Temperature regulation mechanisms in human beings are intricate, variable, and complex. Thermal regulation systems maintain constant body temperature over a wide range of internal heat production and environmental conditions. Body systems function together in a delicate equilibrium. For the body temperature to remain constant, thermal power generation plus thermal power flows into the body must equal the sum of all power flows from the body to the environment. Energy flows from the body can be in radiation, convection, respiration, evaporation, conduction, and excretion (Bartlett & Braun, 1982). Temperature regulation mechanisms include convection, radiation, conduction, and evaporation. This paper describes the physics behind these temperature regulation mechanisms.
Evaporation
The rate of heat removal from the body surface depends on the surrounding environment. If the environmental temperature is above 34 ˚C, evaporation of transpiration is the only effective cooling mechanism. At this temperature, the sweat glands begin to produce perspiration. The evaporation rate is inversely related to the relative humidity of the ambient air (Wendt et al., 2007). Therefore, it would be highest in low-humidity air. Conversely, the rate of cooling would approach zero as relative humidity approached saturation. Therefore, a person can maintain normal body temperature if the surrounding temperature ranges between 115-127 ˚C provided the air has zero humidity, the person can sweat profusely by drinking a lot of water and has no clothing that would hinder evaporation (Bartlett & Braun, 1982). The range of ambient temperatures that determines initial defense is the thermoneutral zone (Mekjavic & Eiken, 2006).
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On the other hand, if the environmental temperature is 48 ˚C and the relative humidity is approaching saturation, a person cannot endure this environment for more than a few minutes. A fan produces cooling by forcing the circulation of ambient saturated air, thus increasing the evaporation rate. If water vapor saturation in the air is less than the saturation concentration at 37 ˚C, cooling occurs by evaporation of water in the respiratory system. Additionally, insensible perspiration occurs at low environmental temperatures when water diffuses through the skin and is evaporated from the surface. Wetting the skin to cool off is counterproductive since wetness reduces the gradient of water concentration in the skin, leading to diffusion. Water loss by respiration and insensible perspiration removes thermal energy from the body through the latent heat of vaporization at a rate of 20W (Bartlett & Braun, 1982). Respiration and evaporation oppose hypothermia (Aguilella-Arzo et al., 2003).
Convection
When a body is in a high-temperature environment that inhibits the evaporation mechanism, cooling occurs via convection by blood circulation in the cardiovascular system (Morrison & Nakamura, 2019). The skin surface is cooler than the core of the body. The temperature gradient below the surface determines the rate at which thermal energy is brought to the surface. When large thermal powers need to be dissipated, the body increases the temperature gradient by dilating the network of blood vessels near the skin’s surface. The warm blood is transported closer to the surface, raising the surface temperature and the thermal gradient below the surface. Subsequent removal of thermal energy from the surface occurs as long as the environmental temperature is lower than the skin temperature. Cutaneous vasodilation occurs concurrently with the evaporation of sweat (Charkoudian, 2003). Conversely, when the body is cold, it reduces the rate of heat loss by constricting blood vessels below the skin surface, so that warm blood is kept at a greater depth. As a result, heat flow through the skin is reduced (Bartlett & Braun, 1982). Intense and prolonged exercise may lead to hypohydration and temperature rise, which result in dehydration and hyperthermia (Kodera et al., 2020).
During surgery, general anesthetics impair thermoregulatory responses. For example, they decrease the threshold for vasoconstriction. Additionally, shivering rarely occurs during surgery (Sessler, 1993). Recent technological developments have made body core temperature regulation possible using selective thermal stimulation (STS) approaches to assist in surgery (Bischof & Diller, 2018).
Extreme underwater environmental conditions lead to heat loss and hypothermia. Localized cold stress can lead to cold-induced injuries, especially on the lower extremities (Lahiri et al., 2020). Freedivers use masks, isothermal suits, and gloves to regulate body temperature. Seawater is cooler than the body. Moreover, water conducts body heat 26 times faster than air. Convection relates to the movement of water surrounding the diver (Aguilella-Arzo et al., 2003).
Radiation
Electromagnetic radiation is transferred to bodies not in contact through radiation. The infrared radiation of thermal energy from the body cools it (Tansey & Johnson, 2015). Radiation is most effective at room temperature for a naked person at rest, while convection is more suitable for a clothed person in motion (Aguilella-Arzo et al., 2003).
Conduction
Conduction is the direct transfer of heat between the body and an object in contact with the body (Tansey & Johnson, 2015). For a deep-sea diver, energy transferred across the skin and the wet suit occurs via conduction. Peripheral factors, the skin’s fat content, determine the rate of conduction. Conductive heat transfer is described by Fourier’s law for solids where heat flow is proportional to the temperature gradient (Aguilella-Arzo et al., 2003).
Conclusion
Temperature regulation mechanisms include convection, radiation, conduction, and evaporation. The rate of heat removal from the body surface depends on the surrounding environment. Respiration and insensible perspiration remove thermal energy from the body through the latent heat of vaporization. When a body is in a high-temperature environment that inhibits the evaporation mechanism, cooling occurs via convection. The infrared radiation of thermal energy from the body cools it. Conduction is the transfer of heat between the body and an object in contact with it.
References
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