The link between autonomic and behavioral thermoregulation

The human thermoregulatory apparatus has both autonomic and behavioral mechanisms at its disposal. Behavioral mechanisms include changing of clothes, moving to warmer/cooler/shaded areas and changing the environment by operating windows or the thermostat. For autonomous thermoregulation the body relies on metabolic responses to increase heat production, and besides sweating also on cardiovascular responses to increase heat loss and modulation of body tissue insulation.

In this edition of Temperature, Schlader et al. identify that the hypo- or hyper- tensive load on the cardiovascular system that is a consequence of autonomous thermoregulation may cause health risks for people that have problems with their heart.¹ This is exemplified by increased mortality during cold spells or heat waves in health-compromised populations; but also mild thermal challenges can have long lasting effects on systolic blood pressure in older adults.² Schlader et al. indicate that instead of undergoing these internal perturbations, the body may minimize the cardiovascular load by behavioral thermoregulation to counteract or even preemptively avoid the thermal challenge.¹

In this particular paper Schlader et al. describe how thermoregulatory behavior, by moving from a cool to a warm environment and vice versa, is preceded by small changes in blood pressure and moderate changes in skin blood flow.¹ Thermal behavior is thus successful in avoiding large internal cardiovascular perturbations in a healthy subpopulation.

Noteworthy, behavior initiated with minimal changes to core temperature, and Schlader et al. conclude that distal skin temperature (i.e., fingertip) may be the primary auxiliary signal for the body to initiate cold-defensive behavior.¹ Based on their data a similar conclusion may be drawn for heat-defensive behavior, however, Schlader et al. discuss possible limitations from the methodology and point out that face and neck skin may have a stronger influence on thermal sensation in warm conditions.¹ All in all, the data shows the strong coupling of modest changes to skin temperature in relation to initiation of thermal behavior. Moreover the behavioral thermopreferendum may work out as a second line of defense (after skin blood flow) to minimize the metabolic and water expenditure for body temperature regulation.

But what if the thermosensory pathway is impaired, such as in older adults or diabetics?³ Could a lack of thermoregulatory response add to cardiovascular problems in these populations? The work of Schlader et al. gives clear clues on how to proceed with this matter and the link between autonomous and behavioral thermoregulation may prove critical especially in those populations who have impaired autonomous means of regulating body temperature. For instance, monitoring of temperature and cardiovascular parameters with wearables may be used to inform individuals, or their medical professionals, that they should show thermoregulatory behavior in order to avoid adverse thermal challenges.

Apart from strong health implications the work of Schlader et al. opens new perspectives with other research disciplines. For instance, current research on indoor environments focuses on design and operation of sustainable buildings, with minimal energy consumption for heating and cooling. The ultimate goal is to make buildings robust for behavioral thermoregulation, for instance by applying local heating or cooling mechanisms, or thermally dynamic environments to try to keep building occupants comfortable.^4,5

In practice a major issue in this type of research is to find a variable to monitor that predicts behavior and comfort on an individual level. Typically temperatures of the hands are used, and this is confirmed as a good choice by the results of Schlader et al., at least for cold conditions. However, a novel finding by them suggests new variables to monitor: “cardiovascular and skin blood flow responses … were not different between the first and final behaviors.”¹ Thus these variables may be investigated further as reliable indicators for upcoming thermal behavior.

The work of Schlader et al. is right at the interface between autonomic and behavioral thermoregulation and aids to elucidate how autonomous and behavioral thermoregulation work in tandem to protect the milieu intérieur of the body. This knowledge in turn may lead to healthier, more comfortable and sustainable environments.