Microvascular responses in rabbit skeletal muscle after fixed volume hemorrhage.

The effect of hemorrhage on the microvascular responses in the tenuissimus muscle was studied by means of intravital microscopy in rabbits anesthetized with urethan. The rabbits were bled 30% of their calculated blood volume within 3 min. Hemorrhage initially caused mean arterial pressure to drop from 70 +/- 7 to 26 +/- 5 mmHg. During the subsequent 30-min observation period it increased to 43 +/- 8 mmHg. The transverse arterioles (TRs), supplying both muscle tissue proper and adjacent connective tissue, gradually constricted to 75% of control over the 30-min period. Terminal arterioles (TEs) branching from the TR in the muscle tissue constricted to 65% in 10 min and then gradually relaxed, eventually reaching 80% of control diameter. The constriction of the TEs was confined to a short sphincterlike structure (10-20 microns) at the origin of the bifurcation. Upon constriction, the diameter of the sphincterlike structure was less than the critical diameter for erythrocyte passage. Given that the effective blood viscosity in the narrow TE is strongly dependent on luminal diameter, the overall effect on blood flow and its distribution in the tenuissimus muscle was a dramatic reduction of volume flow to 20-30% of the control value. During the early phase, the reduced flow was diverted to the connective tissue at the expense of nutrient flow to the muscle tissue. This early blood flow pattern gradually reversed, partially restoring nutrient flow to the muscle fibers.