Once you take that first sip of coffee, the caffeine enters your blood from the stomach and small intestine and begins to stimulate your central nervous system. The receptors in the cells within your heart are stimulated by the caffeine and increases your heart rate. As your heart beats faster — accelerating by about three beats per minute — your blood flow speeds up, too. The increase in your heart rate from caffeine can take effect in as soon as 15 minutes and take approximately six hours to wear off.