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