It is widely accepted among the scientific community that countercurrent heat exchange between blood vessels developed as an advantageous evolutionary trait for preserving body heat in humans and other animals; however, much of the theory behind this is qualitative in nature4,5. In this study, we provide a quantitative model of countercurrent heat exchange in a human finger exposed to freezing temperatures. We seek to compare the impact of changes in various physiologically-based parameters on convective heat loss from the finger. By comparing the net effects of several parameters to that of countercurrent heat exchange, we can make a concrete claim about how important the presence of countercurrent exchange as an evolutionary trait is to the preservation of core body temperature.

In this study, we consider the tissue temperature within a human finger that is losing heat to the surrounding cold blowing air, while also gaining heat from blood vessels that participate in countercurrent heat exchange. To investigate the mechanism of countercurrent heat exchange in a human finger, we used COMSOL, a multiphysics finite element analysis and simulation software, to develop a simple geometry and replicate the heat exchanging properties of the human finger when exposed to extreme cold or freezing external conditions. The two major blood vessels we consider will be separated and surrounded by finger tissue that has its own physical properties. Heat is transferred from the owing blood, through the finger tissue, and is lost to the surrounding cold environment. Using this model, we were able to analyze how the blood and tissue temperature gradients change with position and time over while exposed to external freezing conditions.

The model demonstrated the advantages of countercurrent exchange as a critical trait in mediating convective heat loss in an organism, thereby supporting the accepted idea that it evolved as a mechanism to preserve heat. In transferring much of the heat from the arterial blood to the venous blood, instead of to the tissue, the body retains that thermal energy as opposed to allowing it to escape as convective heat loss. The net effect is a decrease in the average tissue temperature, which equates to a decrease in the flux of outward heat, henceforth maintaining core body temperature at the expense of that of the extremities.

The results of this study provide insight on the relative importance of different physiological responses to the cold. While other physiological adaptations may serve to mitigate convective heat loss and therein preserve core body temperature, the most significant factor affecting heat loss was the distance between vessels. Thus, consistent with qualitative assessments, the evolution of blood vessel geometries has proven mathematically to play a critical role in the conservation of body heat. By understanding the mechanistic underpinnings of how this works, we can form a better basis of understanding for future bio-inspired designs.