Pulmonary and Systemic Circulations and the Coronary Vasculature

Overview: Two Circuits, One Pump

The heart functions as a dual pump that drives blood through two distinct circuits simultaneously. The pulmonary circuit carries deoxygenated blood from the right side of the heart to the lungs and returns oxygenated blood to the left side. The systemic circuit carries oxygenated blood from the left side of the heart to every organ and tissue in the body and returns deoxygenated blood to the right side. Understanding these circuits is foundational for EKG technicians because changes in blood flow — especially to the heart muscle itself — produce the electrical changes you will detect on a 12-lead tracing.

The Pulmonary Circuit

Deoxygenated blood returning from the body enters the right atrium through two large veins: the superior vena cava (from the upper body) and the inferior vena cava (from the lower body). From the right atrium, blood passes through the tricuspid valve into the right ventricle. When the right ventricle contracts, it ejects blood through the pulmonic valve into the pulmonary trunk, which divides into the left and right pulmonary arteries — one for each lung. These are the only arteries in the body that carry deoxygenated blood.

Inside the lungs, blood travels through progressively smaller vessels until it reaches the pulmonary capillaries surrounding the alveoli. Here, gas exchange occurs: carbon dioxide diffuses out of the blood into the alveoli to be exhaled, and oxygen diffuses in. The now-oxygenated blood collects into pulmonary veins — four of them, two from each lung — which drain into the left atrium. These are the only veins in the body that carry oxygenated blood. This completes the pulmonary circuit.

The Systemic Circuit

Oxygenated blood in the left atrium passes through the mitral (bicuspid) valve into the left ventricle, the most muscular chamber of the heart. The left ventricle ejects blood through the aortic valve into the aorta, the body's largest artery. The aorta arches superiorly (the aortic arch), giving off branches to the head and arms, then descends through the thorax and abdomen, supplying organ beds including the digestive tract, kidneys, and lower limbs.

Blood delivers oxygen to tissues through systemic capillary beds, picks up carbon dioxide, and returns through the venous system. Small veins merge into larger ones, ultimately forming the superior and inferior vena cavae, which empty back into the right atrium. This completes the systemic circuit.

Key point: the left ventricle works against high systemic pressure, which is why its walls are significantly thicker than those of the right ventricle. Conditions that increase the workload on the left ventricle — such as hypertension — can cause left ventricular hypertrophy, which produces characteristic changes on an EKG.

The Coronary Vasculature: Supplying the Heart Itself

The heart muscle (myocardium) cannot extract oxygen directly from the blood passing through its chambers. It has its own dedicated blood supply through the coronary arteries, which arise from the base of the aorta just above the aortic valve at openings called the coronary ostia.

Right Coronary Artery (RCA)

The right coronary artery originates from the right coronary ostium and travels in the right atrioventricular (AV) groove. Its major branches include the sinoatrial (SA) nodal artery (in most individuals), the right marginal artery, and, in most people, the posterior descending artery (PDA). Because the RCA feeds the SA node and the AV node in the majority of the population, occlusion of the RCA can cause significant conduction disturbances — bradycardia and heart block — in addition to myocardial damage. RCA occlusion classically produces inferior wall MI, reflected in EKG leads II, III, and aVF.

Left Main Coronary Artery and Its Branches

The left main coronary artery arises from the left coronary ostium and quickly divides into two major branches:

• Left Anterior Descending (LAD) artery: Travels down the anterior interventricular groove toward the apex. It supplies the anterior wall of the left ventricle, the anterior two-thirds of the interventricular septum, and often the apex. The LAD is sometimes called the "widow maker" because occlusion causes a large anterior MI, seen in EKG leads V1–V4. Septal and diagonal branches arise from the LAD.

• Circumflex artery (Cx): Travels in the left AV groove toward the posterior surface of the heart. It supplies the lateral and posterior walls of the left ventricle and the left atrium. Occlusion produces lateral MI, reflected in leads I, aVL, V5, and V6.

Coronary Dominance

The term coronary dominance refers to which artery gives rise to the posterior descending artery. In most individuals the RCA is dominant. In a minority, the circumflex is dominant. This affects which vessel is responsible for inferior and posterior MI patterns.

Clinical Significance for EKG Technicians

Knowing coronary anatomy allows you to recognize why ST-segment changes appear in specific lead groupings. When you see ST elevation in the inferior leads, you will understand that the RCA territory is likely involved. When elevation appears in the anterior leads, the LAD territory is suspect. You will not diagnose MI — that is the physician's role — but accurate lead placement and a technically clean tracing are your direct contribution to patient safety. A misplaced electrode can mimic or mask ST changes, potentially delaying life-saving treatment. Always follow your facility's protocol and immediately report any tracing that appears abnormal to the supervising clinician.