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The Endocrine System • Hormones are substances secreted by cells that act to regulate the activity of other cells in the body. – Hormones affect all cells in the body and are made and secreted by endocrine glands. • Endocrine glands are ductless organs that secret hormones either into the bloodstream or the fluid around cells. • The endocrine glands can be found through out the body and are collectively known as the endocrine system. • Endocrine glands, such as the pancreas, can also be exocrine glands. – Exocrine glands secrete substances through ducts to specific locations inside and outside the body. The Endocrine System Types of Hormones • Hormones can be grouped into two types based on their structure. Hormones can either be amino acid-based hormones or steroid hormones. – Amino acid based-hormones are made of amino acids, either a single modified amino acid or a protein made of 3-200 amino acids, and are water soluble. – Steroid hormones are lipid hormones that the body makes from cholesterol and are fat soluble. • Similar to steroid hormones are thyroid hormones. • Regardless of which type of hormone is being activated, all hormones affect only their target cells. – Target cells are specific cells to which a hormone travels to produce a specific effect. – On the target cells are receptors. Receptors are proteins that bind to specific signal molecules, such as hormones, that cause a cell to respond. • Amino acid-based hormones – Amino acid-based hormones bind to receptor proteins on the cell membrane and are called first messengers. – Second messengers are then activated. A second messenger is a molecule that initiates changes inside a cell in response to the binding of a specific substance to a receptor on the outside of a cell. • Changes to the cell through enzyme action then occur in a cascade fashion. • Steroid and Thyroid Hormones – Steroid and thyroid hormones are similar because they are both fat soluble. – Because these hormones are fat soluble they can pass through the cell membrane. – Thus, these hormones can enter their target cells and bind directly to receptor sites in the cytoplasm or nucleus and directly activate enzymes. • Two other types of chemical messengers that are classified as hormones are neuropeptides and prostaglandins. – Neuropeptides are hormones secreted by the nervous system and tend to affect many cells near the nerve cells that release them. – Prostaglandins are modified fatty acids that are secreted by most cells and tend to accumulate in areas where tissues are disturbed or injured. The Hypothalamus The Pituitary Hypothalamus and Pituitary Gland • Two organs, the hypothalamus and the pituitary gland, control the initial release of many hormones for the endocrine system. – The hypothalamus is the area of the brain that coordinates many activities of the nervous and endocrine systems. • The hypothalamus secretes: • 1. ADH = Antidiuretic Hormone=stimulates reabsorption of water in the body. • 2. Oxytocin- Used during childbirth to bring on labor contractions. • The nerve cells in the hypothalamus that secrete hormones are called neurosecretory cells. – These cells secrete two types of hormones to the pituitary gland. • Releasing hormones stimulate the anterior pituitary to make and secrete hormones. • Release-inhibiting hormones inhibit production and secretion of anterior-pituitary hormones. The Pituitary Gland • 1. Known as the master gland. Controls all other glands. • 2. It is stimulated by hormones from the hypothalamus to target other glands. • 3. It secretes: – A. – B. – C. – D. – E. GH= Growth Hormone ACTH= Adrenocorticotrophic Hormone FSH= Follicle Stimulating Hormone LH=Luteinizing Hormone TSH=Thyroid Stimulating Hormone Acromegaly • Widening of bones The Thyroid The Thyroid Gland • The thyroid gland is located near the larynx and helps maintain a normal heart rate, blood pressure, and body temperature by increasing or decreasing cellular metabolic rates. • It secretes thyroxine. • The thyroid gland is also important for development. Goiter • Abnormal thyroid activity can result in hypothyroidism or hyperthyroidism. • Overproduction of thyroid hormones is called hyperthyroidism and can have symptoms that vary from overactivity to high body temperature. • A deficiency in a thyroid hormone is known as hypothyroidism and can have symptoms that vary from weight gain to retardation. The Adrenal Gland Video The Adrenal Gland • Humans have an adrenal gland located above each kidney. Each adrenal gland has an inner core, the medulla, and an outer core, also called the cortex. • The medulla and the cortex function as separate endocrine glands. – The medulla is controlled by the nervous system, and the cortex is controlled by the anterior pituitary. Cushing’s Syndrome -11month old baby Cushing’s Syndrome • Adrenal Medulla – The adrenal medulla secretes the hormones that stimulate a “flight-or-fight” response to a stress. – In this response, the hormones epinephrine, also called adrenaline, and norepinephrine are released. • These hormones increases heart rate, blood pressure, blood glucose levels, and blood flow into the heart and lungs so the body can respond to the initial stress. • Adrenal Cortex – In the presence of some stresses the pituitary gland will secrete the adrenocorticotropic hormone (ACTH). – This hormone stimulates the adrenal cortex to produce the hormone cortisol. • Cortisol promotes the production of glucose from proteins to help cells make usable energy. The Gonad Video