## Coronary Blood Flow Autoregulation **Key Point:** Coronary blood flow remains relatively constant despite changes in systemic arterial pressure between 60 and 140 mmHg. This autoregulation is achieved through **two independent but complementary mechanisms**: myogenic and metabolic. ### Mechanisms of Coronary Autoregulation | Mechanism | Trigger | Response | Time Scale | |-----------|---------|----------|------------| | **Myogenic** | Increased pressure → stretch of vascular smooth muscle | Vasoconstriction; decreased pressure → vasodilation | Immediate (seconds) | | **Metabolic** | Increased metabolic demand → ↓O₂, ↑adenosine, ↑H⁺, ↑K⁺ | Vasodilation; increased blood flow matches oxygen demand | Gradual (minutes) | **High-Yield:** Both mechanisms work **in parallel** to maintain constant coronary flow (approximately 0.8–1.0 mL/min/g of myocardium) across a wide range of perfusion pressures. ### Metabolic Factors in Coronary Autoregulation 1. **Adenosine** — produced during ATP breakdown; potent vasodilator 2. **Hydrogen ions (H⁺)** — accumulate during anaerobic metabolism; cause vasodilation 3. **Oxygen tension (PO₂)** — hypoxia triggers vasodilation 4. **Potassium (K⁺)** — released from active myocardium; vasodilator 5. **Prostaglandins** — PGI₂ is a vasodilator **Clinical Pearl:** When systemic pressure drops below 60 mmHg (critical perfusion pressure), autoregulation fails and coronary flow becomes pressure-dependent — a critical threshold in cardiogenic shock. ### Why Other Options Are Incomplete - **Baroreceptor reflex** modulates heart rate and contractility but is not the primary mechanism of coronary autoregulation - **Metabolic vasodilation alone** cannot explain the immediate response to pressure changes (myogenic component is essential) - **Parasympathetic innervation** has minimal direct effect on coronary vascular tone [cite:Guyton & Hall Textbook of Medical Physiology Ch 11]
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