Cardio is exercise that makes your heart beat faster and your lungs work harder for an extended period. The word is short for cardiovascular exercise, which refers to your heart and blood vessels working together. When you run, swim, jump rope, or ride your bike, you're doing cardio.

During cardio exercise, your muscles need more oxygen, so your heart pumps faster to deliver oxygen-rich blood throughout your body. Your breathing speeds up too. You might feel your heart pounding and find yourself breathing heavily, but that's exactly what's supposed to happen. Your heart is a muscle, and like any muscle, it gets stronger when you exercise it regularly.

Doctors recommend cardio because it helps keep your heart and lungs healthy, gives you more energy, and helps your body work efficiently. Different activities count as cardio: soccer players do cardio when they sprint up and down the field, swimmers do cardio during laps, and even dancing can be cardio if it gets your heart pumping.

Exercises like lifting weights or stretching mainly strengthen muscles or improve flexibility but usually don't keep your heart rate elevated for long periods, so they aren't considered cardio.