Coronary Circulation (Physiology)
For NEET PG, focus on the unique metabolic and mechanical regulation of coronary blood flow. Unlike other organs, the heart consumes the most oxygen and relies heavily on aerobic metabolism.
1. Regulation of Blood Flow
- Metabolic Autoregulation: This is the dominant mechanism. Adenosine, hypoxia, and nitric oxide (NO) are the primary vasodilators during increased oxygen demand (e.g., exercise).
- Systolic Compression: Coronary flow to the left ventricle is unique because it is restricted during systole due to high intramyocardial pressure. Peak flow occurs during diastole.
- Right Ventricle: Receives flow throughout both systole and diastole due to lower pressure.
2. NEET PG “Must-Know” Concepts
| Factor | Clinical Significance |
|---|---|
| Tachycardia | Reduces diastolic filling time, significantly decreasing coronary perfusion. |
| Oxygen Extraction | Myocardium extracts nearly all  (extraction ratio > 70%) at rest; increased demand requires increased flow, not extraction. |
3. Dominance
- Right Dominant: 85% of people; RCA gives rise to the Posterior Descending Artery (PDA).
- Left Dominant: 8% of people; LCx gives rise to the PDA.
- Codominant: 7% of people.
NEET PG Hint: When you see a question about tachycardia-induced angina, the physiology is: diastolic duration decreases → coronary perfusion time decreases → inadequate blood supply to the subendocardium. Also, remember that adenosine is the key vasodilator for autoregulation, often tested in the context of pharmacologic stress testing.