Local metabolic factors dominate, especially adenosine and hypoxia
Expansion
Metabolic control dominates. The main vasodilator mediators are:
- Adenosine, released when ATP is broken down
- Hypoxia itself
- Potassium ions, carbon dioxide and hydrogen ions
- Nitric oxide from the endothelium
Autonomic effects are comparatively minor and are usually overridden. Sympathetic stimulation raises heart rate and contractility, and the resulting metabolic demand causes vasodilatation despite direct alpha-mediated vasoconstriction.
Two quantitative facts explain coronary vulnerability. Resting oxygen extraction is about 70 per cent, far higher than the body average of 25 per cent, so there is little extraction reserve. And coronary flow reserve, the ratio of maximal to resting flow, is normally fourfold to fivefold; a stenosis that consumes this reserve produces angina on exertion before it limits resting flow.