Pressure-induced local venous constrictor responses in human fingers

Particularly in humans, systemic veins are often exposed to extensive changes in transmural pressure at altered body positions, when centrally conveyed constrictor reflexes provide compensation to maintain a proper venous return. The present aim was to explore whether also purely local mechanisms, of e.g. myogenic and/or axon reflex nature and operating at the venular-small vein levels, may contribute to offset venous pooling at local pressure increases. For such purposes the capacitance vascular responses were recorded plethysmographically in human fingertips, as most of the blood content is here contained in venules and small veins. Local increases in transmural pressure, induced by graded venous outflow obstructions, first induced by a corresponding passive-venous distension which within a few seconds was followed by a venoconstrictor response that could return the local blood content to nearly control level within 25-35 s. On sudden release of venous obstruction this 'active' response was unmasked as a prompt volume 'undershoot' that subsided in another 15-30 s. These microvascular responses could be powerful enough almost to offset the venodistension caused by transmural pressures up to 90-100 mmHg, and at brisk pressure increases they could be quite rapid in onset. As general reflex influences seemed to be excluded. these results in humans suggest that also the capacitance microvessels, like the precapillary resistance ones, can display myogenic responses to offset pressure increases. As the venular-small vein compartment contains up to 40% of the total blood volume such local responses, if generalized, may importantly contribute to venous control in humans.