First, caffeine mimics a naturally occurring chemical called adenosine and binds to the brain's adenosine receptors and the adenosine receptors in other organs of the body. The receptors are blocked by caffeine, which prevents adenosine from binding to them. Adenosine's role in the brain is to slow down nerve impulses and cause drowsiness, so caffeine's presence inhibits this reaction and increases alertness and responsiveness. In their stimulated state, nerve cells can release the hormone epinephrine (adrenaline), which increases heart rate and blood pressure, increases blood flow to muscles, decreases blood flow to the skin and organs, and prompts the liver to release glucose (a form of liquid sugar which supplies the body with energy).