The heart is a muscular pump that drives blood through the body in a repeating cardiac cycle: during diastole the chambers relax and fill with blood, and during systole they contract and eject it, producing the two-part "lub-dub" sound heard here as a double pulse. Contraction timing is set by the heart's own electrical system: the SA node (sinoatrial node) fires first as the natural pacemaker, and the signal passes through the AV node to coordinate atria and ventricles. The beating heart pushes blood through a closed loop of arteries, capillaries, and veins, visualized here as glowing tubes with particles representing red blood cells flowing faster right after each beat.

• Resting heart rate: ~60-100 beats per minute (trained athletes often 40-60 BPM)
• Heart rate under exertion: can exceed 150-190 BPM
• Cardiac output at rest: about 5 liters of blood per minute
• Total blood volume in an adult: roughly 5 liters
• Circulatory system length: about 100,000 km of blood vessels (arteries, veins, capillaries)
• Red blood cells: ~4-6 million per microliter, lifespan around 120 days
• Electrical pacemaker: SA node fires spontaneously to set the base rhythm
• Cardiac cycle at 75 BPM lasts about 0.8 seconds (systole ~0.3s, diastole ~0.5s)