The Endocrine System Anatomy Ch. 9

Hormones Hormones are chemical substances that are secreted by endocrine cells into the extracellular fluids and regulate the metabolic activity of other cells in the body. Nearly all hormones can be classified as either amino acid based molecules (proteins, peptides, and amines) or steroids. All hormones are amino acid based except for sex hormones and the hormones of the adrenal cortex.

A hormone affects only certain tissue cells or organs which are called target cells or organs. For a target cell to respond to a hormone the target cell must have specific protein receptors on the plasma membrane that the hormone can attach to.

The term hormone comes from the Greek word meaning “to arouse”. Hormones bring about their effects on the body’s cells by altering the cells activity. These alterations may include increasing or decreasing the rate of a normal metabolic process

Changes that occur due to hormone binding: Permeability of the plasma membrane Formation of proteins or enzymes in the cell Activation or deactivation of enzymes Stimulation of mitosis (cell division) Increasing secretion

Mechanisms of hormone changes in the cell Direct Gene Activation Used by steroids The steroids diffuse through the plasma membrane of the target cell The steroid enters the nucleus The steroid binds to a specific receptor in the nucleus The new combination of the hormone and receptor binds to a specific site on the DNA The binding with the DNA causes certain genes to form mRNA. The mRNA sends information out of the nucleus and new proteins are made.

Second Messenger System Used by acid based molecules These hormones are not able to enter the target cell so they bind to hormone receptors on the target cells plasma membrane. The hormone which is the first messenger binds to a membrane receptor. This binding sets off a series of reactions that activates an enzyme. The enzyme causes a reaction that will produce the second messenger in the cell. The role of the second messenger is to oversee any changes in the cell and promote the response of the target cell to the hormone. There are many different second messengers.

Control of hormone release Negative feedback mechanisms are the primary means of regulating blood levels of nearly all hormones. There are 3 categories of stimuli that activate endocrine organs: Hormonal stimuli The most common type of stimuli in which endocrine organs are prodded into action by other hormones Humoral stimuli Changing blood levels of certain ions and nutrients stimulate hormone release Neural stimuli Nerve fibers stimulate hormone release

The major endocrine organs The major endocrine organs of the body include the: Pituitary gland Thyroid gland Parathyroid gland Adrenal gland Pineal gland Thymus Pancreas Gonads (testes and ovaries)

The hypothalamus is part of the nervous system but is also recognized as a major endocrine organ. The function of some gland is purely endocrine (pituitary, thyroid, adrenals, and parathyroid). Other glands, such as the pancreas and gonads, have both endocrine and exocrine functions. Endocrine glands are ductless glands which means that hormones that are produced are released into blood or lymph. Exocrine glands release products at the body’s surface or into body cavities through ducts.

Pituitary-hypothalamus relationship The anterior pituitary gland controls the activities of so many other glands that it is sometimes called the master gland. The release of each hormone from the pituitary gland is controlled by hormones produced by the hypothalamus. The pituitary gland is connected to the hypothalamus. The hypothalamus produces 2 hormones, oxytocin and antidiuretic, which are stored and released by the posterior pituitary gland.

Anterior Pituitary hormones Growth Hormone (GH) Growth of skeletal muscles and long bones Prolactin (PRL) Stimulates and maintains milk production in the mother’s breasts. Adrenocorticotropic Hormone (ACTH) Regulates the endocrine activity of the cortex portion of the adrenal gland Thyroid-stimulating Hormone (TSH) Influences the growth and activity of the thyroid gland Follicle-stimulating Hormone (FSH) Stimulates development of sperm and egg cells Lutenizing Hormone (LH) Triggers ovulation of the egg and produces progesterone and estrogen in females Stimulates testosterone production in males

Posterior pituitary hormones Oxytocin Released in significant amounts only during childbirth and in nursing women. Stimulates powerful contractions of the uterus during childbirth Causes milk ejection in a nursing woman Antidiuretic Hormone (ADH) Causes the kidneys to reabsorb more water from forming urine. As a result urine volume decreases and blood volume increases. Increases blood pressure by causing constriction of arterioles. Because of this function it is sometimes called vasopressin.

