Expansion
Adenosine, carbon dioxide, hydrogen and potassium ions, and hypoxia
Expansion
Accumulating metabolites cause local arteriolar dilatation:
- Adenosine, from ATP breakdown; especially important in heart and brain
- Carbon dioxide and hydrogen ions
- Potassium ions, released by repeatedly firing cells
- Lactate and rising osmolality
- Hypoxia itself
- Inorganic phosphate
Two patterns result:
- Active (functional) hyperaemia - blood flow rises in proportion to metabolic activity, as in exercising muscle, where flow can increase twentyfold
- Reactive hyperaemia - after a period of occlusion, flow overshoots because metabolites have accumulated; this is the flush seen when a tourniquet is released
The system is elegantly self-regulating and largely independent of nerves and hormones, which is why it persists in a denervated or transplanted organ. It is also why local metabolic control overrides sympathetic vasoconstriction in the coronary and cerebral beds, a phenomenon known as functional sympatholysis.