Caffeine affects many body systems simultaneously. Sometimes the effects are complementary, but other times the effects counteract one another. Further, if you have caffeine sensitivity, a tiny dose might affect you as severely as a large dose would affect someone else.
Effect
Coffee and other caffeinated products produce a mild euphoric feeling and increase alertness. Caffeine consumption typically leads to alertness because it is a stimulant, which means it affects the central nervous system.
Adenosine Receptors
When caffeine reaches your brain, it interferes with normal adenosine activity. Adenosine receptors in your brain allow adenosine to bind to them, but they also allow caffeine to bind to them. When adenosine binds with the receptors, a sedative effect occurs, slowing brain activity. Adenosine is a byproduct of cellular metabolism, which means the more active your neurons are, the more adenosine they produce. Essentially, the process allows your brain to self-regulate.
Caffeine
When caffeine binds to the adenosine receptors, adenosine can't have its normal sedative effect. Consequently, you feel more alert. This stimulant effect is counterbalanced by caffeine's vasoconstrictive effect on blood vessels in the brain. Caffeine causes the brain's blood vessels to narrow, decreasing blood flow within your brain. However, the stimulant effect is greater than the vasoconstrictive effect, so the overall effect is stimulation.
Cardiovascular Effects
Caffeine also might increase your heart rate and blood pressure, though typically only after high doses. No one knows the exact mechanisms that underlie all the stimulant effects caffeine has on the cardiovascular system, but several theories exist. Caffeine seems to affect brain centers that regulate the cardiovascular system, and it also might stimulate your adrenal gland to release adrenaline, which in turn stimulates your cardiovascular system.
Athletic Performance
Caffeine also seems to stimulate improved physical performance and endurance, though no one is sure exactly how. Caffeine can cause the bronchioles in your lungs to dilate, or widen, so this might allow more air to pass through and lead to a corresponding increase in athletic performance. And caffeine might facilitate the use of fats in the blood for energy, allowing your body to conserve its stores of glucose, which normally fuel activity.
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