Thermoregulation in Humans: A Review of Developmental Changes
by
Sophia Sanders
A REPORT SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
ADVANCED SCIENTIFIC RESEARCH
at
ISLANDS HIGH SCHOOL
Savannah-Chatham County Public School System
Instructors: Megan Heberle, Eric Lind
Spring 2016
Introduction
Thermoregulatory homeostasis, while contributed to by multiple mechanisms, is
ultimately controlled by the core temperature of the human body (Tc). The core body
temperature, though difficult to accurately measure, remains at a general average of 37 ℃ with a
safe range of variation of approximately 2℃ (Brown et al. 1992). Therefore, the body must carry
out automatic regulatory measures in order to maintain homeostasis, allowing it to stay within
the safe ranges of human temperature.
A temperature below 35℃ is considered hypothermic, and is defined as a state of the
body in which heat loss exceeds the body’s ability to create heat. Hypothermia is a dangerous
condition of the body, and can be controlled by behavioral thermoregulation, or conscious
choices that minimize the exchange of heat from the body to the environment (Seely et al. 2008).
Hyperthermia occurs when the body is dissipating heat at a slower rate than it is being
created, which results in overheating at extreme levels. Hyperthermia can also be controlled by a
combination of behavioral and autonomic thermoregulatory functions.
In cold environmental settings, body heat is produced by the metabolism, thus placing
those with higher metabolic rates, such as adolescents, at an advantage in cold climates (Inbar et
al. 2004). While the production of this heat occurs within the metabolism, the heat is transferred
by the blood vessels to the skin, consequently creating the skin temperature that is frequently
used to gauge the thermal efficiency of humans under normal circumstances (Laakso et al.
2011). Skin temperature has a lower basal, or resting, temperature than the core of 34℃ (Laakso
et al. 2011). However, skin temperature also plays a significant role in thermoregulation, as the
basis of thermal perceived comfort occurs due to the skin temperature and is therefore
responsible for the induction of behavioral thermoregulatory measures.
The autonomic measures of thermoregulation fall into four categories: 1) radiation; 2)
conduction; 3) convection; and 4) evaporation. Heat dissipation occurs primarily by evaporation
via perspiration, but can also be accomplished with radiation, conduction, or convection.
Alternatively, heat gain can be accomplished with metabolic heat generation or muscle
contractions that cause heat, as well as with radiation, conduction, or convection measures (Seely
et al. 2008).
The most common autonomic measures for maintenance of homeostasis are vasodilation
and vasoconstriction, depending on the climate. Vasodilation occurs when the blood vessels are
dilated, thus raising skin temperature by bringing warm blood to the surface of the skin in order
to dissipate heat in warm environmental circumstances (Inoue et al. 2004). Vasoconstriction
therefore constricts blood vessels in order to cool the surface of the skin to allow for less
dissipation of heat in order to keep body heat in the body’s core in cold climates (Kurz et al.
1993). These functions both occur automatically by the activation of the trigger stimulus in times
of thermal entropy.
Muscular activity also plays a role in thermoregulation, as there are certain involuntary
functions that help to maintain thermal homeostasis. Shivering, for example, is an involuntary
muscle contraction that is used as a thermal mechanism to generate muscular heat in cold
environments (Ozaki et al. 1997). This is also why exercise causes overheating - the muscle heat
generated by contractions during movement causes the body to warm, thus causing the warming
of the body’s core (Araki et al. 1979).
An evaporative measure of cooling occurs in the form of sweat, but this method of heat
dissipation can be problematic in humid environments or in people with underlying heart
conditions such as low blood pressure or dangerously high heart rates (Inbar et al. 2004). This
method, in overly warm or humid environments, could lead to dehydration and, if extreme, heat
exhaustion.
It is important to note that the temperature of the human body continues in a negative
feedback loop. This means that, in accordance with the set point theory, the body cools beyond
its optimal temperature, warms to bring the temperature back up, and then cools it yet again.
