written by Meg Garvey, PhD
The human body strives to stay in a state called homeostasis, this especially includes body temperature. In fact, the body is capable of maintaining its temperature within a few tenths of a degree during a host of activities and in different environments. Thermoregulation is regulated by the nervous system to either warm or cool the body through a variety of mechanisms influenced by the heat balance equation: metabolism +/- conduction +/- convection +/- radiation - evaporation = heat storage (1). The purpose of this paper is to introduce the concept of thermoregulation and will take a more detailed focus on cooling the body through the evaporation of sweat.
The body’s mechanism for maintaining core body temperature is called thermoregulation, which is controlled by the nervous system. Heat is a byproduct of muscular engagement and cooling mechanisms are triggered when body temperature rises above your baseline, roughly 98.6°F (1).
According to the heat balance equation, heat storage = metabolism +/- conduction +/- convection +/- radiation - evaporation. At rest, heat is transferred to inactive skeletal muscles, which have a resting temperature between 91.5 - 95°F (2), however, during exercise excess heat needs to be transferred instead from the core to the skin via the bloodstream and cutaneous vasodilation. Once it reaches the skin, as referenced in the heat balance equation previously, the body can utilize a few mechanisms to dissipate the heat:
⬢ Conduction - at a comfortable room temperature only 3% of heat is lost through this method; this occurs when heat is transferred from one object to another or down a thermal gradient within the same organism (2).
⬢ Convection - at a comfortable room temperature roughly 15% of heat is lost via this method; this involves moving liquid or gas to illicit heat transfer (3).
⬢ Radiation - it is important to indicate that this is impacted by clothing, so if a nude individual is placed in a room at a comfortable temperature roughly 60% of heat is lost to the environment via radiation, which is the loss or gain of heat through infrared rays (3).
⬢ Evaporation - this is further broken down into insensible water loss and sweating.
⬢ Insensible water loss - is water lost through respiration and diffusion, for the purpose of temperature regulation it also has no governing control mechanisms on the rate of water loss.
⬢ Sweating- unlike insensible water loss, sweating can be ramped up or down via the rate of loss and for every mL of fluid evaporated from the skin 2.43 kJ of heat is lost (3). At rest in a comfortable environment the body loses roughly 25% of its heat by evaporation.
For the purposes of this paper secondary data analysis was conducted on Nix Biosensors ground truth validation data set (data collected between October 2021 and August 2022). This data set included 1157 workout observations among 257 athletes (86.5% female, 77% non-Hispanic white, with ages ranging from 18-67 years) demographics of this population are summarized in Table 1.
As mentioned above, sweating cools the body through the evaporation of liquid from the skin’s surface. The volume of sweat production is directly correlated with the body’s heat production. For example, the more intense an individual’s exertion, the more body heat is produced, and the more sweat the individual produces in an attempt to cool the body. However, intensity is not the only factor that drives thermoregulation. There are many factors that can impact core body temperature - and therefore sweat rate - during activity(4).Any alteration in these factors can affect the rate of sweat production, even when all other conditions are held constant. For example, sweat rate can vary up to 87% when the environmental temperature is increased by 17°F (5).
Some of these factors include:
⬢ Environmental Variables - temperature, humidity, cloud cover, wind speed, altitude, etc.
⬢ Individual Variables- body mass index, fitness level, heat acclimatization, activity intensity, etc.
⬢ Insulation - clothing or equipment that keeps heat close to the body.
The same environmental factors that can trigger a thermogenic response from the body at rest will become increasingly impactful to the effectiveness of heat dissipating mechanisms during exercise. Recalling the list of mechanisms for heat transfer above, for heat dissipation both radiation and convection are reliant on a large temperature differential between the body and the surrounding environment. This is a major reason why we do not see substantial sweat production during colder weather vs. warmer weather. However, when the surrounding temperature approaches and exceeds our core body temperature that temperature gradient is now reversed and radiation and convection facilitate gaining heat from the environment rather than dissipating it. When this situation occurs, the primary mechanism for heat dissipation is sweating.
The effectiveness of sweat hinges on its ability to be evaporated into the surrounding environment. The environment can only evaporate moisture when it has the capacity to do so, dictated by humidity and dew point. When these two conditions are high the water vapor pressure gradient between the skin and the environment is low, lowering the effectiveness of heat dissipation through evaporation. Wind speeds can facilitate evaporation, however, when wind speeds are quite low this also lowers the effectiveness of evaporation. When environmental conditions are not conducive to facilitating the mechanisms of thermoregulation, a critical heat load can be reached resulting in heat illness and injury.
