Anatomy and Physiology of the Heart: Chambers, Valves, and Coronary Circulation

Overview: Why the Heart Matters to Respiratory Therapists

The heart and lungs work as a single cardiopulmonary unit. Every breath you coach a patient to take is useless unless the heart can move oxygenated blood to the tissues. As a respiratory therapist, you must understand cardiac anatomy in detail — not just to pass the NBRC exam, but because cardiac dysfunction directly causes or worsens respiratory failure. Pulmonary edema, cor pulmonale, and cardiogenic shock are daily realities in any clinical setting you will enter.

The Four Chambers

The heart is divided into a right side and a left side by a muscular wall called the interventricular septum (between the ventricles) and the interatrial septum (between the atria). Each side has two chambers: an atrium above and a ventricle below.

Right atrium (RA): Receives deoxygenated blood returning from the body through the superior vena cava (from the head and upper body) and the inferior vena cava (from the lower body). The coronary sinus also drains cardiac muscle venous blood into the right atrium.

Right ventricle (RV): Receives blood from the right atrium and pumps it through the pulmonary trunk into the pulmonary circulation. The RV wall is thinner than the left because it works against lower pressure (the pulmonary circuit is a low-resistance system).

Left atrium (LA): Receives oxygenated blood from the lungs via the four pulmonary veins — the only veins in the body that carry oxygen-rich blood.

Left ventricle (LV): The most muscular chamber. It pumps oxygenated blood into the aorta and out to the entire systemic circulation. Because systemic vascular resistance is much higher than pulmonary resistance, the LV wall is roughly three times thicker than the RV wall.

The Four Valves

Valves keep blood moving in one direction. There are two types: atrioventricular (AV) valves, which sit between atria and ventricles, and semilunar valves, which guard the exits from the ventricles.

Tricuspid valve: Located between the right atrium and right ventricle. It has three leaflets (cusps). It opens during ventricular filling (diastole) and closes during ventricular contraction (systole) to prevent backflow into the RA.

Pulmonary (pulmonic) valve: A semilunar valve at the opening of the pulmonary trunk. It opens when the RV contracts and closes when the RV relaxes, preventing blood from flowing back into the RV.

Mitral (bicuspid) valve: Located between the left atrium and left ventricle. It has two leaflets and is held in place by chordae tendineae and papillary muscles. Mitral valve disease can cause pulmonary congestion and dyspnea — a primary reason patients need respiratory therapy.

Aortic valve: A semilunar valve at the origin of the aorta. It opens during LV systole to allow blood into the aorta and closes during diastole. Aortic stenosis increases LV workload and can lead to heart failure.

The Complete Path of Blood

Trace this pathway and learn it exactly for the NBRC exam:

Body tissues → Superior/Inferior vena cava → Right atrium → (through tricuspid valve) → Right ventricle → (through pulmonary valve) → Pulmonary trunk/arteries → Lungs (gas exchange) → Pulmonary veins → Left atrium → (through mitral valve) → Left ventricle → (through aortic valve) → Aorta → Body tissues.

Key fact: The pulmonary arteries carry deoxygenated blood and the pulmonary veins carry oxygenated blood — the reverse of the naming convention in the systemic circulation.

Coronary Circulation

The heart muscle (myocardium) cannot use blood passing through its chambers for its own oxygen supply. It is fed by the coronary arteries, which arise from the aorta just above the aortic valve at the sinuses of Valsalva and fill primarily during diastole.

Left coronary artery (LCA): Divides almost immediately into two major branches:

• Left anterior descending (LAD) artery: Travels down the front of the heart in the anterior interventricular groove. Supplies the anterior LV wall, anterior two-thirds of the interventricular septum, and the apex. Occlusion here causes an anterior MI — the most common and often the most hemodynamically significant. Impaired septal and LV function can cause acute pulmonary edema, requiring immediate respiratory therapy intervention (oxygen, noninvasive ventilation, or intubation).
• Left circumflex (LCx) artery: Wraps around the left side of the heart. Supplies the lateral and posterior LV wall and the left atrium.

Right coronary artery (RCA): Runs along the right side of the heart. Supplies the right atrium, right ventricle, inferior LV wall, and — in most individuals — the SA node and AV node. Occlusion causes an inferior MI and can result in bradycardia and heart blocks.

Clinical Relevance and Safety

An LAD occlusion (often called a "widow maker" MI) can rapidly impair LV function, causing blood to back up into the pulmonary circulation and producing pulmonary edema. Patients present with severe dyspnea, pink frothy sputum, hypoxemia, and crackles. Respiratory therapists are often first-responders managing the airway and ventilation in these emergencies.

Scope of practice reminder: Respiratory therapists assess breath sounds, administer oxygen and aerosol therapy, manage mechanical ventilators, and perform arterial blood gas sampling. Diagnosing MI or interpreting electrocardiograms for definitive diagnosis falls within the physician's scope. Always communicate your findings to the supervising clinician immediately and document accurately.