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The Endocrine System Before You Read The endocrine system releases chemicals called hormones directly into the bloodstream. These hormones control many of the body’s functions. One important hormone is the human growth hormone. What role do you think this hormone serves in helping the body function? When do you think this hormone is most important to human development? Write your response on the lines below. ◗ Read to Learn Control of the Body Internal control of the body is directed by two systems. In this section you will learn about the endocrine system. Later, you will learn about the other system, the nervous system. The functions of all body systems are controlled by the interaction between the nervous system and the endocrine system. As you will recall, in mammals, a gland is a cell or a group of cells that secretes fluid.The endocrine system is made up of a series of glands, called endocrine glands. Endocrine glands release chemicals directly into the bloodstream. These chemicals relay information to other parts of the body. How do the nervous and endocrine systems interact? The endocrine system and the nervous system work together much of the time. The two systems maintain homeostasis in the body. As you will recall, homeostasis is the ability of a living organism to maintain internal equilibrium or conditions that enable it to survive. Because there are two control systems within the human body, the nervous system and the endocrine system, coordination is needed between the two. The hypothalamus (hi poh THA luh mus) is the part of the brain that connects the endocrine system and the nervous system. The hypothalamus receives messages from other areas of the brain. It also receives messages from the internal organs. When a change in homeostasis is detected, the hypothalamus stimulates the pituitary gland. Identify Main Ideas As you read, highlight the glands of the endocrine system. Use another color to highlight what these glands do. The pituitary gland (pih TEW uh ter ee) is the main gland of the endocrine system. The pituitary gland is located in the skull, just beneath the hypothalamus. The hypothalamus controls the pituitary gland. The two are connected by nerves and by blood vessels. When the hypothalamus receives messages, the pituitary gland releases its own chemicals, or it stimulates other glands to release their chemicals. The pituitary gland controls endocrine glands, including the thyroid gland, the adrenal glands, and glands associated with reproduction. How do hormones travel? The endocrine glands secrete chemicals called hormones into the bloodstream. Remember that a hormone is a chemical that is released in one part of an organism that affects another part of the organism. Hormones carry information to other cells in the body. Hormones give these other cells instructions regarding metabolism, growth, development, and behavior. Once glands release the hormones, the hormones travel in the bloodstream. Hormones then attach themselves to target cells. Target cells have specific binding sites for hormones. These binding sites are located either on the plasma membranes or in the nuclei of these cells. The binding sites on target cells are called receptors. How does human growth hormone (hGH) work? Human growth hormone, hGH, provides a good example of an endocrine system hormone. When your body is actively growing, blood glucose levels are slightly lowered because the growing cells use up the sugar. The hypothalamus detects this low blood glucose level. The hypothalamus then stimulates the production and release of hGH from the pituitary gland into the bloodstream. The hormone hGH binds to receptors on the plasma membranes of liver cells. This, in turn, stimulates the liver cells to release glucose into your blood. Your cells need this glucose to keep growing. 1. Evaluate What is the relationship between the hypothalamus and the pituitary gland? Negative Feedback Control If homeostasis is disrupted, the body responds. The endocrine glands are stimulated. Endocrine glands can be stimulated by the nervous system, by changes in blood chemistry, or by other hormones. One type of internal feedback mechanism generally controls adjustments to the endocrine system. This is called a negative feedback system. In a negative feedback system, the hormones, or their effects, are fed back to suppress or slow the original signal. Once homeostasis is reached, the signal stops. The hormone is no longer released. How does the feedback system work? Most of the endocrine glands operate under a negative feedback system. A gland synthesizes and secretes its hormone. The hormone travels in the blood to the target cells. The needed response occurs in these target cells. Information concerning the hormone level or its effect on these target cells is fed back. The feedback is usually sent to the hypothalamus or the pituitary gland to regulate, or change, the gland’s production of the hormone. How do hormones controlled by the negative feedback system work? Antidiuretic hormone, ADH, is one of the hormones that is controlled by a negative feedback system. If you have lost water because your body has been sweating, you will feel thirsty. You feel thirsty because the water content of your blood is reduced. The hypothalamus is able to sense the concentration of water in your blood. The hypothalamus determines that your body is dehydrated. It responds by stimulating the pituitary gland to release antidiuretic (AN tih di yuh reh tihk) hormone (ADH). Antidiuretic hormone (ADH) reduces the amount of water in your urine. The hormone binds to receptors in the kidney cells. In the kidneys, the hormone ADH promotes the reabsorption of water. ADH