Through this system, the result of one action prompts another that will cause the opposite result
in a loop (Laakso et al. 2011).
Infancy
While the autonomic thermoregulatory activities of infants are ultimately similar to that
of an adult, there are some thermoregulatory patterns that are unique to infants.
Despite the hypothesis that the thermal control of infants is unstable, evidence has been
presented of temperature oscillations of an hour when measured continually during sleep
(Tuffnell, 1993). Oscillation periods of one hour are ordinary in human thermoregulation during
sleep, even for adults, due to the negative feedback cycle associated with the body temperature
of mammals (Arons and Zhang, 2006). While the general pattern of the oscillations are the same
between infants and adults, the differences in temperature of infants tend to be more extreme. In
a sleep study, temperatures of infants ranged from 37.8 degrees Celsius to 36.3 degrees Celsius:
this shows as much as a 1.3 degree variation from ordinary adult body temperature of 37.7
degrees (Brown et al. 1992; Seely et al. 2008).
It is generally accepted that newborn infants lose heat rapidly immediately after birth at a
rate of nearly 3 degrees per minute if not assisted (Waldron and Mackinnon, 2007). It has been
assumed that this is due to the large surface area to volume ratio relative to larger, adult humans
(Smales and Kime, 1978). This is supportive of the hypothesis of instability of thermoregulation
in infancy; however, two or three days after birth, the metabolic rates of infants become adequate
to regulate body temperature, thus establishing stability of body temperature patterns (Smales
and Kime, 1978). After this short period of neonatal temperature instability, infants have shown
an ability to maintain similar temperatures despite differing environmental conditions, thus
presenting evidence of adequate homeothermy (Anderson et al. 1990; Tuffnell, 1993).
Though infants show proficiency at thermoregulation similar to that of adults, it is
arguably more important to infant health that homeothermy is maintained, especially before the
establishment of stable thermoregulatory patterns (Smales and Kime, 1978). While no single
factor has been identified as the cause of Sudden Infant Death Syndrome (SIDS), it has been
established that unordinary body temperature is a contributing factor in SIDS (Tuffnell, 1993).
Practices to maintain infant body heat, like swaddling and the use of heat lamps, are necessary in
the first two to three days of life due to the inability of infants to properly maintain body
temperature, especially in premature infants (Stern, 1980).
Childhood
Under neutral environmental temperature circumstances, the thermoregulatory patterns of
children show no difference from that of an adult (Araki et al. 1979). However, due to children’s
higher surface-area-to-mass ratio relative to adults, certain differences in the exchange of heat
between the body and the environment occur at extreme temperatures. There is a higher rate of
exchange between children’s bodies and their environment than that of an adult, as their larger
surface areas allow for faster acquisition of outside temperature conditions; this causes children
to be less adaptive to climatic heat stress (American Academy of Pediatrics, 2000).
The thermoregulatory ability of children is generally considered to be inferior to that of
an adult, as children exhibit lower evaporative heat loss due to lower heat transfer between their
internal organs and skin (Delamarche et al. 1990). This means that children seemingly have less
ability to dissipate excess heat. However, children have shown no higher frequency in heatrelated injuries than adults; consequently, it has been hypothesized that children dissipate heat
with convective and radiative thermoregulatory strategies (Delamarche et al. 1990). Sweating
rates increase after the age of 13, suggesting that sweating rates increase to levels similar to that
of an adult after a child reaches puberty (Inbar et al. 2004).
Despite higher metabolic heat production in children, the rectal temperature both in
neutral conditions and during exercise are the same between children and adults (Inbar et al.
2004). However, core temperature has been evidenced to be higher in children (Inoue et al.
2004).
Despite these differences and the tendency of children to perform athletic activities at the
same intensity as most adults, there has been no evidence of higher rates of heat related injuries
in children. (Delamarche et al. 1990).
