Saturday, April 21, 2018

Coronary circulation: Fun fact about blood flow to the myocardium

Hello everyone, here is a post busting a myth about coronary circulation.

Many of us have always thought that the heart itself receives blood during diastole which is only partially true. Here is the complete story.

Getting back to basics, let us revise a bit about the cardiac anatomy.

The heart receives its blood supply from the coronary arteries namely, the right and the left coronary artery. They arise at the base of the aorta from the openings called Coronary Ostia located behind the aortic valve leaflets. The left coronary artery divides into the left circumflex and the left anterior descending artery.

As we all know the blood flow in any vessel is determined by the difference of pressure between its two ends. For a coronary artery the two ends are the aorta (where it begins) and the ventricle (where it finally ends). This means the pressure difference between the aorta and the ventricle determines the blood flow in the coronary artery, and blood will only flow in the coronary artery when this pressure difference is positive. (Aorta at higher pressure and ventricle at lower pressure)

Look at this table carefully,

Pressure in mm of Hg
Aorta Left ventricle Right ventricle
Systole 120 120 25
Diastole 80 0 0

Clearly, there is a positive pressure difference between the aorta and left ventricle only during diastole but there is a positive pressure difference between the aorta and the right ventricle at both the times the systole and the diastole!

So we can infer, blood flow to the myocardium via the coronary arteries occurs,
- Only during the diastole to the left ventricle
- Both during diastole and systole to the right ventricle (rather more during the systole than diastole)

Where can we apply this??

Since no blood flows to the sub endocardium of the left ventricle during systole, this region is the most susceptible to ischemic injury and so is the most common site for myocardial infarction.

Hope this cleared up some myths about the coronary circulation!

That’s all for now,

Stay Awesome!
Keep calm and keep studying!

- Ashish G. Gokhale


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