also reduces the amount of water that is excreted in urine. Information about the blood water levels is constantly fed back to the hypothalamus. The hypothalamus can then regulate the pituitary gland’s release of ADH. If the body becomes overhydrated, or has too much water, the hypothalamus stops stimulating the release of ADH. What does the hypothalamus do when you are dehydrated? How do hormones that are controlled by a negative feedback system control blood glucose levels? Another example of a negative feedback system involves the regulation of blood glucose levels. Unlike most other endocrine glands, the pancreas is not controlled by the pituitary gland. After you have finished eating a meal, your blood glucose levels are high. When the blood glucose levels are high, the pancreas releases the hormone insulin. Insulin signals the liver and muscle cells to take in glucose. This lowers the blood glucose levels. When the blood glucose levels become too low, the pancreas releases another hormone called glucagon. Glucagon signals cells in the liver to release stored glycogen as glucose. Hormone Action Once hormones are released by an endocrine gland, they travel to target cells, and they cause a change. There are two basic types of hormones. They are grouped according to how they act on their target cells. The two groups are steroid hormones and amino acid hormones. What are steroid hormones? Hormones that are made from lipids, or fats, are called steroid hormones. Steroid hormones are lipid-soluble. As a result, they can diffuse freely into a cell through the cell’s plasma membrane. The hormones bind to a hormone receptor inside the cell. The hormone, bound to its receptor, forms a hormone-receptor complex. The hormone-receptor complex travels to the cell nucleus. In the cell nucleus, this hormone-receptor complex starts the process for making specific messenger RNA (mRNA) molecules. The mRNA molecules move out to the cytoplasm. The mRNA molecules transport instructions from DNA in the nucleus for the synthesis of the required proteins while in the cytoplasm. What are amino acid hormones? The second group of hormones is made from amino acids. Remember that amino acids can be strung together in chains. Proteins are made from long chains of amino acids. Some hormones are short chains of amino acids while other hormones are long chains. Once amino acid hormones are secreted into the bloodstream, they bind to receptors. These receptors are embedded in the plasma membrane of the target cell. From the plasma membrane, they open ion channels in the membrane. Ions have electrical charges. These ion channels route signals down the surface of the membrane to activate enzymes within the cell. The enzymes change the behavior of other molecules inside the cell. 3. What are the two groups of hormones? ________________________ Adrenal Hormones and Stress The adrenal glands help prepare your body for stressful, difficult situations. The adrenal glands are located on top of the kidneys. These glands consist of two parts—an inner portion and an outer portion. The outer portion secretes steroid hormones. These include glucocorticoids (glew ko KOR tuh koydz) and aldosterone (ahl DOS tuh rohn). These steroid hormones cause an increase in available glucose. They also raise blood pressure. As a result, they help the body combat fear, very hot or very cold temperatures, bleeding, infection, disease, and other common anxieties. The inner portion of the adrenal gland secretes two amino acid hormones. One is epinephrine (eh puh NEH frun). Epinephrine is often called adrenaline. The other hormone is called norepinephrine. Remember the fight-orflight response? During this response, the hypothalamus relays impulses to the nervous system. The nervous system stimulates the adrenal glands to increase the output of both epinephrine and norepinephrine. These two hormones stimulate heart rate, blood pressure, and the rate of breathing. They increase the efficiency of muscle contractions and they also increase blood sugar levels. Thyroid and Parathyroid Hormones The thyroid gland is located in the neck. This gland regulates metabolism, growth, and development. The main metabolic and growth hormone of the thyroid is thyroxine. Thyroxine affects the rate at which your body uses energy. It also determines how much food you need to eat. The thyroid gland secretes calcitonin (kal suh TOH nun). This hormone regulates calcium levels in the blood. The body needs the mineral calcium for blood clotting, the formation of bones and teeth, and for normal nerve and muscle function. Calcitonin binds to the membranes of kidney cells. Calcitonin then causes the kidneys to excrete more calcium. Calcitonin also binds to bone-forming cells. It causes these bone-forming cells to increase calcium absorption and to make new bone. 4. How are epinephrine and norepinephrine involved in the fight-or-flight response? Parathyroid hormone (PTH) is involved in mineral regulation. This hormone is produced by the parathyroid glands. Parathyroid glands are attached to the thyroid gland. The release of PTH leads to an increase in the rate that minerals are absorbed in the intestine. The three minerals affected are calcium, phosphate, and magnesium. PTH causes the release of calcium and phosphate from bone tissue. PTH also increases the rate at which the kidneys remove calcium and magnesium from urine and return these two minerals to the blood. Hormones associated with the endocrine system control many different body functions. Different hormones play more important roles during various stages of growth and development. They are the main biological influence on your behavior and on your development. 5. What three minerals are regulated by PTH? Activity: