Thermal windows and heat exchange

Endothermic animals, such as birds and mammals, regulate their body temperature balancing the amount of heat metabolically produced and that exchanged with the environment. Thus, whenever ambient temperature rises and/or metabolic heat production increases (such as during exercise), adjustments to promote heat dissipation are activated to defend body temperature. One of such adjustments modulates heat transfer from inner organs to specific body parts, which in turn alters the temperature gradient between the animal and the environment and, therefore, affects heat dissipation. On the contrary, when ambient temperature or heat production decreases, heat transfer to these areas is down-regulated and heat loss prevented. Therefore, the underlying mechanism of such response is a temperature dependent change in vascular perfusion, i.e., vasoconstriction and vasodilation.¹

Although the response just described may occur in any body region, there are cases in which specific body parts assemble features that optimize heat exchange. These regions are referred to as “Biological Thermal Windows” and are characterized by enlarged surface area, poor insulation, rich vascular bed and, most importantly, provided with the ability to alter blood flow under different conditions.¹ These features combined allow for thermal windows to modulate some of the physical parameters involved in heat transfer, which can be pinpointed on the appropriate heat transfer equations. As changes in vascular perfusion are not energetically demanding, thermal windows broaden the temperature range in which endothermic organisms can regulate body temperature with minimum costs (the thermoneutral zone), particularly by extending its upper limit.

Usually, biological thermal windows are manifested as body appendages as ears, tails, and bills. Accordingly, familiar examples include the tail of rodents,¹ the ears of jackrabbits² and elephants,³ and the toucan bill.⁴ For this latter case, the dramatic changes in vascular perfusion and, therefore, in surface temperature of the thermal window (bill), is herein illustrated by means of infrared imaging technology (Slide 1). Graphically, changes in surface temperature of a generalized thermal window vs. ambient temperature sits in the middle area of changes observed for body regions covered with non-vascular insulative materials, as hairs and feathers, and regions in which blood flow is kept continuously high (Slide 1).

Slide 1.

Body temperature, heat exchange, and thermal windows.

Teaching Slide

A PDF of this Teaching Slide can be downloaded from the publisher's website.