ECG Basics Part 1: Start here Lesson 1 of 22

The heart: chambers, valves and blood flow

Before you read an ECG, know what it is watching: a four-chambered muscle that works as two pumps in step.

Where the heart sits

The heart lies behind the sternum (breastbone), between the lungs, with more of it to the left of the midline. Its tip, the apex, points down, to the left and forward, and usually lies near the fifth intercostal space in the mid-clavicular line. The right ventricle is the chamber nearest the front of the chest; the left atrium lies furthest back.

Four chambers

The two atria on top are thin-walled receiving chambers. The two ventricles below are thick-walled pumps. A muscular wall, the septum, separates right from left: the interatrial septum between the atria and the interventricular septum between the ventricles. Right and left always mean the patient's right and left, so on a front-view diagram the right heart is on your left.

Four heart chambers, valves and blood flowSchematic frontal section of the heart. The right and left atria sit side by side, separated by the interatrial septum, above the right and left ventricles, which are separated by the interventricular septum. Low-oxygen blood from the superior and inferior venae cavae enters the right atrium, passes the tricuspid valve into the right ventricle, rises through the right ventricular outflow and the pulmonary valve into the pulmonary trunk, and divides into the right and left pulmonary arteries to the lungs. Oxygen-rich blood returns from the lungs in the pulmonary veins to the left atrium, passes the mitral valve into the left ventricle and leaves through the aortic valve into the aorta to the body. The pulmonary valve sits higher than the aortic valve. The left ventricular wall is thicker than the right and forms the apex.RLSuperior vena cavaPulmonary trunkPulmonary valveRight atriumInteratrial septumTricuspid valveInferior vena cava(enters from below)Right ventricleInterventricular septumAorta(to body)Right and leftpulmonary arteries(to lungs)Left atriumPulmonary veins(from lungs)Aortic valveMitral valveLeft ventricleApexLow-oxygen bloodOxygen-rich blood
Schematic four-chamber view seen from the front (the patient's right is on your left). Dashed arrows: low-oxygen blood; solid arrows: oxygen-rich blood. Not to scale.

Four one-way valves

ValveLets blood pass fromLeaflets
TricuspidRight atrium → right ventricle3
PulmonaryRight ventricle → pulmonary trunk3 (semilunar cusps)
Mitral (bicuspid)Left atrium → left ventricle2
AorticLeft ventricle → aorta3 (semilunar cusps)

The tricuspid and mitral valves sit between the atria and the ventricles, so they are called atrioventricular (AV) valves. Thin cords, the chordae tendineae, tie their leaflets to papillary muscles in the ventricle wall, which stops the leaflets turning inside out when the ventricles squeeze. The pulmonary and aortic valves guard the two vessels that leave the heart.

The route of one drop of blood

  1. Blood returning from the body enters the right atrium through the superior and inferior venae cavae.
  2. It passes the tricuspid valve into the right ventricle.
  3. The right ventricle pumps it through the pulmonary valve into the pulmonary trunk and the right and left pulmonary arteries.
  4. In the lungs it picks up oxygen and gives off carbon dioxide.
  5. The pulmonary veins (usually four) bring it back to the left atrium.
  6. It passes the mitral valve into the left ventricle.
  7. The left ventricle pumps it through the aortic valve into the aorta and out to the body.
Pulmonary and systemic circulationsLoop diagram. The right heart (right atrium to right ventricle) pumps low-oxygen blood through the pulmonary arteries to the lungs; oxygen-rich blood returns in the pulmonary veins to the left heart (left atrium to left ventricle). The left ventricle pumps oxygen-rich blood through the aorta to the body; low-oxygen blood returns in the venae cavae to the right atrium.RLPulmonary circulationLungsblood picks up oxygenBodyorgans use the oxygenSystemic circulationRight heartRight atriumRight ventricleLeft heartLeft atriumLeft ventriclePulmonaryarterieslow-oxygenPulmonaryveinsoxygen-richAortaoxygen-richVenae cavaelow-oxygenLow-oxygen bloodOxygen-rich blood
The right heart pumps blood through the lungs; the left heart pumps it around the body. Pulmonary arteries carry low-oxygen blood and pulmonary veins carry oxygen-rich blood.
Pulmonary arteries carry low-oxygen blood and pulmonary veins carry oxygen-rich blood. Arteries are named for carrying blood away from the heart and veins for bringing it back, whatever its oxygen content.

Two pumps, one beat

Both atria contract together, then both ventricles contract together. At rest each ventricle pushes out about the same volume with every beat, roughly 70 mL in an adult. The left ventricle pumps into the high-pressure circulation of the whole body, so its wall is normally about two to three times thicker than the right ventricle's. That larger muscle mass dominates the normal QRS complex, as later lessons show.

The heart's own blood supply

Heart muscle is fed by the coronary arteries, which arise from the root of the aorta just above the aortic valve: the right coronary artery, and the left main coronary artery, which divides into the left anterior descending and circumflex arteries. In most people the right coronary artery also supplies the AV node, one reason an inferior myocardial infarction can come with AV block (Lesson 18: ST segment and QT; Lesson 16: PR interval and AV block).

Electricity comes first

Every contraction is triggered by an electrical signal a fraction of a second earlier. The ECG records that signal, not the squeezing itself. The next lesson follows the signal along the heart's own wiring.

Key points

  • Four chambers: two atria receive blood, two ventricles pump it.
  • Right heart → lungs; left heart → body. Four valves keep the flow one way.
  • The left ventricle has the thickest wall and dominates the normal QRS.
  • The ECG shows the electrical trigger, not the contraction.
Check yourself

  1. Put these in the order blood meets them: left atrium, right ventricle, aorta, lungs, right atrium, left ventricle.

    Answer: right atrium → right ventricle → lungs → left atrium → left ventricle → aorta.

  2. Which valve does blood cross as it leaves the left ventricle?

    Answer: the aortic valve.

  3. Which vessels bring oxygen-rich blood into the left atrium?

    Answer: the pulmonary veins.

The ECG Lab includes a simplified Heart view that lights up the atria, the conduction system and the ventricles in time with each simulated beat.

See it move in the live simulatorNormal sinus rhythm

References

  1. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021.

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