Thyroid hormones Thyroxine (T4) & Triiodothyronine (T3) Calcitonin Both control the rate at which glucose is burned and converted into body heat and energy. Every tissue in the body is a target Important for normal tissue growth and development The major metabolic hormones Difference is in the number of bound iodine atoms. Calcitonin Decreases blood calcium levels by causing calcium to be deposited in the bones

Parathyroid hormones Parathyroid Hormone (PTH) The most important regulator of calcium levels in blood. Stimulates the breakdown of bone matrix to release calcium into the blood and increase blood calcium levels.

Adrenal Cortex Hormones Aldosterone (outer most of the cortex) Regulates mineral content of the blood, particularly sodium and potassium. The target is the tubules of the kidneys that either reabsorb minerals or allow them to be flushed out of the body as urine. When aldosterone levels rise the kidneys reabsorb more sodium and secrete more potassium. When more sodium is reabsorbed then more water is absorbed. Aldosterone regulates both water and electrolyte balance.

Aldosterone also assists in the regulation of blood pressure. Renin is an enzyme produced in the kidneys when blood pressure drops and causes the release of aldosterone. Blood pressure increases. A hormone released by the heart called atrial natriuretic peptide (ANP) prevents aldosterone release and therefore reduces blood pressure.

Cortisone and Cortisol (middle layer of the cortex) Increase blood glucose levels (hyperglycemic) Decrease inflammation and reduce pain by inhibiting pain causing molecules called prostaglandins Often prescribed as drugs to suppress inflammation.

Androgens and Estrogens (inner most cortex) Sex hormones Androgens (males) Estrogens (females)

Adrenal medulla hormones Epinephrine (adrenaline) Norepinephrine (noradrenaline) Both are released as part of the sympathetic division of the ANS (fight or flight) Both are pumped into the blood stream in enhance and prolong the effects of the sympathetic division. Increase heart rate, blood pressure, and blood glucose levels, and dilate the small passageways of the lungs The result is more oxygen and glucose in the blood. Both hormones help the body to deal with short term stressful situations.

The hormones of the medulla help the body to cope with brief short term stressors. The hormones of the cortex help the body to deal with prolonged stressors.

Pancreas hormones Insulin Glucagon High levels of glucose in the blood cause the release of insulin into the blood Insulin is responsible for the storage of glucose as glycogen when blood levels are too high Insulin is hypoglycemic because it decrease the levels of glucose in blood. Glucagon Low levels of glucose in the blood cause the release of glucagon into the blood Glucagon causes the breakdown of glycogen Glucagon is hyperglycemic because it increases glucose levels. Target organ is the liver

Pineal hormone Melatonin Levels rise and fall during the night and day Peak levels occur at night making us drowsy and sleepy. The lowest levels occur during the daylight hours peaking around noon. Sleep trigger that plays a role in establishing the body’s day and night cycle. Helps to coordinate the hormones of fertility. Inhibits the reproductive system until adult body size has been reached.

Thymus hormone The thymus is largest in infants and children and decreases in size as you reach adulthood. Thymosin Are essential for normal development of T- lymphocytes (white blood cells). Important in the immune response of children.

gonads Ovaries Estrogen Development of sex characteristics in women (primarily growth and maturation of reproductive organs). Responsible for appearance of secondary sex characteristics. Development of breasts Pubic hair Widening of the hips Increased body fat

Progesterone Acts with estrogen to bring about the menstrual cycle and breast development. Quiets the muscles of the uterus so that an implanted embryo will not be aborted. Helps prepare breast tissue for lactation.

Testes Androgens: male sex hormones Testosterone Promotes the growth and maturation of the reproductive organs Causes secondary sex characteristics Facial hair Development of bones and muscles Lowering voice Continuous production of sperm