You run up a flight of stairs and feel your pulse racing and your legs aching. You are using one of the hardest working muscles in your body–your heart. Your heart adapts to the circumstances you face by beating faster or slower, sending more or less blood to parts of the body. As the heart beats it sends blood through vessels to the entire body. Blood vessels called arteries bring blood from the heart to capillaries throughout the body. Blood vessels called veins collect blood from capillaries and return it to the heart. Capillaries are where an exchange takes place. Nutrients and oxygen move out of the blood in capillaries into the cells of the body. Carbon dioxide and other substances move from the cells into the capillary blood. If you laid the body's blood vessels end to end they would measure about 60,000 miles—far enough to circle the earth more than twice. Your body contains about 5 liters of blood, and your heart beats about 100,000 times every day. Your heart is a vital organ—isn't it time to love your heart?