Expansion
Myogenic and metabolic mechanisms hold flow steady despite changing pressure
Expansion
Two mechanisms:
- Myogenic (Bayliss effect) - vascular smooth muscle contracts when stretched by a rise in pressure, narrowing the vessel and limiting flow
- Metabolic - a rise in flow washes out vasodilator metabolites such as adenosine, carbon dioxide, hydrogen and potassium ions, so the vessel constricts; a fall in flow lets them accumulate and the vessel dilates
The result is a plateau of nearly constant flow between roughly 60 and 160 mmHg mean arterial pressure.
Autoregulation is strongest in the brain, kidney and heart, the organs least able to tolerate interruption, and weakest in skin and skeletal muscle, where flow is dictated by thermoregulatory and metabolic demand.
In chronic hypertension the entire curve shifts to the right, which is why an abrupt reduction of pressure to apparently normal values can cause cerebral or renal hypoperfusion.