Coronary Circulation

CORONARY CIRCULATION

• Two coronary arteries (right & left) – Arise from root of aorta.
• Blood flow is non-uniform during cardiac cycle.

Coronary blood flow:

• Over 70% of total flow – Occurs during diastole.
• Almost 5%  of total cardiac output – Occurs at rest – 250 ml per minute.

Regulation factors:

By 2 mechanisms:

1. Chemical regulation:

• Very important mechanism.
• Mainly by local intrinsic regulation (Eg: autoregulation)
• By production of vasodilating metabolites, in response to minimal ischemia.
• Vasodilatory chemical stimuli – Low PO2, High PCO2, H+, K+, lactic acid, ADP, AMP & adenosine.

2. Neural regulation: 

• Coronary arteries are supplied by autonomic sympathetic & relatively few parasympathetic (vagal) fibers.

Effects of autonomic sympathetic fibers:

• Have both alpha & beta sympathetic (adrenergic receptors), beta receptors predominating. 
• Beta-receptor stimulation – Produces vasodilation.
• Alpha receptor stimulation – Produces vasoconstriction.

Effect of parasympathetic (vagal) fibers:

• Indirect efect (predominate) – Vasoconstriction.
  • Due to decreased heart rate & cardiac contractility→ Resulting in decreased vasodilator metabolites formation.
• Direct effect – Vasodilatation.

Factors influencing:

1. Perfusion pressure:

• Coronary blood flow is directly related to perfusion pressure.
• Perfusion pressure = Arterial pressure – venous pressure.

Arterial pressure:

• Coronary arteries arise directly from base of aorta.
• Left ventricle directly pushes blood into coronary arteries.
• Hence, coronary arterial pressure is same as aortic pressure.

Venous pressure:

• Majority of veins from heart drain into right atrium directly.
• Hence, coronary venous pressure is same as right atrial pressure.

Coronary perfusion pressure:

• Coronary perfusion pressure during systole = Systolic aortic pressure – right atrial pressure.
• Coronary perfusion pressure during diastole = Diastolic aortic pressure – right atrial pressure.
• Coronary perfusion pressure is greater during systole.
• As systolic aortic pressure is more than diastolic aortic pressure.
• Even though perfusion pressure is more during systole, blood flow is more during diastole.

2. Resistance:

• Coronary blood flow is inversely proportional to coronary vascular resistance.
• Coronary blood flow = Coronary perfusion pressure/vascular pressure.
• High blood flow resistance – During systole.
• Due to myocardial contraction & vessel squeezing.
• Hence, coronary blood flow is less during systole & more during diastole.

Don’t Forget to Solve all the previous Year Question asked on CORONARY CIRCULATION