Why the Inner Layer Suffers First | Visual AI Learning Guide

Look at the cross-section of the ventricular wall. The coronary arteries sit on the outside, on the epicardial surface, and their branches dive inward. Blood has to travel from outside to inside. The subendocardium, the layer just under the inner lining, is at the end of that journey. It gets the least supply. Now add two more pressures. During each contraction, the muscle squeezes the vessels running through it, and that compression is strongest in the inner layer, so flow there can drop or even reverse briefly. And by the law of Laplace, wall stress is highest at the inner surface, so the subendocardium also demands the most oxygen. Least supply, greatest demand. That is why, when perfusion falls, the inner layer becomes ischemic first, and why necrosis spreads outward from there.

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Coronary arteries run along the epicardial surface and send branches inward, so blood must travel from the outside of the ventricular wall toward the inner lining. The subendocardium — the layer just beneath the endocardium — sits at the end of that path and is therefore the last region to receive perfusion and the first to lose it when flow falls. Two additional factors compound this. First, intramyocardial pressure is highest in the subendocardium during systole, because contracting muscle compresses the vessels running through it; this external compression can briefly reduce or even reverse flow in the inner layer. Second, wall stress is greatest at the inner surface according to the law of Laplace, so the subendocardium also has the highest oxygen demand. The result is a transmural gradient: the subendocardium has the least favorable supply and the greatest demand. When coronary perfusion pressure drops or flow is limited, the subendocardium becomes ischemic first, and with prolonged or severe reduction the necrosis spreads outward from the subendocardium toward the epicardium in a wavefront pattern.