Under certain stressful situations, the brain sends impulses to the two small adrenal glands that lie on top of the kidneys. They produce two chemicals called adrenaline and noradrenaline, which affect the body in various ways. These chemicals are released into the bloodstream to help the body react to stressful and potentially dangerous situations. This is often called the "fight-and-flight response." The heart rate and blood pressure increase, the breathing gets faster and deeper, sugar is released into the blood from the liver providing energy and fuel for the muscles, and the person feels sweaty and nervous. These chemicals can be measured in the blood. The brain senses, feels, hears, or sees the danger and the adrenal glands produce the chemicals that enable the person to either fight or flee.
High levels of adrenaline and noradrenaline in the blood for long periods increase the blood pressure, and this increases fat deposits in arteries. Sudden increases in these chemicals are thought to inflame the artery wall, causing cracking of the surface layer of the fat. Blood clots form on the cracked fat layer, blocking off the artery, and cause a heart attack.
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