How does extreme heat affect the human body?

In response to high environmental temperatures, the body’s thermoregulation mechanisms include sweating and cutaneous vasodilation – the widening of blood vessels to direct blood flow towards the skin surface and away from the gastrointestinal tract.

To compensate for reduced central blood volume (amount of blood in heart, lungs, and surrounding vessels) and decreased blood pressure, the heart contracts faster and more forcefully. In people with pre-existing cardiovascular conditions, this strain on the heart may place them at risk of ischemic heart disease, heart attacks, arrhythmias, and other adverse outcomes. 

After a prolonged period of reduced blood flow, the gut epithelial barrier becomes ischemic (lacking blood supply) and “leaky” – allowing toxins from gut-colonizing bacteria to enter the bloodstream. These endotoxins induce a strong inflammatory response affecting the entire body. At the same time, thermal damage to the blood vessel lining triggers blood clotting pathways, which lead to blockage of blood vessels and dysregulated bleeding (consumptive coagulation).

It is thought that together, sustained inflammation and coagulation pathways cause serious multi-organ dysfunction in heat stroke. The brain, kidneys, liver, lungs, heart, gut, and skeletal muscle are most affected.

Residence greatly impacts vulnerability to heat exposure.

 The urban poor are more likely to live in dilapidated or public housing that lack cooling and insulation systems. Low-income workers are more likely to engage in heavy physical labor outdoors, which increases heat exposure and susceptibility to heat stroke.

Lastly, these residents may lack access to quality healthcare, social support, and labor protections such as time off from work – impeding recovery in the event of heat illness.

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References

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Garcia, Christian K, et al. “Exertional Heat Stroke: Pathophysiology and Risk Factors.” BMJ Medicine, vol. 1, no. 1, Oct. 2022, p. e000239, https://doi.org/10.1136/bmjmed-2022-000239. Accessed 20 Oct. 2022.

Hoffman, Jeremy S., et al. “The Effects of Historical Housing Policies on Resident Exposure to Intra-Urban Heat: A Study of 108 US Urban Areas.” Climate, vol. 8, no. 1, 13 Jan. 2020, p. 12, www.mdpi.com/2225-1154/8/1/12/htm, https://doi.org/10.3390/cli8010012.

Liu, Cuiqing, et al. “Cardiovascular Response to Thermoregulatory Challenges.” American Journal of Physiology-Heart and Circulatory Physiology, vol. 309, no. 11, 1 Dec. 2015, pp. H1793–H1812, https://doi.org/10.1152/ajpheart.00199.2015.

“Urban Heat Islands in the Northwest | USDA Climate Hubs.” Www.climatehubs.usda.gov, www.climatehubs.usda.gov/hubs/northwest/topic/urban-heat-islands-northwest.