Step by Step
LCA
Left coronary artery
Divides into two branches: the Left Anterior Descending (LAD), which supplies the anterior left ventricle and the interventricular septum — often called the "widow maker" since it's the most critical vessel to lose — and the Circumflex artery, which supplies the lateral and posterior left ventricle.
RCA
Right coronary artery
Supplies the right atrium, right ventricle, and — in about 85% of people (called "right dominant") — the SA node and AV node as well.
Fill
Coronary arteries fill during diastole
Unlike most vessels, coronary arteries fill with blood during diastole, when the heart muscle is relaxed. During systole, the contracting myocardium squeezes the coronary vessels closed, temporarily cutting off their own blood supply.
MI
Myocardial infarction (MI)
An MI occurs when a coronary artery becomes blocked, causing ischemia (lack of blood flow) and eventually cell death in the heart muscle it supplies. LAD occlusion is considered the most dangerous, given how much of the left ventricle it supplies.
Applied Walkthrough
1
A patient suffers a blockage in their Left Anterior Descending (LAD) artery. Since the LAD supplies the anterior left ventricle and interventricular septum, this blockage puts a large portion of the heart's pumping capacity at risk — explaining why LAD occlusions are often called the "widow maker."
2
Notice that coronary arteries themselves only fill with blood during diastole, when the heart muscle is relaxed — during systole, the contracting heart muscle actually squeezes these vessels shut.
3
This means a rapid heart rate (which shortens the relative time spent in diastole) can reduce the total time available for coronary blood flow, which is part of why certain heart conditions worsen with very fast heart rates.
4
If blood flow to the LAD's territory is cut off long enough, the affected heart muscle undergoes ischemia and eventually cell death — a myocardial infarction, with especially serious consequences given how much of the left ventricle the LAD supplies.
Exam Application
Exams test whether you can match each coronary artery (LCA, LAD, Circumflex, RCA) to the specific heart structures it supplies, and whether you understand the counterintuitive fact that coronary arteries fill during diastole rather than systole.
⚠ Common Trap
The most common trap is assuming coronary arteries fill during systole (when the heart is actively pumping blood everywhere else) — in reality, the contracting myocardium during systole squeezes the coronary vessels closed, so they can only fill during diastole when the heart muscle relaxes.
✓ Quick Self-Check
1. What are the two branches of the left coronary artery?
The Left Anterior Descending (LAD) and the Circumflex.
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2. What does the LAD supply, and why is it called the "widow maker"?
The anterior left ventricle and interventricular septum; it's called this because occlusion is especially dangerous given how much of the heart it supplies.
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3. What does the right coronary artery typically supply?
The right atrium, right ventricle, and (in about 85% of people) the SA and AV nodes.
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4. During which phase of the cardiac cycle do coronary arteries fill with blood?
Diastole.
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5. What is a myocardial infarction?
Tissue death in the heart muscle caused by a blocked coronary artery cutting off blood flow (ischemia).
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