Distensibility of single capillaries and venules in the rat and frog mesentery

The distensibility characteristics of single microvessels in the mesentery of frogs and rats has been determined by a new technique capable of resolving a change in radius of about 0.01 micron in vivo. Two water insoluble immiscible oils were micro-injected into occluded segments of the vessels. Since the volume of the oil remained constant, the position of the interface at different applied pressures could be used to determine changes in vessel diameter. The stress/strain relationship of capillaries and venules in the frog mesentery and venules in the rat mesentery were determined and found to be similar to each other. Vessel diameters increased by up to 15% over the whole pressure range used but in a markedly non-linear manner, stiffening at higher strains. It is suggested that the basement membrane has the appropriate properties to explain this behaviour.