Human Heart Model: Real 3D Heart, Valves, Blood Flow & Pulse Lab

The heart has four chambers: two thin-walled atria on top that receive blood, and two thick-walled ventricles below that pump it out. The right atrium receives deoxygenated blood returning from the body through the venae cavae and passes it down into the right ventricle, which pumps it the short distance to the lungs through the pulmonary artery. The left atrium receives oxygenated blood coming back from the lungs through the pulmonary veins and passes it into the left ventricle, which pumps it at high pressure out through the aorta to the whole body. A muscular wall called the septum separates the two sides, so oxygenated and deoxygenated blood never mix.

Double circulation means blood passes through the heart twice on every full trip around the body, in two separate loops. In the pulmonary circuit the right heart pumps deoxygenated blood to the lungs to load oxygen and unload carbon dioxide, and the oxygenated blood returns to the left heart. In the systemic circuit the left heart pumps that oxygenated blood out to the tissues and the deoxygenated blood returns to the right heart. Mammals and birds need this design because it keeps oxygen-rich and oxygen-poor blood completely separate and lets each circuit run at its own pressure: low for the delicate lungs, high for the body. A single loop would lose too much pressure passing through the lungs to supply an active, warm-blooded animal.

The four valves are one-way doors that keep blood flowing forward and stop backflow. The two atrioventricular valves sit between each atrium and ventricle: the tricuspid on the right and the mitral on the left. The two semilunar valves guard the ventricle exits: the pulmonary valve into the pulmonary artery and the aortic valve into the aorta. The lub-dub is the sound of valves snapping shut. The first sound, lub, is the atrioventricular valves closing as the ventricles start to contract, stopping blood from flowing back into the atria. The second sound, dub, is the semilunar valves closing as the ventricles relax, stopping blood from flowing back from the arteries.

The cardiac cycle is the repeating sequence of contraction and relaxation in one heartbeat. Systole is contraction, diastole is relaxation. First the atria contract and top off the ventricles. Then the ventricles contract; pressure rises and the AV valves slam shut (lub). With all valves briefly closed, pressure climbs until it exceeds the arteries, the semilunar valves open, and blood is ejected into the pulmonary artery and aorta. The ventricles then relax; pressure falls below the arteries, the semilunar valves close (dub), and with all valves shut the ventricles keep relaxing before the AV valves reopen and they fill again. At rest near 72 beats per minute each cycle lasts about 0.8 seconds.

The left ventricle has a far thicker, more muscular wall because it pumps blood at much higher pressure. It drives the systemic circuit, pushing oxygenated blood all the way around the body against considerable resistance, reaching a peak of roughly 120 millimetres of mercury. The right ventricle only pushes blood the short, low-resistance trip through the lungs, at a peak of about 25 millimetres of mercury, so it needs far less muscle. Crucially, both ventricles eject the same volume per beat, about 70 millilitres; the difference is the pressure they generate, not the amount they pump.

Cardiac output is the volume of blood the heart pumps each minute, equal to heart rate times stroke volume. Stroke volume is the blood ejected by a ventricle in one beat, about 70 millilitres at rest. At a resting rate near 72 beats per minute, cardiac output is roughly 72 times 70, about 5 litres per minute, close to the body total blood volume. During exercise the muscles demand more oxygen, so cardiac output rises sharply: heart rate climbs, driven by the nervous system and adrenaline, and stroke volume also increases as the heart fills more fully and contracts harder. Together these can lift cardiac output to 20 to 25 litres per minute in a fit adult.

These are the three main vessel types, each suited to its job. Arteries carry blood away from the heart and have thick, elastic, muscular walls to handle the high pressure of each beat; the largest is the aorta. Arteries branch into arterioles and then capillaries, microscopic vessels one cell thick where oxygen, nutrients, and carbon dioxide diffuse between blood and body cells. Capillaries join into venules and veins, which return blood to the heart at low pressure, with one-way valves and squeezing muscles helping it back against gravity. Note that arteries are defined by direction, not oxygen: the pulmonary artery carries deoxygenated blood to the lungs.

Oxygen-poor blood from the body enters the right atrium through the superior and inferior venae cavae, passes the tricuspid valve into the right ventricle, and is pumped through the pulmonary valve into the pulmonary artery to the lungs. Oxygen-rich blood returns through the pulmonary veins to the left atrium, passes the mitral valve into the left ventricle, and is pumped through the aortic valve into the aorta and out to the body.

The P wave is the electrical signal spreading across the atria, just before they contract. The QRS complex is the signal spreading rapidly through the ventricles, just before they contract. The T wave is the ventricles recovering (repolarising) before the next beat. The gap between P and QRS is mainly the deliberate delay at the AV node, which lets the atria finish emptying.

The coronary arteries are the heart’s own blood supply. The right and left coronary arteries branch from the aorta just above the aortic valve and run over the surface of the heart; the left splits into the left anterior descending and circumflex arteries. If fatty plaque and a clot block one, the heart muscle it supplies is starved of oxygen and starts to die — a heart attack, or myocardial infarction.

Press the tips of your index and middle fingers gently on the inside of your wrist, just below the base of your thumb (the radial artery), or gently on one side of your neck beside the windpipe (the carotid artery) — never both sides at once. Do not use your thumb, which has its own pulse. Count the beats for 15 seconds and multiply by 4 to get beats per minute. Each beat you feel is the artery stretching as the left ventricle pushes blood out.

For teenagers and adults a normal resting heart rate is about 60 to 100 beats per minute; young children have faster rates. Trained endurance athletes often rest at 40 to 60 beats per minute because exercise makes the heart muscle stronger and the left ventricle larger, so each beat pumps more blood. With a bigger stroke volume, fewer beats give the same cardiac output of about 5 litres per minute. A rough guide to the maximum heart rate is 220 minus your age.

Yes. The chambers, valves, papillary muscles and great vessels come from the Human Reference Atlas, an open 3D atlas made by the NIH-funded HuBMAP program from the Visible Human male data set (shared under a CC BY 4.0 license). SciSim simplified the meshes for the web and added the colors, the heartbeat, the valve motion and the blood-flow markers, which are teaching animations rather than a medical simulation.