Venoarteriolar Reflex

The maintenance of stable blood flow despite changes in the posture of the whole subject or a limb is important. A major change in position causes a change in blood and pulse pressure and activation of arterial baroreflexes. A lesser change in position changes venous volume and activation of venous baroreflexes. When only a limb is moved, these baroreflexes are not activated. Instead, when venous transmural pressure is increased by 25 μmHg (for example, by lowering a limb), there is a reduction in blood flow by 50%. This abrupt reduction in the flow is often termed "the venoarteriolar reflex." This response exists in all tissues of the limbs, including subcutaneous adipose, muscle, and skin and hence it is an important vasoregulatory mechanism. The existence and importance of the reflex is unquestioned, but the mechanism of this "reflex" is uncertain. Investigators have attempted to define the neural pathway, citing intactness of the reflex with acute spinal blockade but disappearing after sympathectomy.