Distensibility of capillaries in the bat wing.

Preliminary experiments in our laboratory have shown that the distensibility characteristics of the capillary compartment in the bat wing depended upon its location in the vascular tree. The capillaries were then divided into arteriolar, middle and venular segments (according to their proximity to precapillary sphincters or nonmuscular venules). The bat was enclosed in an airtight box, one wing protruding through a slit and extended over a glass plate for microscopic observations. Continuous recordings of the diameter of the capillary segments were obtained; after 5 min of control recordings, the box pressure was raised in steps of 25 mm Hg to a maximum of 100 mm Hg and then returned to control level. The duration of each step was 4 min. Each increase of the pressure led to the dilatation of the capillary, but its arteriolar segment appeared to be more distensible than the middle and venous ones. After shifting the box pressure, the diameter increase was gradual and capillary distensibility decreased with increasing pressure (the venular segment showed the most prominent reduction in distensibility). These findings suggest the existence of a longitudinal gradient of distensibility in the capillary compartment.