Can models of human thermoregulatory responses predict how the body will react in nature?
Heatstroke is a condition that occurs when the human body temperature rises to, or above 40°C, because of natural exposure to extreme environmental heat (classic heatstroke) or strenuous exercise in hot environments (exertional heatstroke). Untreated heatstroke symptoms can lead to permanent damage to vital organs, muscles, and the brain. Modelling the body’s response to heat exposure could help predict and prevent instances of heatstroke.
The human body’s core temperature ranges between 36.7°C and 37.5°C during homeostasis. When exposed to heat, the body’s core temperature is maintained within this range through thermoregulation and the heat stress response. These processes create a homeostatic balance within the body, where it maintains functional despite changes in the environment (equilibrium).
During thermoregulation, the body responds to heat through negative feedback with changes in behaviour and the autonomic nervous system to remove heat. Heat is removed from the body via the circulatory system, through the skin and then from the skin to the environment through convection, radiation, and/or evaporation.
In the heat stress response, the body reacts to environmental changes and generation of heat in the muscles on a cellular level. Heat detection triggers a stress response in the body and causes the production of heat shock proteins. These proteins are created using energy that is normally reserved for growth. The proteins work to identify other proteins in the body that have been impacted by heat (and responded by unfolding or misfolding). A similar response occurs in the endoplasmic reticulum and the mitochondria within the muscle cells, which signal damaged proteins and respond by triggering the unfold protein response. These two responses work together to maintain cell function or trigger apoptosis (programmed cell death).
The mechanisms that help control body temperature can vary due to several factors. These include environment, such as one of high humidity or long-term high heat; amount of body fat, with the body retaining more heat the more adipose tissue it has; body mass, with the rise in core body temperature being inversely correlated to total body mass; clinical factors that disrupt body processes such as sweating (e.g. burns); and other variables such as fitness, age and acclimatisation.
Humans can gradually acclimatise to heat or change in exercise levels, which will reduce heat stress. Impacts of acclimatisation include lower heart rate, earlier sweating of higher volumes and faster trigging of the heat stress response. These changes are reversible and varied.
Due to the complexity of the human response to stress and the impact of factors, such as acclimatisation, it is unclear what level of environmental heat triggers the heat stress response, its duration, the variability in body performance, and what factors lead to failure to maintain homeostatic balance.
Adapted from: Bouchama, A., Abuyassin, B., Lehe, C., Laitano, O., Jay, O., O’Connor, F. G., & Leon, L. R. (2022). Classic and exertional heatstroke. Nature Reviews Disease Primers, 8(1), 1-23.
Questions for students:
- List the way(s) in which the body will respond to a change in temperature.
- Describe how homeostasis responds to change using a negative feedback loop.
- Select one response a human body has to temperature change and explain the role it plays in controlling heat exchange.
Key words for students to understand:
- Heatstroke
- Classic heatstroke
- Exertional heatstroke
- Thermoregulation
- Heat stress response
- Homeostatic balance
- Endoplasmic reticulum
- Equilibrium
- Mitochondria
- Negative feedback
- Convection
- Radiation
- Evaporation
- Heat shock proteins
- Unfold protein response
- Apoptosis
- Adipose tissue
- Acclimatisation