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What are hormones?
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A hormone is a chemical messenger that is secreted or released into our bloodstream by specific organs known as glands.
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Hormones regulate or control many processes in our body,
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including blood glucose levels, water content in blood, general growth and blood pressure, just to name a few.
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As an example, thyroxine, which is a hormone secreted by the thyroid gland, helps to regulate metabolism.
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In other lessons you'll learn about a few of these hormones in more detail.
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Several hormones are secreted by the pituitary gland, located right below the hypothalamus in your brain.
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Instructions are given by the hypothalamus to the pituitary gland, according to current conditions in our body.
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The secreted hormones travel through the bloodstream to target organs, where either a specific response is directly produced,
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or stimulates or causes the target organ to secrete its own hormones.
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We can think of this like a post office system.
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Hormones or post must reach the correct target organs or addresses for the right response to occur.
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Feeling thirsty?
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When the hypothalamus detects low levels of water in blood, it signals or tells the pituitary gland
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to release a hormone called the antidiuretic hormone or ADH into your bloodstream.
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ADH travels to your kidneys, in this case the target organ
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and causes water to be absorbed so urine becomes more concentrated and output is decreased.
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Drinking more water increases the water content in your blood,
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which then causes the hypothalamus to signal the pituitary gland to secrete less ADH.
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Less ADH means your kidneys will absorb less water, causing urine to become less concentrated and output is increased.
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Ever felt suddenly frightened?
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When threatened, the brain activates neurons that stimulate the adrenal gland, located right above your kidneys to rapidly secrete adrenaline.
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Adrenaline then acts on your heart and lungs to increase heart rate and breathing rate.
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This along with other physiological responses is known as the Fight or Flight response and is activated very quickly.
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At the same time the hypothalamus signals the pituitary gland to secrete the adrenocorticotropic hormone,
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which travels to your adrenal glands to secrete a hormone known as cortisol.
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Cortisol acts in target organs to allow the body to adapt long-term to stress.
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So as a review: Hormones are chemical messengers secreted by glands into the bloodstream
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and produce a specific response when they reach the target organ.