Physiology of the Neonate
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal in Principles of Physiology for the Anaesthetist, 2020
The neonate loses heat by radiation, convection, evaporation and conduction. Various studies have estimated that radiation, convection, evaporation and conduction account for 39%, 34%, 24% and 3%, respectively, of heat loss in newborns in incubators. Radiant heat loss decreases as the environmental temperature rises. Because the newborn has a large surface area/volume ratio, radiant heat loss is greater with smaller neonates. Convective heat loss (the transfer of heat by the movement of surrounding air) is proportional to the temperature difference between the body surface and the air and the velocity of movement of the air. Evaporative heat loss occurs from the body surface and the respiratory tract. Evaporation from the skin is related to relative humidity and amount of sweating. Although the newborn has six times more sweat glands per unit area than an adult, its ability to sweat is less, the peak response being about one-third that of adults. Full-term neonates sweat when the rectal temperature is between 37.5°C and 37.9°C, and at ambient temperatures greater than 35°C. Premature infants of less than 30 weeks’ gestation do not sweat at all, as the sweat glands are immature.
Human Perspiration and Cutaneous Circulation
Flavia Meyer, Zbigniew Szygula, Boguslaw Wilk in Fluid Balance, Hydration, and Athletic Performance, 2016
Increased body temperature, either through active or passive thermal stress, is transferred to the environment via non-evaporative and evaporative heat loss mechanisms. Non-evaporative heat loss (i.e., dry heat loss) mechanisms include conduction, convection, and radiation, and in humans, they are the predominant mechanisms of heat loss at rest. The effectiveness of these mechanisms is dependent on the temperature gradient between the environment and the skin. Skin temperature is primary modulated by increasing/decreasing the blood flow to the skin, which allows the transfer of heat from the internal core of the body to the ambient environment. The evaporative heat loss mechanisms include increased ventilation (i.e., panting), saliva spreading, and perspiration. While evaporative heat loss can occur through panting or saliva spreading in other mammals and birds (Robertshaw 2006), sweating coupled with increased cutaneous vasodilation is the predominant heat dissipation mechanism in humans (White 2006).
Aircrew equipment – Thermal protection and survival
Nicholas Green, Steven Gaydos, Hutchison Ewan, Edward Nicol in Handbook of Aviation and Space Medicine, 2019
Protective clothing for cold should: Prevent heat loss from body by insulation.Provide insulation by trapping a large volume of dry air in small sections; minimise internal convection and cold air ingress (windproof).Enable adjustments in insulation to cater for increased/decreased metabolic heat production (use of layers, openings).Wick moisture away from body.Be vapour permeable to prevent accumulation and condensation of moisture under the clothing which reduces clothing insulation.Include head protection (due to the absence of cold constrictor fibres in blood vessels of scalp).
Heat-induced hypervolemia: Does the mode of acclimation matter and what are the implications for performance at Tokyo 2020?
Published in Temperature, 2020
Lorenz S. Kissling, Ashley P. Akerman, James D. Cotter
Spectators and athlete support staff may not be aware that these warm-humid environments have a disproportionately larger impact on the fittest athletes, but relatively less than people resting or working at low intensities. Specifically, (i) dry heat loss mechanisms (convection and radiation) can transfer little more than resting rates of heat production in warm environments, such that (ii) the additional transfer needed to sustain exercise requires evaporation, but the high ambient vapor pressure limits the gradient for evaporation and thus evaporative heat loss will become maximal at modest work rates, whilst (iii) any extra speed provides negligible additional convective or evaporative transfer (i.e., velocity-vs-heat loss is an exponential relation). So, whereas endurance performance is most impaired for less fit individuals in non-humid heat stress [16], the opposite may be true in humid heat because of the relatively larger importance of evaporation for elite endurance athletes. The heat stress elicited in the Tokyo Olympics (and Paralympics) will therefore concomitantly reduce the performance of sustained/repeated exercise and increase the risk of exertional heat illness [17]. Plans to move competition into the earlier hours have been proposed [18], but even at midnight the WBGT could be ~30°C [2], which would likely still impair performance and carry a risk of exertional heat illness [17].
Evaluating the impact of solar radiation on pediatric heat balance within enclosed, hot vehicles
Published in Temperature, 2018
Jennifer K. Vanos, Ariane Middel, Michelle N. Poletti, Nancy J. Selover
A rise in core temperature (Tc) is the best determinant of heat injury occurring, and the rate at which an individual's Tc rises and the value at which hyperthermia and illness occur differs due to attributes such as body size, gender, and age, among other factors [30–32]. A Tc value of 40°C is related to the most extreme form of heat illnesses and may be of greater consequence to the very vulnerable (i.e., children, elderly), and a value of 42°C is shown as the critical thermal maximum for adults [33]. Heatstroke is represented by a continuum of symptoms that may ultimately lead to death [34]; survivors may also suffer brain damage [35,36], with one study [37] finding hyperactivity, attention deficit, and epilepsy in child heatstroke survivors. The avenues and magnitudes of heat loss (evaporative, radiant, convective, and/or conductive exchanges) depend on the environment as well as human physiology/behavior, and are part of an intricate thermoregulatory feedback in the human body [38]. Thermal balance models can help estimate heat stress and strain in an enclosed space, whether environmentally- or forensically-based [3,27], and further extend the modeled data for more effective communication to parents and caregivers.
Relationship between heat loss indexes and physiological indicators of turnout-related heat strain in mild and hot environments
Published in International Journal of Occupational Safety and Ergonomics, 2023
Huipu Gao, A. Shawn Deaton, Roger Barker, Xiaomeng Fang, Kyle Watson
Figure 4 shows the comparative effects of different turnout systems on predicted core temperature at the end of wear sessions conducted in mild (Figure 4a) and hot (Figure 4b and c) environmental conditions. Statistically significant differences in core temperature rise were observed depending on the THL and Ref ratings of the composite systems used in the construction of otherwise identical turnout suits. The differences observed depend on the temperature and humidity of the test environment. Composites with bi-component moisture barriers exhibit lower evaporative resistance in mild conditions because of water uptake in the moisture barrier component in the system [5]. At 25 °C, 65% RH, all study turnout garments, with the exception of system C, produce a similar effect on core temperature rise (p < 0.01), as shown in Figure 4a. In mild conditions, therefore, moisture absorption in the moisture barrier layer contributes to apparent heat loss, offsetting the evaporative resistance difference in bi-component moisture barrier (A, B, D and E) systems. This phenomenon makes these systems perform similarly with regard to heat loss and heat strain.
Related Knowledge Centers
- Bladder
- Body Temperature
- Hypothermia
- Hyperthermia
- Vagina
- Uterus
- Homeostasis
- Zoology
- Ecophysiology
- Rectum