Effect of pial arteriolar constriction on red cell velocity in pial venules and on venular diameter

Erythrocyte (RBC) velocities in pial venules were measured with high speed microcinematography, during constriction of pial arterioles, and following recovery of the arterioles. Constriction of arterioles was elicited by local application of BaCl₂. In 24 of 27 paired measurements, in 18 mice, RBC velocity in the venule, diminished during constriction of the arterioles. The diminution in RBC velocity occurred whether or not the venule changed diameter. The data indicate that constriction of pial arterioles can result in a diminution of cortical blood flow, manifest by a corresponding reduction in RBC velocity through efferent venules draining the cortex. In half the cases, venular diameter remained unchanged during paired velocity measurements, while in half the trials venules became smaller when the arteriole constricted. A diminution in the diameter of the venule is interpreted as passive collapse, since venules of this size (30 μm or less) are devoid of smooth muscle. Passive collapse would be indicative of a fall in intraluminal pressure in venules, during the arteriolar constriction, and is further evidence of an effect of the latter on overall hemodynamics in the cerebral microcirculation.