Cardiovascular Anatomy, Physiology, and the Cardiac Cycle

Why Cardiac Anatomy and Physiology Matter to EMS Providers

Every skill you will perform in cardiac emergencies—from interpreting a rhythm strip to delivering a shock or pushing medications—depends on a clear mental picture of how the heart is built and how it works. Errors in assessment often trace back to a shaky foundation in physiology. Build that foundation now.

Gross Structure of the Heart

The heart is a hollow, muscular organ located in the mediastinum, slightly left of center in the chest, resting on the diaphragm. It is surrounded by the pericardium, a tough fibrous sac with an inner serous layer that secretes a small amount of lubricating fluid. The wall of the heart itself has three layers: the outer epicardium, the thick muscular myocardium, and the smooth inner lining, the endocardium.

The heart has four chambers: the right atrium and right ventricle (the right heart) and the left atrium and left ventricle (the left heart). A muscular wall called the interventricular septum separates the two ventricles; the interatrial septum separates the two atria. The left ventricular wall is significantly thicker than the right because it must generate enough pressure to pump blood through the entire systemic circulation.

Valves and Great Vessels

Four valves ensure one-way blood flow through the heart.

• Tricuspid valve — between the right atrium and right ventricle (three leaflets)
• Pulmonic valve — between the right ventricle and the pulmonary artery (semilunar)
• Mitral (bicuspid) valve — between the left atrium and left ventricle (two leaflets)
• Aortic valve — between the left ventricle and the aorta (semilunar)

The great vessels connecting to the heart are the superior and inferior vena cavae (returning deoxygenated blood to the right atrium), the pulmonary artery (carrying deoxygenated blood from the right ventricle to the lungs), the pulmonary veins (returning oxygenated blood from the lungs to the left atrium), and the aorta (carrying oxygenated blood from the left ventricle to the body).

Blood Flow Through the Heart

Deoxygenated blood from the body → superior and inferior vena cavae → right atrium → through the tricuspid valve → right ventricle → through the pulmonic valve → pulmonary artery → lungs (gas exchange) → pulmonary veins → left atrium → through the mitral valve → left ventricle → through the aortic valve → aorta → systemic circulation.

Remember: the right heart handles venous (deoxygenated) blood; the left heart handles arterial (oxygenated) blood.

Coronary Circulation

The myocardium has its own dedicated blood supply. The left and right coronary arteries arise from the base of the aorta, just above the aortic valve, from small openings called the coronary ostia. The left coronary artery (LCA) divides quickly into the left anterior descending (LAD) artery, which supplies the anterior wall and septum, and the left circumflex (LCx) artery, which supplies the lateral and posterior walls of the left ventricle. The right coronary artery (RCA) supplies the right ventricle and, in most people, the inferior wall of the left ventricle and the sinoatrial and atrioventricular nodes.

Clinical safety point: Occlusion of the LAD is sometimes called the "widow maker" because it deprives the largest mass of the left ventricle of blood. Any patient with chest pain, diaphoresis, nausea, or jaw/arm discomfort should be evaluated for acute coronary syndrome and transported without delay.

The Cardiac Conduction System

The heart's electrical system initiates and coordinates every contraction.

1. Sinoatrial (SA) node — located in the right atrium; the normal pacemaker, generating impulses at 60–100 beats per minute at rest.

2. Atrioventricular (AV) node — located at the junction of the atria and ventricles; introduces a brief delay (~0.12 seconds) allowing the atria to finish contracting before the ventricles fire.

3. Bundle of His — a bundle of specialized fibers that carries the impulse from the AV node into the interventricular septum.

4. Right and left bundle branches — divide from the bundle of His and travel down each side of the septum.

5. Purkinje fibers — spread throughout the ventricular walls, delivering the impulse rapidly to the myocardium and causing coordinated ventricular contraction.

If the SA node fails, the AV node can pace at 40–60 bpm; if that fails, ventricular tissue can pace at 20–40 bpm—a slow, unreliable escape rhythm.

The Cardiac Cycle: Mechanical Events

One complete heartbeat is called the cardiac cycle. It has two main phases:

• Systole — the ventricles contract. Pressure rises, the mitral and tricuspid valves close (producing the first heart sound, S1), and the aortic and pulmonic valves open, ejecting blood.
• Diastole — the ventricles relax and fill. The aortic and pulmonic valves close (producing S2), and the mitral and tricuspid valves open as blood flows from the atria into the ventricles.

Stroke volume is the amount of blood ejected per beat; cardiac output equals stroke volume multiplied by heart rate. These concepts directly guide your interpretation of a patient's hemodynamic status.

Correlating Electrical and Mechanical Events

On a normal ECG (electrocardiogram):

• P wave — SA node fires; atria depolarize and contract.
• PR interval — conduction delay through the AV node.
• QRS complex — ventricles depolarize and contract (systole begins).
• T wave — ventricles repolarize (relaxation, diastole resumes).

A rhythm strip lets you see the electrical side of every cardiac cycle. Your job as an EMS provider is to connect those electrical signals to what the heart's muscle is—or is not—doing mechanically, and to assess whether the patient has a pulse that matches the rhythm.

Safety point: Always treat the patient, not the monitor. A rhythm may look organized on screen while the patient has no pulse (pulseless electrical activity, PEA). Confirm perfusion with a pulse check.