ENHANCEMENT IN THE EFFECTIVE THERMAL-CONDUCTIVITY IN RAT SPINOTRAPEZIUS DUE TO VASOREGULATION

This study was undertaken to gain a better understanding of the counte rcurrent heat exchange of thermally significant blood vessels in skele tal muscle by measuring the vascular structure and flow in an exterior ized rat spinotrapezius muscle and estimating the enhancement in the e ffective thermal conductivity of the muscle. Detailed anatomic measure ments of the number density and length of countercurrent vessel pairs between 45 and 165 mu m diameter were obtained. Moreover, diameter and blood pow in the 1A to 3A vessels were measured for muscles in which pharmacological vasoactive agents were introduced allowing one to vary the local blood flow Peclet number from I to 18 in the major feeding arteries. These combined measurements have been used to estimate the r ange of possible enhancement in the effective thermal conductivity of the tissue. The newly derived conduction shape factor in Zhu et al. [2 3] for countercurrent vessels in two-dimensional tissue preparations w as used in this analysis. Our experimental data indicated that the val ue of this conduction shape factor was about one-third to two-thirds t he value for two countercurrent vessels of the same size and spacing i n an infinite medium. The experiment also revealed that the Weinbaum-J iji expression for k(eff) was valid for the spinotrapezius muscle when the largest vessels were less than 195 mu m diameter. A fivefold incr ease in k(eff) was predicted for 195 mu m diameter vessels. Vasoregula tion was also shown to have a dramatic effect on k(eff). A tissue that exhibits only small increases in k(eff) due to countercurrent convect ion in its vasoconstricted state can exhibit a more than fivefold incr ease in k(eff) in its vasodilated state.