Caffeine is a stimulant of the central nervous system. As caffeine enters the body it metabolizes in three molecules that have different effects. These molecules are theobromine, paraxanthine and theophylline, which are responsible for increased flow of oxygen, increased athletic performance and increased heart rate and ability to focus. Caffeine blocks the effect of adenosine in the brain and other body parts, therefore adenosine neurotransmitters can not bind to nerve cell receptors. Since caffeine and adenosine are very similar in structure, caffeine can easily bind to adenosine receptors. By blocking the effects of adenosine, adenosine cannot do its job. Adenosine slows down the rate of nerve activity and is responsible for regulating neurotransmitters such as dopamine. By inhibiting adenosine, the caffeine increases brain activity. The increase of spontaneous nerve firing and release of neurotransmitters stimulate nerve cells, and the hormone epinephrine causes the heartbeat to increase, muscles to contract easily, respiration to increase and one's mood to change. The intake of caffeine also increases dopamine levels in the body, increasing the feel-good sensation.
Caffeine’s effect is quick to leave the body. However, if your body is highly exposed to caffeine, it will learn to tolerate caffeine’s effects and the body will become sensitized to adenosine, causing blood pressure to drop.
In the body, caffeine is metabolized in the liver by an enzyme called CYP1A2. CYP1A2 controls the speed at which caffeine is released from the body. If there is an abundance of CYP1A2 enzymes, caffeine will be released fast, transforming them into other substances. If there aren’t plenty of enzymes, caffeine will be present in the body longer. CYP1A2 metabolizes other drugs along with caffeine, therefore when they compete for the enzyme, they block the enzymatic metabolism and the speed at which they are released from the body decreases.