CorpusHuman Anatomy

Heart — Internal Anatomy

Coronal section, anterior view, with blood flow

The heart is drawn as if cut in the coronal plane and viewed from the front, so the patient's right side is on the viewer's left. Blue marks deoxygenated blood in the right heart, the venae cavae and the pulmonary arteries. Red marks oxygenated blood in the pulmonary veins, the left heart and the aorta. The muscular wall (myocardium) is thin in the atria and the right ventricle and thickest in the left ventricle, which pumps against systemic pressure.

Red cells stream along the route of the circulation. One heartbeat lasts 1.2 s. In systole, the ventricular cavities shrink, the tricuspid and mitral valves close and the pulmonary and aortic cusps open. In diastole this reverses, and atrial contraction tops up the ventricles at the end of diastole.

In the body, the right ventricular outflow tract and pulmonary trunk lie in front of and to the left of the aortic root, and the two outflow tracts cross like a spiral. The diagram shows this crossing: the conus sweeps in front of the left ventricular outflow to reach the pulmonary valve, the right pulmonary artery passes behind the ascending aorta and SVC, and the left pulmonary artery passes in front of the descending aorta. Use the guided walkthrough to follow one red cell through the heart.

Step-by-step

  1. 1 · Systemic venous return

    Deoxygenated blood from the body returns to the heart through two large, valveless veins. The superior vena cava drains the head, neck and upper limbs. The inferior vena cava drains everything below the diaphragm. Both empty into the right atrium, as does the coronary sinus, which is not shown.

  2. 2 · Right atrium

    The right atrium collects venous blood at a low pressure of about 2–6 mmHg. Its wall contains the SA node, the heart's pacemaker. The interatrial septum separates it from the left atrium. On the septum is the fossa ovalis, a remnant of the fetal foramen ovale.

  3. 3 · Tricuspid valve

    In diastole the tricuspid valve opens and blood flows into the right ventricle. Atrial contraction adds the last 20% of filling. At the start of systole the cusps close (part of S1), held by the chordae tendineae, which are tensed by the papillary muscles.

  4. 4 · Right ventricle

    The thin-walled right ventricle contracts, generating about 25 mmHg. The moderator band carries the right bundle branch to the anterior papillary muscle. Blood is driven up the smooth conus arteriosus (infundibulum), which sweeps in front of the left ventricular outflow.

  5. 5 · Pulmonary valve

    Once right ventricular pressure exceeds pulmonary artery pressure, the three cusps of the pulmonary valve open. They snap shut at the end of ejection, producing P2. P2 is delayed by inspiration, which causes physiological splitting of S2.

  6. 6 · Pulmonary trunk and arteries → lungs

    The pulmonary trunk divides under the aortic arch into the right and left pulmonary arteries. The right one passes behind the ascending aorta and SVC. These are the only arteries in the adult that carry deoxygenated blood. In the alveolar capillaries, blood gives up CO₂ and takes up O₂.

  7. 7 · Pulmonary veins

    Four valveless pulmonary veins bring oxygenated blood (PO₂ ≈100 mmHg) back to the left atrium. Muscle sleeves around their openings into the atrium are the usual triggers of atrial fibrillation.

  8. 8 · Left atrium

    The left atrium is the most posterior chamber and lies directly in front of the oesophagus. It holds pulmonary venous blood at about 6–12 mmHg. This pressure is what the pulmonary capillary wedge pressure estimates.

  9. 9 · Mitral valve

    The two-cusped mitral valve opens in diastole. In systole its anterior and posterior leaflets close (M1, the loudest part of S1). Chordae from the anterolateral and posteromedial papillary muscles stop the leaflets prolapsing.

  10. 10 · Left ventricle

    The thick-walled left ventricle and the interventricular septum contract together, generating about 120 mmHg. The ventricle ejects about 70 mL per beat, an ejection fraction of about 60%. The cavity shrinks and the wall thickens with each beat.

  11. 11 · Aortic valve

    The aortic valve opens when left ventricular pressure exceeds aortic diastolic pressure (about 80 mmHg). When it closes (A2) it produces the dicrotic notch. Its right and left aortic sinuses give rise to the coronary arteries, which fill mainly in diastole.

  12. 12 · Aorta and arch branches

    Blood enters the ascending aorta and arch. From the arch, the brachiocephalic trunk, left common carotid and left subclavian arteries supply the head and upper limbs. The descending aorta carries the rest of the cardiac output to the trunk and lower limbs.

Labelled parts