Structure of the heart
The heart is a muscular pump about the size of a fist, sitting between the lungs. It has four chambers: twoatria on top that receive blood, and two ventricles below that pump it out. A wall called the septum separates the right side (deoxygenated blood) from the left side (oxygenated blood). Diagrams show the heart as if you were facing the person, so the right side of the heart appears on the left of the picture.
Four valves keep blood moving one way. The tricuspid and mitral(bicuspid) valves sit between each atrium and ventricle. The pulmonary and aorticvalves sit where blood leaves the ventricles.
The pathway of blood
- Deoxygenated blood from the body returns through the venae cavae to the right atrium.
- It passes through the tricuspid valve into the right ventricle.
- The right ventricle pumps it through the pulmonary valve into the pulmonary artery and on to the lungs.
- In the lung capillaries it picks up oxygen and gives up carbon dioxide.
- Oxygenated blood returns through the pulmonary veins to the left atrium.
- It passes through the mitral valve into the left ventricle.
- The left ventricle pumps it through the aortic valve into the aorta, which branches to every organ.
Double circulation
Mammals and birds have a double circulation: blood passes through the heart twice on each full trip. The pulmonary circuit carries blood between the heart and lungs at low pressure, which protects the delicate lung capillaries. The systemic circuit carries blood to the rest of the body at high pressure, so it can reach the brain, the feet and everywhere between. That is why the left ventricle has a much thicker muscular wall than the right.
The cardiac cycle
Each heartbeat follows the same sequence. In atrial systole the atria contract, topping up the ventricles. In ventricular systole the ventricles contract: the pressure rise slams the tricuspid and mitral valves shut (the first heart sound, “lub”) and forces the pulmonary and aortic valves open. Indiastole the muscle relaxes, the pulmonary and aortic valves shut (the second sound, “dub”), and blood flows in from the veins to refill the chambers. The beat is started by the heart’s own pacemaker, thesinoatrial node, in the wall of the right atrium.
Heart rate and cardiac output
Cardiac output is the volume of blood each ventricle pumps per minute:
cardiac output = heart rate × stroke volume
At rest, 70 beats per minute × 70 mL ≈ 4.9 L per minute: roughly all of your blood every minute. During hard exercise, adrenaline and nerve signals raise both the rate and the strength of each beat, and output can reach 15–20 L per minute in a fit adult, or more in trained athletes.
Arteries, veins and capillaries
- Arteries carry blood away from the heart. They have thick, elastic, muscular walls to cope with high pressure.
- Veins carry blood back to the heart at low pressure. They have thinner walls and valves to stop backflow.
- Capillaries are only one cell thick, so oxygen, nutrients and waste can pass between blood and tissues.
Key terms
- Atrium / ventricle
- Upper receiving chamber / lower pumping chamber of the heart.
- Systole / diastole
- Contraction / relaxation of the heart muscle.
- Stroke volume
- Volume of blood pumped by one ventricle in one beat.
- Septum
- Muscular wall dividing the left and right sides of the heart.
Common misconceptions
- “Arteries always carry oxygenated blood.” The pulmonary artery carries deoxygenated blood. Arteries are defined by direction: away from the heart.
- “Deoxygenated blood is blue.” It is dark red. Blue is just a diagram convention.
- “The two sides of the heart beat one after the other.” Left and right contract together; it is the atria and ventricles that take turns.
Educational model only. This simulator is a simplified diagram, not a medical tool. It cannot diagnose or assess any heart condition, and the values shown are typical textbook figures. If you have concerns about your heart or health, speak to a qualified healthcare professional. See our educational and medical disclaimer.
Key takeaways
- The right side of the heart pumps blood to the lungs; the left side pumps it to the body.
- Valves keep blood flowing one way and make the “lub-dub” sounds as they shut.
- Each beat is atrial systole, ventricular systole, then diastole.
- Cardiac output = heart rate × stroke volume.
Model note: the diagram is schematic, not to scale, and the vessels are rearranged so the two circuits are easy to follow. One trip round the loop takes about 8 beats here; in a resting person it takes about a minute.