How does my heart beat speed up and slow down?

When the heart is working right, it's an astoundingly efficient machine. The blood vessels supplying the muscles in your body are open and supple like rubber bicycle inner tubes that expand as needed. The blood vessels leaving your heart shuttle blood throughout your body from one source to another no matter how much you need. Your heart is so powerful that it can change pumping levels to whatever situation you're in, whether you're running on a treadmill or away from any angry flock of geese.

Imagine clenching your fist 60 to 70 times per minute for your entire life-which is essentially what your heart does-without ever becoming exhausted. That pumping processes five liters of blood when you're resting. When you decide to pop in your 1980s Richard Simmons workout tape-or when our ancestors were being chased by a saber-tooth cat-you immediately begin to pump 20 liters of blood.

Your heart is equipped to handle the drastic change. When you step on the gas pedal and get moving, a healthy heart starts pumping more efficiently by squeezing out more blood per beat. But the heart rate only starts to increase when it's forced to do so, which is why doctors measure your heart rate as you push yourself and relax after exercise to get clues about your health.