Significance of vessel size and type in vascular heat transfer.

This study was undertaken to gain a better understanding of the fundamental mechanisms of micro- and macrovascular heat transfer by experimentally identifying those vessels most important in the process. Tissue temperature fields around thermally nonequilibrated vessels were determined using a small temperature sensor that was guided through the rabbit thigh to generate a detailed temperature map. The measurements revealed that the lower limit of vessel size for thermal nonequilibration was 100 microns for arteries and 400 microns for veins. Local temperature fields were found around four of the five (80%) arteries that were greater than 300 microns in diameter but in only 3 of the 12 (25%) veins greater than 400 microns. These experimental results are in good agreement with previously published theoretical studies (5) in which it was concluded that thermal equilibration in the branching countercurrent vascular network of the rabbit limb occurs in vessels an order of magnitude larger than the capillaries. In those studies the smallest vessels capable of carrying heat were predicted to be 50 microns ID with the major blood tissue heat exchange occurring in vessels greater than 100 micron ID. These findings contrast with the view that most heat transfer occurs in the capillaries and suggest that vascular heat transfer analysis must take into account the vascular architecture of the 50- to 1,000-micron vessels where most heat transfer occurs.