How Your Heart Pumps Blood
Your heart is a muscular pump the size of your fist that beats about 100,000 times a day. It runs two loops at once: it sends oxygen-poor (blue) blood to the lungs to refuel with oxygen, and it pushes oxygen-rich (red) blood out to the whole body. Press Play and follow the bright tracer cell on its full round trip — changing color as it picks up and drops off oxygen — while the chambers squeeze and the valves snap shut. Watch the ECG and the ventricular / aortic / atrial pressure graph move in lock-step, drag the heart-rate slider, or switch on Exercise mode and see stroke volume and cardiac output climb.
What's happening
The Science in Plain Language
1. Two loops, one pump. Blood does not travel in a single circle — it runs a figure-eight through the heart. The pulmonary circuit carries blood from the heart to the lungs and back (a short trip to pick up oxygen). The systemic circuit carries blood from the heart out to the whole body and back (a long trip to deliver that oxygen). The right side of the heart drives the lung loop; the left side drives the body loop.
2. Follow one blood cell. Oxygen-poor blood arrives from the body through the vena cava into the right atrium, drops through the tricuspid valve into the right ventricle, and is pumped through the pulmonary valve and pulmonary artery to the lungs. In the lungs it swaps carbon dioxide for oxygen — turning from blue to red — and returns through the pulmonary veins to the left atrium. From there it falls through the mitral valve into the left ventricle, which pumps it through the aortic valve into the aorta and out to every cell in your body.
3. A confusing-but-important detail. Arteries usually carry oxygen-rich blood and veins carry oxygen-poor blood — but the lung vessels are the exception. The pulmonary artery carries blue (deoxygenated) blood to the lungs, and the pulmonary veins carry red (oxygenated) blood back. "Artery" just means "away from the heart," not "oxygen-rich."
4. The cardiac cycle: fill, then pump. Each heartbeat has two phases. During diastole the ventricles relax and fill with blood. During systole they contract and squeeze that blood out. Systole itself has hidden steps: first an isovolumetric contraction where all four valves are shut and pressure shoots up with no blood leaving, then ejection once left-ventricular pressure climbs past the ~80 mmHg sitting in the aorta and the aortic valve flies open. The familiar "lub-dub" sound is not the muscle — it is the one-way valves snapping shut. The "lub" is the tricuspid and mitral valves closing as the ventricles start to squeeze; the "dub" is the pulmonary and aortic valves closing as the ventricles relax (that closure leaves the small dicrotic notch you can see on the aortic pressure trace).
5. Reading the graphs (the Wiggers diagram). The two live traces in the panel are the classic tools cardiologists use. The ECG is the heart's electrical signal: the small P wave is the atria depolarizing (their squeeze), the tall QRS complex is the ventricles depolarizing an instant before they contract, and the T wave is the ventricles repolarizing as they relax. The pressure trace stacks three curves in millimeters of mercury: the left-ventricular pressure spikes from a few mmHg up toward ~120 mmHg, the aortic pressure swings between roughly 120/80 (that is exactly what a blood-pressure cuff measures), and the low atrial pressure rides gently underneath. Every spike lines up with a beat.
6. Why the left ventricle is the thickest. Look at the diagram: the left ventricle has by far the thickest muscular wall. It has the hardest job — pushing blood past your toes, your fingertips, and up to your brain against the resistance of the entire body. The right ventricle only has to push blood a few centimeters to the nearby lungs, so its wall is much thinner. When blood pressure is measured, the top number (systolic) is largely the force of that left-ventricle squeeze.
7. Output, and what "Exercise" changes. Each beat ejects a stroke volume of about 70 mL at rest. Multiply that by the heart rate and you get cardiac output — roughly 5 liters per minute, close to your entire blood volume every minute. Switch on Exercise and two things happen together: the heart beats faster and squeezes harder, so both stroke volume and rate rise and cardiac output can climb to 15–25 L/min to feed working muscle. Systolic pressure rises to meet the demand, which is why you can watch the numbers move on the panel.