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Thermal Physiology and Thermoregulation
Published in James Stewart Campbell, M. Nathaniel Mead, Human Medical Thermography, 2023
James Stewart Campbell, M. Nathaniel Mead
Shivering generates muscular heat as part of the body's natural response to cold. The shivering reflex manifests as rapid rhythmic contractions of the skeletal muscles and is triggered whenever the core temperature drops below a certain set point. The increased muscular activity warms the body by increasing metabolic heat generation. Maximum shivering can increase basal body heat production by four or five times, producing about 375 watts (0.5 HP) of heat.20 The hypothalamus serves as the primary driver for shivering.21 Shivering can also be a response when developing a fever: the hypothalamic set point for core temperature is raised, causing the body temperature to rise by creating the sensation of feeling cold and ramping up physiologic heat production and conservation until the new core temperature set point is reached.
Working in Extreme Temperatures
Published in Ronald Scott, of Industrial Hygiene, 2018
If the body response described above is inadequate to counteract a cold environment, the body core temperature drops. The International Society of Physiologists defines the start of hypothermia as the dropping of the core temperature to 96°F (35°C). The skin becomes cold as circulation is withdrawn from the skin. If this withdrawal is extreme, skin tissue may die from lack of oxygen. As the body temperature drops further, severe shivering is evidence of a problem. At 95°F heart rate slows and central nervous system functions become less effective. The individual is drowsy, fatigued, forgetful, and uncoordinated. By 94°F speech is difficult, vision is less acute, and disorientation sets in. Consciousness is lost when the core temperature drops to between 86 and 90°F. When the core temperature drops below 86°F (30°C), weakened heart beat and accompanying slower respiration are potentially lethal.
Introduction to heat transfer
Published in Tariq Muneer, Jorge Kubie, Thomas Grassie, Heat Transfer, 2012
Tariq Muneer, Jorge Kubie, Thomas Grassie
In addition to postural response to temperature, the human body also possesses a number of additional mechanisms for reducing heat loss, and also for generating heat internally. In response to cold, and the movement of cool air over our bodies, the erector pili muscles raise the hairs on our bodies to trap air, providing an insulating envelope and therefore reducing heat loss. Shivering is generally initiated in response to a drop in skin temperature. If the body core temperature also begins to fall, shivering will become more violent as the body tries to raise its internal temperature. In terms of heat generation, shivering is more effective than voluntary muscular contraction. In shivering, as the limbs do not effectively move or do work, all the energy used is dissipated as heat within the muscle as opposed to the 60–70% conversion gained through voluntary contraction (Clark and Edholm, 1985). A schematic diagram of the major human thermoregulatory systems is shown in Fig. 1.5.1.
Physiological and thermoregulatory effects of oral taurine supplementation on exercise tolerance during forced convective cooling
Published in European Journal of Sport Science, 2022
Richard Simmonds, James Cole, Jamie Tallent, Owen Jeffries, Nicola Theis, Mark Waldron
Exercise in cold environments presents a significant challenge to thermoregulation, with exaggeration of the thermal gradient from core-to-environment facilitating heat loss, primarily via convective and conductive pathways (Young & Castellani, 2001). Cold exposure elicits reflex increases in efferent skin sympathetic activity, leading to peripheral vasoconstriction and reductions in skin blood flow to delay heat losses (Castellani & Young, 2016). These thermoregulatory reflexes occur when skin surface temperature decreases ∼3–4°C below a thermoneutral threshold of ∼34°C (Greaney, Stanhewicz, Kenney, & Alexander, 2015). During prolonged cold exposure, shivering becomes important for thermoregulation to generate metabolic body heat (Castellani & Young, 2016). Initial vasoconstriction leads to increased intra-thoracic pressure, central blood pressure, cardiac output and myocardial oxygen demand (Doubt, 1991). Thus, coupled with the thermogenic (shivering) responses, the primary effects of cold exposure increase the net O2 cost at rest and exercise (Cheuvront & Haymes, 2001). Higher rates of glycogenolysis are also observed in cold, driven by increased activation of the sympathetic nervous system due to cold exposure (Jacobs et al., 1985; Martineau & Jacobs, 1988). Collectively, these acute physiological responses to moderately cold exposure could have negative effects on exercise efficiency, potentially leading to an earlier onset of fatigue and impaired endurance performance (Galloway & Maughan, 1997).
Technical textiles for military applications
Published in The Journal of The Textile Institute, 2020
R. G Revaiah, T. M. Kotresh, Balasubramanian Kandasubramanian
Cold exposure elicits several defence mechanisms in the body that try and boost core temperature in chilly weather. The body begins to generate additional heat through tensioning muscles, leading to involuntary shivering. Shivering that begins in the torso region subsequently spreads to limbs. It has been reported that shivering can raise metabolic heat production up to three times (Spurr, Hutt, & Horvath, 1957). Involuntary shivering, however, is highly undesirable for military personnel involved in fine motor activities (such as deep-sea divers and helicopter pilots). The hypothalamus, located near pituitary glands, acts as body’s thermostat, reduces blood supply to the extremities through vasodilation in a bid to keep core warm at any cost – sacrificing the extremities should the need arise (Cheung, 2015). This leaves extremities vulnerable to frostbite, which in worst case may lead to amputation (Handford, Thomas, & Imray, 2017; Morrison, Gorjanc, & Mekjavic, 2014; Shenaq & Gottlieb, 2017). A case study by Lorentzen and Penninga (2018) presents the severity of frostbite as cause for tissue loss, amputation and morbidity in arctic Greenland. Another field study on chilblain at HA Himalayan regions of India delineates the effectiveness of available pharmacological measures (Singh et al., 2018). Several interesting strategies adopted by living organisms for survival in the cold have been reported (Finegold, 1986). However, human survival entirely depends on the insulation around the body.