finally blending into the surrounding tissue in adulthood. It secretes the hormone Thymosine, which changes the lymphocytes into T cells which attack foreign substances in the body. Imbalance in thymosine reduces the body’s immunity to disease. The Hypothalamus is the size of an almond and is a small region of the brain, composed of grey matter. It is considered a part of the endocrine system because it signals the adrenal glands to secrete hormones. It produces Antidiuretic Hormone and Oxytocin which are stored and released in the neurohypophysis. It also secretes regulatory hormones which control hormone release by the pituitary gland. (Health Encyclopedia). In addition to these main endocrine glands, other organs such as the stomach, small intestine, heart and placenta secrete hormones which participate in endocrine activity. (Seer’s Training web site). In conclusion, the endocrine system plays a pivotal role in the control and coordination of the processes of the body. References. Adam. (2001) Endocrine System. Retrieved on 16 November 2007 from http://www.besthealth.com/besthealth/bodyguide/reftext/html/endo_sys_fin.html Health Encyclopedia (2007) Body by Design VI. The Endocrine System. Retrieved on 16 November 2007 from http://www.faqs.org/health/Body-by-Design-VI/The-Endocrine-System.html Rodriquez. Endocrine System. Retrieved on 16 November 2007 from http://blue.utb.edu/biology/Rodriquez/A&PII%20Lecture/endocrine_system.htm Seer’s Training web site. Introduction to the Endocrine System. Retrieved on 16 November 2007 from http://training.seer.cancer.gov/module_anatomy/unit6_1_endo_intro.html University of Maryland Medical Center web site. Endocrinology Health Guide. Retrieved 16 November 2007 from http://www.umm.edu/endocrin/index.htm Read More