Press a cardboard tube over a partner's chest and you can hear each heartbeat. Count the beats for 30 seconds, then multiply by two to get the beats per minute. That simple calculation gives you the resting heart rate — the starting point before any exercise.

Your heartbeat sends a pressure wave through every artery, so you can count beats per minute at more than one place on the body. The wrist (radial artery), neck (carotid artery), and inner thigh (femoral artery) each connect to a different artery in a different region. Checking multiple pulse sites — and using a stethoscope to match the pulse to the heart sounds — confirms how the rate holds consistent across the body.

The number of beats per minute varies from person to person. Gender may play a role in resting heart rate. Females in one test averaged 85 beats per minute while males averaged 79.

Your heart beats faster when your body works harder. Walking up and down stairs for three minutes raises the number of beats per minute. Music playing during the same activity can push the heart rate even higher.

Chemicals can alter the number of beats per minute, even in animals other than humans. Daphnia are small freshwater creatures with transparent bodies, which means you can watch their hearts directly under a microscope. You dissolve three over-the-counter medications into separate water solutions — pseudoephedrine (a decongestant), aspirin (a pain reliever), and Benadryl (an antihistamine) — then place new groups of daphnia in each one. A video camera attached to the microscope records the heartbeats in slow motion, and comparing each group to the plain-water control shows which drugs speed the heart up and which slow it down.