Adolescents
Adolescents, as they have gained the sweating ability not seen in children, largely have
the same thermoregulatory mechanisms as adults (Brokenshire et al. 2009). Adolescents are
generally more involved in athletic activities, but no excessive heat injuries due to physiological
inferiority have been evidenced. However, adolescents can have shortcomings in their behavioral
thermoregulatory measures, as adolescents tend to be less responsible in hydration and nutrition
without prompt (American Academy of Pediatrics, 2011). The criteria for efficient
thermoregulation depend on the same risk factors as with adults, such as weather, exertion, level
of fitness, and hydration, thus suggesting that adolescents have very little to no difference from
adults in thermoregulatory mechanisms (Brokenshire et al. 2009).
Young Adults
As humans grow into young adults, their surface area to body mass ratio, which accounts
for younger humans’ differences in thermoregulatory mechanisms, becomes equivalent to that of
an adult and an elder, as there is no significant differences in the mean heights of young adults
and the elderly (Direrro et al. 1999). Along with the similarity of young adults to both more
developed humans, young adults exhibit similar rectal temperatures and sweating rates (Viveiros
et al. 2011). Some characteristics are shared between the infantile, the young adult, and the
elderly. In a study done by Inbar (2004) in which the thermoregulatory practices of three age
groups were studied, all were similar in their dry heat gain, mass-related evaporative heat loss,
and metabolic heat production (Inbar et al 2004).
Despite these similarities, young adults seem to present certain superiorities over other
age groups in regards to thermoregulatory efficiency. For example, young adults were evidenced
to have the highest rates of sweating sensitivity and sweating efficiency in comparison to
children and the elderly, consequently causing their high rate of evaporative heat loss (Inbar et
al. 2004). The higher rate of sweating sensitivity could be due to the suggested higher rate of
skin-blood flow among young adults, as this could be a signal of a more efficient dispersal of
heat (Viveiros et al. 2011).
Alternatively, it has been oppositely evidenced that young adults have a higher threshold
for both vasoconstriction and heat production, thus showing signs of lower sensitivity to
environmental temperatures (Frank et al. 2000). Foundings in another study, however, show a
more immediate reaction of vasodilation in higher temperature than that of other age groups
(Schellen et al. 2010).
There is also noted to be higher maximum intensity of temperature for young adults,
which does support the hypothesis of the superior thermoregulatory methods of young adults
(Frank et al. 2000).
Shortcomings of young adult’s high sweating sensitivity are also apparent, shown in the
evidence in young adults of the highest thermal strain among age groups, possibly due to the
higher amount of heat that requires dissipation in order for young adults to achieve thermal
balance (Inbar et al. 2004).
Adulthood
As most general studies of thermoregulation occur on adult humans, the most information
is available based on the thermoregulatory practices of adults relative to other age groups.
However, effective and ethical ways to measure thermoregulatory responses of humans to
thermal extremes and substances such as caffeine and alcohol are rare. To remedy this lack of
ability to measure these responses, several thermal models have been developed in order to
theoretically map the thermal responses to both thermal and nonthermal stimuli. These models
include the Stolwijk model and the Fiala model (Laakso et al. 2011). The Stolwijk model is
beginning to be replaced by the Fiala model, as the Stolwijk human thermal model is lacking in
its accountability of blood flow, clothing, and conductive and radiative heat transfer (Foda et al.
2011).
The factors of thermoregulation that were most often attributed to thermal efficiency is
skin blood flow, vasodilation, sweat rate, air movement, and clothing (Laakso et al. 2011). It has
been considered that the autonomic thermoregulatory responses such as blood flow and
vasodilation are more heavily affected by the core temperature (Tc), while the subjective comfort
of the same person is affected more heavily by skin temperature (Tsk) (Frank et al. 1998).
