The atria contract and squeeze the last of the blood down through the open atrioventricular valves into the ventricles — the atrial kick that tops them off.
Hold up a fist: that is roughly the size of your heart, and it beats about $100{,}000$ times a day without a rest. It is really two pumps side by side. The right pump sends oxygen-poor blood the short trip to the lungs; the left pump sends oxygen-rich blood the long trip around the whole body. Each pump has a small upper chamber that receives blood (an atrium) and a big lower chamber that pushes it out (a ventricle).
Blood runs in a figure-eight of two loops. In the pulmonary circuit the right ventricle pushes blood to the lungs and it comes back to the left atrium, now loaded with oxygen. In the systemic circuit the left ventricle pushes blood to the body and it comes back to the right atrium, now low on oxygen. Four valves act as one-way doors so blood never flows backward: the tricuspid and mitral valves between atria and ventricles, and the pulmonary and aortic valves at the ventricle exits. The left ventricle wall is thickest because it must generate about $120\ \text{mmHg}$ to drive blood around the body, while the right ventricle needs only about $25\ \text{mmHg}$ for the lungs.
One heartbeat is the cardiac cycle: contraction is systole, relaxation is diastole. The atria contract first, then the ventricles. As the ventricles squeeze, the atrioventricular valves slam shut — the first heart sound, lub ($\text{S}_1$). Pressure builds until it tops the arteries, the semilunar valves open, and blood is ejected. As the ventricles relax and pressure falls, the semilunar valves snap shut — the second sound, dub ($\text{S}_2$). How much blood the heart moves per minute is the cardiac output, $CO = HR \times SV$: at rest about $72 \times 70\ \text{mL} \approx 5\ \text{L/min}$.
Press Play and watch the chambers contract in order — atria, then ventricles — while the valves open and close and the pressure curve climbs to its $120\ \text{mmHg}$ peak. Drag the Beats per minute slider up to the Exercise preset and see the cardiac-output readout climb as the rate rises. Rotate the heart and follow a single blue blood cell from the right ventricle up the pulmonary artery toward the lungs, then a red one from the left ventricle into the aorta — that is the double circulation in motion.
| Phase | What happens |
|---|---|
| 1. Atrial systole | The atria contract and push the last of the blood through the open atrioventricular (tricuspid and mitral) valves into the ventricles — the atrial kick. The ventricles are now full ($\approx 120\ \text{mL}$ end-diastolic volume). |
| 2. Isovolumetric contraction | The ventricles begin to contract. Pressure rises and the AV valves slam shut — the first heart sound (S1, lub). For an instant all four valves are closed and the volume cannot change. |
| 3. Ventricular ejection | Ventricular pressure tops the pressure in the arteries, the semilunar (pulmonary and aortic) valves open, and blood is ejected — to the lungs from the right, to the body from the left. Left-ventricle pressure peaks near $120\ \text{mmHg}$. |
| 4. Isovolumetric relaxation | The ventricles relax; pressure falls below the arteries and the semilunar valves snap shut — the second heart sound (S2, dub). Again all valves are briefly closed. |
| 5. Ventricular filling (rapid) | Pressure in the ventricles drops below the atria, the AV valves open, and blood that has been collecting in the atria rushes in passively. Most filling happens here, before the atria even contract. |
| 6. Reduced filling (diastasis) | Filling slows as the ventricles near full. Meanwhile the atria keep refilling — the right atrium from the vena cavae, the left atrium from the pulmonary veins — ready for the next atrial systole. |
Mammals use double circulation: blood passes through the heart twice per lap. The pulmonary circuit (right heart → lungs → left heart) sends deoxygenated blood to pick up oxygen; notice that the pulmonary artery is the one artery that carries deoxygenated blood, and the pulmonary veins the one veins that carry oxygenated blood. The systemic circuit (left heart → body → right heart) delivers oxygen to every tissue. Two loops keep the two kinds of blood separate and let each run at its own pressure — gentle for the delicate lungs, forceful for the far-flung body.
The heartbeat is myogenic: it starts inside the heart itself, not from the brain. The sinoatrial (SA) node in the right atrium is the natural pacemaker; its electrical wave spreads across the atria (making them contract), pauses at the atrioventricular (AV) node, then races down the septum and through the ventricle walls so the ventricles contract from the bottom up. The nervous system and hormones such as adrenaline only speed up or slow down this built-in rhythm — raising heart rate and the force of contraction during exercise.
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 vena 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.
Key takeaway: atria receive and ventricles pump; the right side sends deoxygenated blood to the lungs, the left side sends oxygenated blood to the body.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.
Key takeaway: two loops, pulmonary and systemic, re-pressurize blood between the lungs and the body and stop the two kinds of blood from mixing.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.
Key takeaway: valves enforce one-way flow; lub is the AV valves closing as ventricles contract, dub is the semilunar valves closing as ventricles relax.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.
Key takeaway: the cycle alternates systole and diastole, with the valves opening and closing in a fixed order so blood always moves forward.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.
Key takeaway: the left ventricle is thicker because it pumps blood at high pressure around the whole body, while the thinner right ventricle pushes blood gently through the nearby lungs.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.
Key takeaway: cardiac output equals heart rate times stroke volume, about 5 litres per minute at rest, rising several fold in exercise as both factors increase.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.
Key takeaway: arteries carry blood away from the heart at high pressure, capillaries are thin-walled exchange sites, and veins return blood at low pressure using valves.