Skip to main content

Emergency? Call 911.Emergency steps

The Heart Lab

Try “cardiac arrest”, “recovery”, “left ventricle”, or “hydration”.

    Explore your heart

    Inside the engine

    Select each chamber, valve, and vessel to see what it does, why it matters, and how it helps you perform.

    Interactive anatomy

    Your heart, part by part

    Blood circulation

    Body → right heart → lungs → left heart → body

    Blood makes this loop over and over, all day, every day. Step through each stage or let the tour play.
    Circulation loopBlood travels from the body to the right heart, to the lungs, to the left heart, and back to the body.BodyRight heartLungsLeft heart

    Stage 1: Body

    Muscles and organs take oxygen out of the blood and add carbon dioxide. Oxygen-poor blood heads back to the heart through the veins.

    How it works

    The science, simply

    Start with the basics. Open “Go deeper” for the advanced explanation.

    How the heart pumps blood

    Each heartbeat has two main phases. First the atria squeeze, topping up the ventricles. Then the ventricles squeeze, pushing blood out to the lungs and body. Then the heart relaxes and fills again.

    Go deeper

    An electrical signal starts in the sinoatrial (SA) node in the right atrium and spreads across the atria. It pauses briefly at the atrioventricular (AV) node, giving the ventricles time to fill, then travels down specialized pathways so the ventricles contract together. The relaxing phase is called diastole; the contracting phase is called systole.

    Oxygen delivery

    Oxygen you breathe in moves from your lungs into your blood. Your heart pumps that oxygen-rich blood to your muscles, which use the oxygen to make energy.

    Go deeper

    Most oxygen in blood is carried by hemoglobin inside red blood cells. In tiny vessels called capillaries, oxygen moves from blood into tissues where it's used by mitochondria to produce energy (ATP). During exercise, working muscles extract more oxygen from each unit of blood.

    Blood circulation

    Blood travels in two loops: a short loop through the lungs to pick up oxygen and a long loop through the body to deliver it.

    Go deeper

    The pulmonary circulation (right ventricle → lungs → left atrium) runs at lower pressure. The systemic circulation (left ventricle → body → right atrium) runs at higher pressure, which is why the left ventricle wall is thicker.

    Heart rate

    Heart rate is how many times your heart beats per minute. It goes up when you exercise and down when you rest. Healthy people can have quite different heart rates.

    Go deeper

    The autonomic nervous system controls heart rate. Sympathetic ("fight or flight") activity and adrenaline speed it up; parasympathetic ("rest and digest") activity, mainly through the vagus nerve, slows it down. A heart-rate reading alone can't rule out heart disease.

    Cardiac output

    Cardiac output is how much blood your heart pumps each minute. It's heart rate multiplied by the amount pumped per beat.

    Go deeper

    Cardiac output = heart rate × stroke volume. During hard exercise, both rise, so cardiac output can increase to several times its resting value. Stroke volume depends on how well the ventricle fills, how forcefully it contracts, and the pressure it pumps against.

    Exercise adaptation

    With regular training, your heart can pump more blood with each beat, so it may not need to beat as fast at rest or at the same workload.

    Go deeper

    Endurance training can increase blood volume, stroke volume, muscle capillary density, and mitochondrial content. Some athletes develop changes in heart size or wall thickness. Telling these healthy adaptations apart from heart conditions requires clinical evaluation.

    Sources

    Clinical review: pendingUpdated 2026-10-25

    Try the Blood Flow ExplorerTest yourself in the quiz