Thermoregulation is Impaired by Aging

The old are more vulnerable to all stresses, and heat is no exception. Older people make up a majority of the fatalities in heatwaves. This topic isn't frequently discussed in comparison to other aspects of aging, however. Impairment of the physiological response to heat is a dysfunction of high level processes in numerous organs, not just the skin, and results from a long chain of cause and consequence under the hood. That chain linking low-level molecular damage to high level outcomes is poorly explored, to say the least. This is one of the reasons why targeting the repair of that low-level damage of aging is a more effective strategy for the treatment of aging as a medical condition.

Tens of thousands of deaths have been caused by heat waves across Europe since 2000. There are an estimated 1,500 heat-related deaths every year in the United States. A health center in Paris recorded 2,814 deaths during the 2003 heat wave, 81% of these were in people older than 75 years. Exposure to hotter than usual temperatures poses a thermoregulatory challenge to the human body, particularly when this occurs suddenly, precluding opportunities for acclimatization. Nevertheless, heat illness can be managed through simple behavior changes such as drinking more water and seeking shelter in air-conditioned buildings. Such behavioral strategies rely on effective efferent-afferent physiological responses, but these have been shown to decrease with aging.

Aging impacts thermoregulation in several ways. Older adults (≥ 50 years) store 1.3 to 1.8 times more body heat when exposed to the same heat load than younger adults (19-30 years) during both exercising and passive heat exposure in both humid and dry conditions. The higher heat storage in the older individuals is due to a reduction in heat loss caused by an attenuated sweat response and increased dry heat gain.

Sweating is a critical mechanism for heat loss in humans, particularly when ambient temperature is above skin temperature as dry heat exchange results in heat gain in these situations. Sweating function declines with age at differing rates. Sudomotor function declines first in the legs, followed by progressive decrements in the upper body. Loss of sweating capacity comes from reduced function of each sweat gland rather than a reduction in the number of sweat glands, and is thought to be caused by local rather than central factors. Older adults have a higher core temperature threshold for the onset of sweating. The delayed onset of sweating coupled with the inability to increase and maintain a high sweat rate will delay the effect of cooling from sweat reducing its effectiveness, resulting in a higher core temperature and greater heat strain in the elderly.

With aging, the cardiovascular system experiences functional and structural changes. Total blood volume decreases, reactive oxygen species increase, and nitric oxide availability reduces, yielding a decrease in endothelial-dependent dilation and a reduced blood flow. Older adults increase their skin blood flow (SkBF) ~2-3 times less than their younger counterparts during passive and active heat exposure. Attenuated SkBF will reduce dry heat loss, and therefore increase heat strain on the body. The elderly will struggle to dissipate heat effectively compared with their younger counterparts, resulting in increased thermal and physiological strain.


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