Describe the regulation of the coronary circulation.

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College Answer

Some answers suffered from listing things rather than describing things as the question required. Better answers included a description of metabolic, physical and neuro-humoral factors and the relative importance of each. Many described detailed anatomy which was not necessary.


  • Coronary blood flow
    • 5% of cardiac output, or 50-120ml/100g of myocardial mass
    • 75% of the left main flow and 50% of RCA flow occurs in diastole
    • In systole, LV blood flow is reduced due to the high chamber pressure during contraction
    • For the RV, the systolic chamber pressure is lower, and blood flow is less affected 
    • Thus, diastolic time is more important for LV perfusion, and it can be compromised by tachycardia
  • Coronary blood flow is automatically regulated to meet metabolic demand
    • Myocardial oxygen extraction ratio is already very high (60-70%).
    • Thus, the myocardium cannot increase its oxygen extraction efficiency to meet increased metabolic demand
    • Thus, coronary arterial blood flow increases to match myocardial oxygen demand, and the oxygen extraction ratio remains stable.
    • With exercise, coronary blood flow can increase several-fold
  • Mechanisms of coronary blood flow autoregulation
    • Metabolic substrates and byproducts are thought to act as vasoactive mediators in the coronary circulation
    • Multiple agents are considered important, including adenosine, O2, CO2, lactate, pH, and potassium ions.
    • ATP-sensitive potassium channels also open in response to decreased ATP, resulting in smooth muscle membrane hyperpolarisation and thus relaxation
    • Relative importance:
      • Adenosine and nitric oxide are most important during conditions of hypoxia or ischaemia
      • ATP-sensitive potassium channels are most important under normal loading conditions, and regulate coronary flow in response to sustained changes in systemic arterial pressure
      • Potassium ions have greatest importance in regulating brief changes in coronary flow, rather than sustained changes
  • Other influences on coronary blood flow
    • Myogenic autoregulation (intrinsic arterial smooth muscle property)
    • Autonomic nervous system
      • α1-adrenergic receptor activation stimulates vasoconstriction
      • β-adrenergic receptor activation produces vasodilation
      • Muscarinic receptor stimulation produces coronary vasodilation
    • Various pharmacological agents with coronary vasoactive properties include:
      • Vasodilators (adenosine, GTN, dipyridamole)
      • Vasoconstrictors (vasopressin, COX inhibitors


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