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Physiology · Pathway

Blood Flow Through the Heart Diagram

The path blood takes through the heart, in order: from the venae cavae through the right side to the lungs, then back through the left side to the aorta.

Blood passes through the heart twice on each trip round the body: through the right side on its way to the lungs, and through the left side on its way back out to the body. Dashed arrows carry oxygen-poor blood and solid arrows oxygen-rich, numbered in order. A section seen from the front, the patient's right on your left. Not to scale.

Also called Path of blood through the heart or Circulation through the heart.

The path of blood through a section of the heart seen from the front, in eight numbered stretches. Dashed arrows, for oxygen-poor blood: down the superior vena cava and up the inferior vena cava into the right atrium, through the tricuspid valve into the right ventricle, up through the pulmonary valve into the pulmonary trunk, and out along the right and left pulmonary arteries to the lungs. Solid arrows, for oxygen-rich blood: in from the lungs along the pulmonary veins to the left atrium, through the mitral valve into the left ventricle, up through the aortic valve into the aorta, and round the arch to its branches and the descending aorta.112345678Superior vena cavafrom the upper bodyRight pulmonary arteryto the right lungRight pulmonary veinsfrom the right lungPulmonary valveRight atriumTricuspid valveInferior vena cavafrom the lower bodyRight ventricleAortato the bodyLeft pulmonary arteryto the left lungLeft pulmonary veinsfrom the left lungLeft atriumAortic valveMitral valveLeft ventriclePulmonarytrunkOxygen-poor bloodOxygen-rich blood
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Step 1 of 11

Two circuits through one heart

The right side of the heart takes oxygen-poor blood from the body and pumps it to the lungs; the left side takes oxygen-rich blood from the lungs and pumps it to the body. Follow the arrows in order: dashed while the blood is oxygen-poor, solid once it is oxygen-rich.

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Step 1 of 11: Two circuits through one heart

Download the diagramChecked against its sources on 29 October 2026.

Step by step

The steps of the Blood Flow Through the Heart

  1. 01Two circuits through one heart

    The right side of the heart takes oxygen-poor blood from the body and pumps it to the lungs; the left side takes oxygen-rich blood from the lungs and pumps it to the body. Follow the arrows in order: dashed while the blood is oxygen-poor, solid once it is oxygen-rich.

  2. 02Into the right atrium

    Oxygen-poor blood returns from the body in the two venae cavae. The superior vena cava brings it from the head, neck, arms and chest, and the inferior vena cava from the legs, abdomen and pelvis. Both empty into the right atrium, as does the coronary sinus, which drains the heart's own muscle.

  3. 03Through the tricuspid valve

    While the ventricles are relaxed, blood flows from the right atrium through the open tricuspid valve into the right ventricle. At the end of this filling, the atrium contracts and pushes in more.

  4. 04Out through the pulmonary valve

    The right ventricle contracts. The rising pressure shuts the tricuspid valve, whose cusps are held by the chordae tendineae, and opens the pulmonary valve, so the blood is pushed up into the pulmonary trunk.

  5. 05To the lungs

    The pulmonary trunk divides into the right and left pulmonary arteries, which carry the oxygen-poor blood to the two lungs. Inside the lungs they branch again and again, down to the smallest vessels, the capillaries.

  6. 06In the lungs

    In the lungs the blood runs through capillaries in the walls of the alveoli, the tiny air sacs. Carbon dioxide moves out of the blood into the air in the alveoli, to be breathed out, and oxygen moves from the air into the blood, to be carried by the red blood cells. The blood leaves the lungs oxygen-rich.

  7. 07Back to the left atrium

    Oxygen-rich blood returns from the lungs in the pulmonary veins, usually two from each lung, which open into the left atrium; the right ones pass behind the right atrium to get there. The loop from the right ventricle to the lungs and back to the left atrium is the pulmonary circuit.

  8. 08Through the mitral valve

    As on the right, while the ventricles are relaxed, blood flows from the left atrium through the open mitral valve into the left ventricle, and the atrium's contraction tops it up at the end.

  9. 09Out through the aortic valve

    The left ventricle contracts at the same time as the right. The mitral valve shuts and the aortic valve opens, and blood is pushed into the aorta at a much higher pressure than on the right side, which is why the left ventricle's wall is the thickest.

  10. 10Round the body

    The aorta arches over the heart. Its first branches carry blood to the heart muscle itself, the arch's branches to the head and arms, and the descending aorta to the rest of the body. In the tissues' capillaries the blood gives up oxygen and takes up carbon dioxide, and the veins bring it back to the venae cavae: the systemic circuit.

  11. 11Both sides beat together

    The two sides work in step: both atria contract together, then, after a short pause, both ventricles. So at each beat the right ventricle pumps blood into the lungs while the left pumps blood into the body, and the four valves keep it all moving one way.

Parts

What each part of the blood flow through the heart does

Superior vena cava
Brings oxygen-poor blood from the head, neck, arms and chest down into the right atrium.
Inferior vena cava
Brings oxygen-poor blood from the legs and from the organs of the abdomen and pelvis up into the right atrium.
Right atrium
Collects oxygen-poor blood from the venae cavae and from the heart's own veins, and passes it through the tricuspid valve to the right ventricle.
Tricuspid valve
Opens to let blood from the right atrium into the right ventricle, and shuts when the ventricle contracts, so the blood goes on to the lungs instead of back into the atrium.
Right ventricle
Pumps oxygen-poor blood through the pulmonary valve to the lungs. The lungs' blood vessels are at low pressure, so it pumps at a much lower pressure than the left ventricle and its wall is thinner.
Pulmonary valve
Opens as the right ventricle contracts, letting blood into the pulmonary trunk, and closes as it relaxes, so the blood cannot fall back.
Pulmonary trunk
The wide artery that leaves the right ventricle through the pulmonary valve and soon divides into the right and left pulmonary arteries.
Pulmonary arteries
Carry oxygen-poor blood from the pulmonary trunk to the lungs, one to each lung, where they branch down to the capillaries round the air sacs.
Pulmonary veins
Usually four, two from each lung. They carry the oxygen-rich blood from the lungs to the left atrium.
Left atrium
Collects oxygen-rich blood from the pulmonary veins and passes it through the mitral valve to the left ventricle.
Mitral valve
Opens to let blood from the left atrium into the left ventricle, and shuts when the ventricle contracts, so the blood goes out to the body instead of back towards the lungs.
Left ventricle
Pumps oxygen-rich blood through the aortic valve into the aorta and round the whole body. It works at the highest pressure, so its wall is the thickest of the four chambers.
Aortic valve
Opens as the left ventricle contracts, letting blood into the aorta, and closes as it relaxes, so the blood cannot fall back into the ventricle.
Aorta
The body's largest artery. Its arch gives off the arteries to the head and arms, and it runs on down through the chest and abdomen, branching to every other part of the body.

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