Question 10

Describe the adult coronary circulation (50% of marks) and its regulation (50% of marks). 

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College Answer

This question was generally well answered. Some candidates did not mention the factors which 
are peculiar to the coronary circulation and answered in a generic manner, as if for any vascular 
bed. The coronary circulation has a high O2-ER and flow-dependence. Many candidates seemed 
to lack a perspective that metabolic demand dominates control of the coronary arterial flow. 
The phasic nature of flow was best shown with a diagram and whilst "publication-level" graphs 
are not expected, the graph drawn must be factually correct and convey the principal similarities 
and differences. While this topic is covered in both Guyton and Ganong, additional detail can be 
found in the Mosby physiology monograph series: Cardiovascular Physiology, 10th Edition by 
Pappano and Wier,( the replacement of Berne and Levy).


Adult coronary circulation:

  •  Coronary vascular anatomy:
    • Coronary arteries arise from the sinuses of Valsalva at the aortic root
    • Left main
      • Divides into left anterior descending and left circumflex 
      • Supplies most of the septum and LV
    • Right coronary
      • Supplies the RV, the sinoatrial node
    • Coronary sinus
      • Drains into the right atrium; opening is between the IVC and the tricuspid valve
      • Venous blood oxygen saturation here is ~ 30%
  • 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

      The illustration which (I think) they wanted is probably something like this diagram from the old 6th edition of BarashCoronary blood flow from orignal articles by Green Gregg and Wiggers

Physiological control of coronary circulation:

  • 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
  • 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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