The autonomic regulatory measures range in efficiency even between core and skin
temperature. While peak skin temperature was most strongly affected by blood flow, it was least
affected by sweat rate and vasodilation. The opposite is true, however, for core body
temperature. Parts of this study data may be erroneous, as the model used for this study is unable
to account for evaporative heat loss created by sweating due to lack of accountability for
humidity and wind velocity. There is also a lack of accountability for the possibility of the
heating of some tissues, as opposed to all or none, in extreme heat stress. (Laakso et al. 2011)
As there are four modes of thermal exchange; radiation, evaporation, conduction, and
convection, there is a possibility of several outcomes that could be overlooked within the
parameters of the conventional thermal study (Seely et al. 2008). For example, one study
introduced the notion of body heat conductivity being largely attributed to the amount of
chromosomal Q-heterochromatin in their genome (Ibraimov et al. 2007). This is a possibility that
has not been explored much in other studies of its kind, as the hormones most frequently to
thermoregulation is epinephrine and norepinephrine (Frank et al. 1998). Hormonal
considerations such as these also create considerations of the differing hormone levels presented
in age groups and genders, as with more specific research, connections could be found between
these hormones, human characteristics, and thermoregulation could be established.
Body Proportions
Body proportions can also play a significant role in thermoregulation efficiency.
However, contrary to conventional belief, there was an established decrease in skin temperature
with increase in body fat, creating a negative correlation. This could be attributed to the higher
surface area created by more skin exposure to air, thus allowing for effective dissipation of heat.
Oppositely, there is a positive correlation between body fat and heat stress, which is likely
connected to core temperature. An increase in body fat contributes to change in the conduction
and blood-flow related heat transfer with the environment, as fat requires less blood than an
equivalent amount of muscle due to its lower density. (Zhang et al. 2001)
Despite body fat role in physical regulatory factors, it has little influence on basal
metabolic heat and its production, though it is supposedly based upon height, weight, age, and
gender (Zhang et al. 2001).
Outside of fat content, body proportions such as the relative length of limbs can play a
role in heat transfer. A significant difference has been expressed between length of appendages
such as legs (particularly the thigh) and effective dissipation of heat. Individuals with shorter
limbs were shown to maintain their heat more effectively due to their lover relative surface area
for heat exchange. (Tilkens et al. 2007)
Elderly
As humans age, their regulatory systems, including thermoregulatory mechanisms, peak
in adulthood and begin to deteriorate afterwards. Therefore, the elderly are considered to be less
efficient thermoregulators than adults (Kurz et al. 1993). In several experiments done by
separately-operating researchers, older adults showed to be more hypothermic in a surgery
situation when placed under anesthesia than a younger adult, and were cited to take an
exceptionally long time to reach normal body temperature post-operatively due to their lack of
heat-producing abilities (Ozaki et al. 1997; El-Gamal et al. 2000). This is partially attributed to a
lower basal metabolic rate, and consequently lower resting temperature, as well as a slower rate
of core heating, or, in the case of hyperthermia, heat removal (Ferrero et al. 2008). Older adults
were cited to have required an extra 30 minutes to begin vasoconstriction as a warming
mechanism in cold operating-room conditions as compared to younger patients, and had a lower
mean skin temperature when they did begin to warm (Ozaki et al. 1997). The impairment of
organ and regulatory systems in elderly humans, such as muscular atrophy or dysfunctional
cardiac systems, put seniors at a dangerously increased risk of hyper or hypothermia (Sato et al.
2009).
Not only does increasing age correlate with a decrease in thermoregulatory efficiency,
but it also alters the methods of thermoregulation due to a shift in body mass, body proportions,
neurotransmitter speed, and other differences in physiology (Inbar et al. 2004). Similar problems
exists within the thermoregulatory systems of the elderly that exists for children; higher
vulnerability to temperature extremes due to differences in both the efficiency and methods of
temperature maintenance than that of an adult (Tsuzuki et al. 2002). For example, both children
and seniors have a higher sweating threshold, meaning that their body core temperature must
achieve a higher level to trigger the thermoregulatory mechanism of perspiring (Basu et al.
2002). This delayed response of sweating puts the affected age groups, seniors and children, at a
higher risk of heat-related injury than an adult or adolescent.
