Fundamental Concepts of Human Thermoregulation and Adaptation to Heat: A Review in the Context of Global Warming

Abstract

The international community has recognized global warming as an impending catastrophe that poses significant threat to life on earth. In response, the signatories of the Paris Agreement (2015) have committed to limit the increase in global mean temperature to < 1.5 °C from pre-industry period, which is defined as 1950-1890. Considering that the protection of human life is a central focus in the Paris Agreement, the naturally endowed properties of the human body to protect itself from environmental extremes should form the core of an integrated and multifaceted solution against global warming. Scholars believe that heat and thermoregulation played important roles in the evolution of life and continue to be a central mechanism that allows humans to explore, labor and live in extreme conditions. However, the international effort against global warming has focused primarily on protecting the environment and on the reduction of greenhouse gases by changing human behavior, industrial practices and government policies, with limited consideration given to the nature and design of the human thermoregulatory system. Global warming is projected to challenge the limits of human thermoregulation, which can be enhanced by complementing innate human thermo-plasticity with the appropriate behavioral changes and technological innovations. Therefore, the primary aim of this review is to discuss the fundamental concepts and physiology of human thermoregulation as the underlying bases for human adaptation to global warming. Potential strategies to extend human tolerance against environmental heat through behavioral adaptations and technological innovations will also be discussed. An important behavioral adaptation postulated by this review is that sleep/wake cycles would gravitate towards a sub-nocturnal pattern, especially for outdoor activities, to avoid the heat in the day. Technologically, the current concept of air conditioning the space in the room would likely steer towards the concept of targeted body surface cooling. The current review was conducted using materials that were derived from PubMed search engine and the personal library of the author. The PubMed search was conducted using combinations of keywords that are related to the theme and topics in the respective sections of the review. The final set of articles selected were considered "state of the art," based on their contributions to the strength of scientific evidence and novelty in the domain knowledge on human thermoregulation and global warming.

Keywords: acclimatization; and acclimation; exercise; fluid; global warming; heat; hydration; thermoregulation; work.

Figure 1

Central regulation of body temperature. Body temperature is regulated autonomously in the limbic system, which includes the hypothalamus. (1) The brain receives afferent signals on the state of body temperature from core (Tc) and skin (Tsk) temperatures. Tc is sensed from temperature of blood flowing to the brain and Tsk is derived from thermal sensitive nerves distribute all over the surface of the body. (2) The signals from Tsk and Tc are matched against the temperature set-point, which is about 36.8 °C in resting condition. (3) A departure from the set-point would activate a “thermostat” response through the autonomic nervous system to recalibrate body temperature back to the set-point. (4) The recalibration of body temperature may involve changes in behavior, blood flow distribution, basal metabolic rate adjustments, and the induction of sweating if heat loss is needed. (5) The temperature set-point can also fluctuate due to the influence of circadian rhythm, adaption to physical training, heat acclimatization, and pyrogens.

Figure 2

Effects of physical work and exercise on fluid homeostasis and cardiovascular (CVS) functions. Intense physical work can increase metabolic heat production by >10-fold, leading to an increase in heat storage and the activation of the sweating response to dissipate heat. Metabolic heat produced in the muscle is transferred to venous blood and transported to the skin to be dissipated to the environment. The diversion of venous blood to the skin reduces the volume of venous blood returning to the heart, leading to lower stroke volume and a higher stress load on CVS to maintain cardiac output and to meet the demand for muscle blood flow. The increased demand on the CVS is further burdened by the loss of plasma volume (PV) due to sweating, which can range about 1–3.5 L/h. The loss of PV can be defended by the influx of fluid from extravascular compartments, for up to about 2% loss of body weight. Beyond this level, and in the absence of adequate fluid replacement, excessive sweating can deplete PV, leading to lower blood volume to meet the demands of skin and muscle blood flow. Under these circumstances, physical work would need to slow down or cease due to the decrease in cardiac output. At the extreme, fainting may occur due to insufficient blood flow to the brain. LV = left ventricle and Tc = core temperature.