The purpose of this section is to introduce a few factors as they relate to the mechanisms of thermoregulation. This list is not exhaustive, nor does it incorporate the intricacies of their influence both on thermoregulation as a whole or the variations in sweat production from day to day.
⬢ Fitness level - individuals who are physically trained have training adaptations to the cardiovascular system that are adventitious to heat dissipation. Cardiovascular strength facilitates the transfer of heat from the core to the skin via cutaneous vasodilation.
⬢ Body Mass Index - adipose tissue is metabolically inactive, yet serves as an insulator for the body while skeletal muscle produces heat and baroreflexor demands of skeletal muscle during exercise may compete with cutaneous vasodilation.
⬢ Heat acclimation - an athlete who has been training in a heated environment for 7-14 days will produce training adaptations that will allow them to more efficiently dissipate heat than those athletes who are not heat acclimated.
⬢ Gender - research indicates that women have been underrepresented in heat related exercise research in the past decade (6) and therefore the impact of gender is not fully known. Some studies have indicated that heat tolerance is influenced by gender (7) while others indicate that even though female reproductive hormones impact elements of thermoregulation, core body temperature does not fluctuate between genders during exercise (8).
Similar to clothing blocking the efficiency of radiation, it could also prevent evaporation, especially if it was designed to keep an individual warm in colder weather.
Sweat is produced in sweat glands. There are three types of sweat glands: eccrine, apocrine, and apoeccrine. Eccrine glands are the most plentiful as they cover the whole skin surface. The eccrine sweat glands are involved in evaporative cooling and mainly secrete water and electrolytes. Apocrine glands are found only in parts of the body that contain hair follicles (i.e. armpits and groin). These glands are responsible for what is called “stress sweat.” Apocrine glands produce a thicker, oily sweat than the watery sweat produced in the eccrine glands. The last gland is the apoeccrine gland, which are a mix of apocrine and eccrine glands and do not open up in hair follicles (9).
As it is important to the context of fluid evaporation, discussing the contents of sweat is prudent to this topic. Sweat is more than just water, it's also more than just salt. Athletes, especially those not training under the guidance of a coach, need to consider their electrolyte losses as well. Electrolytes are utilized in the body to conduct electrical signals, these signals do things such as contract muscle fibers, which include both voluntary muscles (like your quadriceps and hamstrings while running) and your involuntary muscles (such as your heart). These are the key electrolytes associated with sweating:
⬢ Sodium - assists with muscle contraction and relaxation, conducts nerve impulses, and assists in water balancing outside of the cell.
⬢ Potassium - assists in maintaining normal fluid balance inside the cell along with supporting normal blood pressure and contracting muscles.
⬢ Chloride - works with sodium and potassium to maintain proper fluid balance, but also works with CO2 (carbon dioxide) to maintain the body’s acid-base balance.
It is important to articulate that the cumulative amount lost during a workout, as well as the electrolytes lost, can vary amongst teammates on the same ride on the same day and amongst a single athlete doing a similar workout in similar conditions on different days. As mentioned previously environmental factors are not the only influences on thermoregulation. There are a whole host of physiological factors such as nutrition, fitness level, skeletal muscle mass, body size, etc. that can have an impact on the amount of sweat that is produced on a given day. As many of these factors are under researched it becomes increasingly important, especially for coaches, to look at their athletes individually rather than having a one-size-fits-all hydration approach; particularly in elevated temperatures when it becomes increasingly more difficult to dissipate excess body heat.
The human body has a variety of mechanisms to utilize in order to maintain its core temperature. During exercise, excess heat is brought to the skin and is transferred to the surrounding environment through either conduction, convection, radiation, or evaporation and is regulated by the heat balance equation. While there are many factors that influence thermoregulation, environmental considerations can quickly increase or decrease the effectiveness of these methods of heat dissipation. Understanding the influences of thermoregulation, both external and internal, can assist the athlete in making decisions that help keep them performing optimally, and more importantly avoid heat related illness or injury.
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(8) Charkoudian, N., & Stachenfeld, N. S. (2014). Reproductive hormone influences on thermoregulation in women. Compr Physiol, 4(2), 793-804.
(9) Groscurth, E. (2002). Anatomy of sweat glands. CURRENT PROBLEMS IN DERMATOLOGY-BASEL-, 30, 1-9.