Unlike children, however, the problems of the senior thermoregulatory system are
concentrated in different areas. In staunch contrast to the heat production of children, older
individuals have a lower rate of metabolic heat production as well as a lower rate of heat gain
through the environment, thus yielding them less heat energy to produce necessary heat in
circumstances of extreme cold (Sato et al. 2009; Inbar et al. 2004; Tsuzuki et al. 2002). This
slowing of the basal metabolic rate of seniors can be at least partially attributed to the change
that occurs in the production of thyroid hormones with age (Sato et al. 2009). These advantages
that children have, considering their superiority over even adults in terms of metabolic heat
production, aid in the conclusion that the elderly are the most disadvantaged in regards to
thermoregulatory methods apart from infants.
The metabolic limitations of the elderly lead to a higher risk of seniors acquiring
hyperthermia or hypothermia in extreme temperature conditions (Ozaki et al. 1997). The slowing
of metabolic rate with age not only affects the rate of metabolic heat production, but also limits
the amount of lean body mass present in the body of the average senior (Frank et al. 2000).
Seniors were shown to have the highest percentage of body fat on average, thus contributing to
the lack of lean body mass presented by the elderly (Inbar et al. 2004). The higher fat content
and consequent lack of lean body mass, in conjunction with the elderly’s reduced sensitivity to
temperature extremes, causes a lack of shivering in the seniors that in turn results in less heat
generation (Frank et al. 2000).
A general consensus of the researchers studying thermoregulation of humans relative to
age is that the elderly population generally has a significantly lower threshold for thermal
mechanisms such as vasoconstriction despite a diminished sensitivity to thermal discomfort
(Frank et al. 2000; Inbar et al. 2004; Ozaki et al. 1997). The neurotransmitter norepinephrine,
which controls the deployment of regulatory systems in response to thermal stimulus, has shown
to be slower on the uptake in the elderly human’s nervous system (Frank et al. 2000; Thompson
et al. 2004).
In conjunction, the maximum levels of elderly’s temperature gain is lower than a younger
person’s, such as their mean skin temperature maximum and the maximum vasoconstriction
intensity (Inbar et al. 2004; Frank et al. 2000). This lowered sensitivity is especially problematic
for older people, as thermal regulatory mechanisms are triggered primarily by thermal comfort
levels and strong emotion (Grassi et al. 2003).
Due to this, older people are slower to recover from periods of thermal stress in both
overly warm and overly cool circumstances, including the influence of a drug such as anesthetic.
In an anesthetic setting, elderly humans are more likely to become hypothermic due to an
extremely high threshold for vasoconstriction, thus causing body temperature of seniors to
remain cool during the presence of anesthesia in their bloodstream as well as take a longer time
for temperature to rise back to ordinary levels (Kurz et al. 1993).
Despite lower thresholds for thermoregulatory action, however, impaired perception in
the elderly reduces their sympathetic nervous system response to trigger vasoconstriction, or
vasodilation in warm settings, due to a lack of sensitivity in the autonomic nervous system as a
person ages (Khan et al. 1992). Even outside of thermoregulatory measures, a decrease in
sensitivity to other bodily needs is evident through the tendency of seniors towards dehydration
due to their diminished sensitivity (Takamata et al. 1999).
Oxygen consumption rate also has a lower peak than that of a younger person, which in
turn leads to a lower skin blood flow and thus poor circulation of heat throughout the body of the
senior (Okazaki et al. 2002, Thompson et al. 2004). This is hypothesized to be due to the aged
skin, and therefore structurally compromised capillaries in the aged skin (Thomas et al. 1999).
However, this can be improved by regular aerobic exercise, though this applies to all ages.
The differences in the thermoregulation of seniors even affects their vital signs, which are
listed as the (1) respiratory rate, (2) pulse, (3) temperature, and (4) blood pressure (Chester et al.
2011). While the changes are not defined, it is implied within the study that the differences,
while changed with age, also depend on outside factors such as lifestyle, weight, and diet. It is
also considered that aging generally has a negative effect on plasticity that could hereby affect
the vital signs.
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