Cessation and onset of muscle capillary flow at simultaneously reduced perfusion and transmural pressure

The effect of simultaneously reduced perfusion and transmural pressure on muscle capillary flow was investigated in the tensor plagiopatagii muscle of unanesthetized bats. Perfusion and transmural pressure were simultaneously changed by suction applied to a sealed box containing the body of the bat, while the wing with the muscle under observation outside the box was exposed to atmospheric pressure. Flow ceased in almost all of the muscle capillaries at box pressures ranging from -46 to -96 mmHg (n = 40; median value: -66 mmHg). When flow in the muscle had ceased, flow was still present at a reduced level in capillaries on top of the muscle and in the web. No signs of obstruction to flow in the feeding or collecting vessels were found. The results are not in accordance with the phenomena causing cessation of flow as predicted by the theories of critical closure or interfacial tension. To restore flow in the muscle capillaries suction to the box had to be reduced by 14 mmHg (medium value; range 4 to 35 mmHg). The latter data conforms to the possible existence of yield stress in small vessels.