Fear is one of the most extreme emotional feelings and, like all extreme emotions, is accompanied by physiological changes in the body. This illustration shows how the body prepares itself for "fight or flight" when in a highly emotional state. The initial signal comes from the brain, which spurs the body to release adrenaline into the blood stream. This then triggers off a series of interrelated responses in the body. The mere thought of fear activates the frontal lobe of the cerebral cortex, which stimulates the hypothalamus into action. The hypothalamus, positioned in the brain, activates the suprarenal medulla. The suprarenal medulla releases adrenaline into the bloodstream and numerous responses in the body ensue. The pupils of the eyes dilate. Hair stands on end. If the skin is broken, blood will readily coagulate to prevent severe loss. The chest expands to increase the volume of inhaled air. The bronchioles relax, allowing a greater volume of oxygen to enter the lungs. The heart dilates, increasing the blood output. Blood pressure rises. Muscles contract. Blood vessels near the surface of the skin contract, causing the skin to pale. Other blood vessels dilate, and the liver releases glucose, which provides fuel for the muscles. And the bladder empties stored urine in cases of extreme fear.