The Endocrine System The Endocrine System is one of main controlling systems in the body. The endocrine system works closely with the nervous system in maintaining a steady state within the body. While the nervous system tends to produce a rapid response by using electrical/chemical stimuli, the endocrine system works much slower and over a longer period of time. The endocrine system produces and releases hormones into tissue fluids and into the bloodstream. Hormones act as messengers that act to regulate organs in other parts of the body. Hormones are designed to act only on specific tissues within the body, creating a physiologic response by the receptor organ. Hormones are regulated by a negative feedback system, which keeps the hormones in balance by activating and curtailing the release of hormones. Disorders in an endocrine gland result in a physiological imbalances. Hypothalamus The nervous and endocrine systems are linked by the hypothalamus gland, which stimulates or inhibits the activity of the pituitary gland. Pituitary The pituitary gland is called the master gland, in that it releases hormones that affect the workings of other glands in the body. It is located within the brain, just behind the point in which the optic nerves cross. The pituitary is broken down into two parts, the anterior lobe, which releases hormones when stimulated by the hypothalamus, and the posterior lobe, which stores hormones, produced by the hypothalamus and are released when they are stimulated by the nervous impulses. Hormones controlled by the anterior lobe of the pituitary include: Human Growth Hormone (Somatotropin) - act by increasing amino acid uptake and promoting protein synthesis in most tissues. Thyroid Stimulating Hormone - stimulates the thyroid gland to produce hormones. Adrenocorticotropic Hormone - stimulates the adrenal cortex of the adrenal glands. Prolactin - stimulates the production of breast milk in females. Follicle Stimulating Hormones - stimulates the development of eggs in the ovaries and sperm in the testes. Luteinizing Hormone - causes sex hormone secretion in both males and females and also causes ovulation in females. Hormones controlled by the posterior lobe of the pituitary include: Antidiuretic Hormone - helps the body to conserve water by promoting the reabsorption of water from the kidneys. Oxytocin - causes contraction of the uterine muscles and promotes the release of milk from the breast. Pineal Gland The pineal gland is a small structure located in the mid-brain. The pineal produces the hormone melatonin, which helps to regulate the sleep cycles. Thyroid Gland The thyroid gland is the largest endocrine gland in the body. Made up of two lobes that sit on either side of the larynx, the thyroid gland releases thyrozine, which increases energy metabolism and protein metabolism. Parathyroid Gland Embedded in the rear of the thyroid gland, are four tiny bodies called the parathyroid gland. The parathyroid glands are responsible for calcium metabolism, particularly the release of calcium from the bone tissues, thus increasing the amount of calcium circulating in the bloodstream. Thymus The thymus gland is a collection of lymph tissue that lies in the center of the chest, just above the heart. This gland is critical for the development of immunity and the development of T-lymphocyte (T-cells). Adrenal Glands The adrenal glands are located just above the kidneys. The Adrenals are made up of two separate parts that have independent functions. The inner area called the medulla releases two hormones, adrenaline and norepinephrine, which controls our fight-or-flight response. Physiological actions that occur when this gland is stimulated include: The contraction of the walls of the arterioles, which causes blood pressure to rise. Conversion of glycogen in the liver to glucose, in order to produce that emergency energy required. An increase of the heart rate. An increase in metabolic rate of body cells. Dialation of the bronchioles The outer portion of the adrenal gland, the adrenal cortex, secretes three types of hormones. They include: Glucocordiocoids - maintain the carbohydrate reserves in the body, as well as to suppress the inflammatory response of tissues. Mineralocorticoids - help to regulate the electrolyte balance in the body by controlling the reabsorption of sodium and secretion of potassium by the kidneys. Sex hormones - are secreted in small amounts. While slight in their effects, this source of sex hormones becomes important as a woman reaches menopause. Islets of Langerhans The Islets of Langerhans are specialized cells that are located in the pancreas which secrete the hormones insulin and glucagon. The correct amounts of insulin and glucagon are required in to regulate blood sugar levels. Insulin is required to transport glucose (a form of simple sugar) into the cells to be used for energy. Insulin is also used to lower the level of sugar in the blood by increasing the rate at which the liver converts glucose into glycogen for storage in fatty tissues. Glucagon, on the other hand, causes the liver to convert glycogen back to glucose and release it back into the blood stream for use by the cells for energy. Ovaries The ovaries produce two hormones estrogen and progesterone that are responsible for the stimulation and preparation of the uterus for pregnancy. Testes The testes produce the hormone testosterone, which is responsible for development and maintenance of primary and secondary male sexual characteristics. Kidney While the kidneys are not typically thought of as secreting hormones, the kidneys actually do secrete two different hormonal substances: Renin - helps to regulate blood pressure. Erythropoieten - stimulates the production of red blood cells.