The normal human resting heart rate has an upper threshold dependent on age. Heart rate increases also as a response to various stimuli, such as fever, exercise or pain, as well as substances like nicotine, caffeine or alcohol. Generally, the heart rate increases as a compensation for change of blood flow. This can happen when the blood volume decreases through dehydration or bleeding. Another common cause of tachycardia is an increase in metabolic demands of tissues. Despite the rapid heartbeat, the tachycardic heart becomes less efficient in pumping blood and provides less blood flow to the body.