The Gonads • Gonads are the gamete-producing organs that also produce a group of steroid sex hormones. – Gonads, ovaries in females and the testes in males, are regulated by sex hormones, which begin production at puberty. • Puberty is the adolescent stage during which the sex organs mature and secondary sex characteristics appear. • The production of sex hormones is stimulated by the release of two hormones by the pituitary. – The first hormone is luteinizing hormone (LH). This hormone stimulates ovulation and the release of progesterone in females and the release of androgens, such as testosterone, in males. – The second hormone is follicle-stimulating hormone (FSH). This hormone stimulates the growth and maturation of the ovarian follicles in females and sperm production in males. The Pancreas Video The Pancreas • The pancreas contains both exocrine and endocrine cells. The endocrine cells are called islets of Langerhans. • These cells secrete hormones that regulate the level of sugar in the blood. The hormone insulin is one of the hormones produced. – Insulin lowers the blood sugar level by stimulating body cells to store glucose or use it for energy. • A condition called diabetes mellitus occurs when cells are unable to obtain glucose from the blood. This results in a high glucose level in the blood • There are two types of diabetes: Type I and Type II. – Type I occurs when immune cells attack and destroy the islet of Langerhans cells. – Type II occurs when cells don’t have sufficient insulin levels or when the organism’s cells have become less responsive. • A condition called hypoglycemia occurs when excessive insulin is stored and not properly delivered to body cells. • This leads to a lowered blood glucose concentration, which can cause such symptoms as overactivity and dizziness. Other Glands • There are several other glands in the endocrine system, including thymus gland, the pineal gland and the parathyroid glands. • Thymus Gland – The thymus gland is located beneath the sternum and plays a role in the development of the immune system by secreting thymosin. • This amino acid-based hormone stimulates formation of T cells. Thymus Video • Pineal Gland – The pineal gland is located near the base of the brain and helps regulate sleep patterns by secreting melatonin. Pineal Gland Video • Parathyroid Gland – The parathyroid glands is made up of four glands embedded in the two thyroid glands. • These glands secrete the parathyroid hormone, (calcitonin) which stimulates the transfer of calcium ions from the bones to the blood. The Parathyroids • Digestive Cells – Some of the digestive organs also act as endocrine cells by secreting a variety of hormones that control digestive processes. Two hormones that are secreted are gastrin and secretin. • Gastrin is released by the stomach when food is eaten. • Secretin is released by the small intestine and helps stimulates the release of various digestive fluids from the pancreas. • The endocrine system plays an important role in the maintenance of a stable internal environment, or homeostasis. • Maintenance of homeostasis is controlled by feedback mechanisms. A feedback mechanism is one in which the last step in a series of events controls the first. • Feedback mechanisms can be either negative or positive. • Negative Feedback – When the final step in a series of events inhibits the initial signal in the series it is called negative feedback. – An example of negative feedback is the regulation of thyroid hormones. Negative Feedback Positive Feedback Positive feedback occurs when the release of an initial hormone stimulates release or production of other hormones or substances. An example of positive feedback is the stimulation and increase in luteinizing hormone by estrogen. • Antagonistic Hormones – Antagonistic hormones work together in pairs to regulate the levels of critical substances. – Example: Both glucagon and insulin regulate blood sugar levels. Quiz 1.What are the chemical messengers of the endocrine system called? A. neurons B. hormones C. blood cells D. carbohydrates • Answer • D. Hormones 2. X and Y are hormones. X stimulates the secretion of Y, which exerts negative feedback on the cells that secrete X. Suppose the level of Y decreases. What should happen immediately afterwards? F. Less X is secreted. G. More X is secreted. H. Secretion of Y stops. J. Secretion of X stops. • Answer • G. More X is secreted 3.Endocrine glands A. function only after puberty. B. function only before puberty. C. release products through ducts. D. release products into the bloodstream. • Answer • D. Release products into the bloodstream 4. What happens after food is eaten? F. Blood glucose levels increase. G. Blood glucose levels decrease. H. Blood glucose levels remain the same. J. Blood glucose levels decrease then increase. • Answer • F. Blood glucose increases 5. Which hormones are primarily responsible for the changes in blood glucose levels about 2 hours after food is eaten? A. insulin B. estrogen and progesterone C. epinephrine and norepinephrine D. aldosterone and parathyroid hormones • Answer • A. insulin 6.Neurotransmitters : nervous system :: hormones : F. feedback system G. endocrine system H. circulatory system J. respiratory system • Answer • G. Endocrine system 7. Thyroid stimulating hormone (TSH) is a hormone that stimulates the release of the thyroid hormones from the thyroid gland. At what time would you expect thyroid hormone levels to be at their lowest? A. 0 hours B. 4 hours C. 8 hours D. 12 hours • Answer • D. 12 hours