As you move, muscles in your legs help send the blood back up to the heart and lungs. Without gravity, blood tends to collect around the heart and in the upper part of the body. The amount of blood pushed through the heart by each heartbeat is called the stroke volume. Although stroke volume increases with activity, it usually remains relatively stable. For this experiment, we'll assume stroke volume is constant at about 75 milliliters per beat.
Cardiac output is the amount of blood pumped by the heart in a unit of time.
Different conditions affect cardiac output on Earth and in space. Since cardiac output changes it is called a variable. Subject sits in a chair for 2 minutes. Using the index and middle finger, pulse-taker locates the subject's pulse by applying light pressure to the wrist to measure heartbeat. At a cue from the timer, the pulse-taker counts the number of heartbeats in 30 seconds, multiplies the number by two and reports the heartbeats per minute to the recorder.