Video narration
Why does your heart start pounding and your breathing get quicker when you climb a flight of stairs? Your legs are working harder, and they need more oxygen-the useful part of the air you breathe-to keep going. With each breath, air reaches your lungs, where oxygen moves into your blood. Your blood carries it to the muscles climbing those steps. At the same time, those muscles send carbon dioxide, a waste gas, back through the blood toward your lungs, so you can breathe it out. The pounding isn't a random alarm. Your body is responding to the extra work by speeding up delivery. Your heart pumps faster, moving oxygen-rich blood along, while your breathing quickens to bring in more oxygen and breathe out more carbon dioxide. So as you climb, your heart pounds because your muscles need more oxygen; your heart and lungs speed up delivery and carry carbon dioxide away.