J Appl Physiol 109: 1288–1289, 2010;
doi:10.1152/japplphysiol.01013.2010.
Invited Editorial
Optimizing heat dissipation for every environment: the cool ability of the skin
to locally regulate sweating
N. Charkoudian
Department of Physiology and Biomedical Engineering, Mayo Clinic, Rochester, Minnesota
Address for reprint requests and other correspondence: N. Charkoudian,
Dept. of Physiology and Biomedical Engineering, Mayo Clinic College of
Medicine, JO4184W, 200 First St. SW, Rochester, MN 55905 (e-mail:
charkoudian.nisha@mayo.edu).
1288
In this issue of the Journal of Applied Physiology, Wingo and
colleagues provide important, novel insight into the complexities
of local–reflex thermal integration in humans by evaluating the
independent roles of skin blood flow and local temperature in the
effects of local cooling on sweating rate (10). Although it is
known that local cooling can decrease sweating in a given area of
skin, the same cooling also decreases skin blood flow, so it was
not known whether it is the change in temperature or the change
in blood flow that modulates the sweating response. Using an
elegant study design, Wingo and colleagues demonstrated that
each variable can independently affect the local rate of sweating
during whole body hyperthermia.
The investigators used a combination of local control of skin
temperature, along with intradermal microdialysis, for local
pharmacological manipulation of skin blood flow. Throughout
body heating, they then measured skin blood flow and sweating
responses 1) between sites with differing skin blood flows but
similar local temperatures (34°C), and 2) between sites with
differing local temperatures (20 and 34°C) but similar skin
blood flows. This design allowed Wingo and colleagues (10) to
evaluate the independent roles of local temperature and of skin
blood flow in the local cooling-mediated decreases in sweating
that had previously been observed. In keeping with the complex and redundant nature of human integrative physiology,
they report that low skin blood flow and low local temperature
are each able to independently decrease sweating during whole
body heating. This was true both when evaluated as absolute
sweating rate and when quantified as the sensitivity (slope) of
the sweating-body temperature relationship.
This capacity of the skin for “local” thermoregulation allows
for optimal use of the thermoregulatory resources available to an
individual in a given environment. Excessive sweating can lead to
dehydration, particularly in prolonged endurance events, which
can negatively affect exercise performance (2). Cooler environments have regularly been shown to be associated with better
performance in events such as marathons, and the mechanisms
identified by Wingo and colleagues (10) in the present study likely
contribute to this phenomenon, due to minimization of excessive
sweating in cooler areas of skin. Furthermore, heat stress is
associated with a decrease in orthostatic tolerance: people are
more likely to experience symptoms, including dizziness, lightheadedness, and tachycardia or even syncope in hot environments
compared with cooler environments (3). The changes in orthostatic tolerance are related to several factors, including altered
cutaneous ␣-adrenergic vasoconstriction during reflex increases in
skin blood flow (3). Cooling some or all of the skin surface can
ameliorate symptoms of orthostatic intolerance during hyperthermia (9). This helpful effect of surface cooling is likely due, at least
in part, to the ability to conserve plasma volume and central blood
volume by minimizing sweating and skin blood flow in the cooled
areas.
The complexity of the control of sweating during exercise is
beyond the scope of this brief discussion, including metabolic
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of maintaining thermal homeostasis requires rapid and complex integration of local and
systemic (reflex) thermal variables to optimize the distribution
of heat dissipation which, in humans, is primarily via sweating
and cutaneous vasodilation. During environmental heat exposure and/or exercise, appropriate physiological heat dissipation
prevents life-threatening hyperthermia. However, the need to
dissipate heat must be balanced by requirements for blood flow
in other areas (for example, muscle blood flow during exercise)
and by the overall need to maintain arterial blood pressure.
Similarly, the integrated thermoregulatory response must be
appropriate to all aspects of the thermal environment.
For example, if a person is running outdoors on a cold day, core
temperature might increase substantially, but exposed areas of
skin might have very low temperatures. If skin temperature is low,
the thermal gradient between the core and skin is increased, and
the need for skin blood flow and sweating is decreased. Indeed,
skin temperature and core temperature have independent and
combined effects on the reflex responses of sweating and skin
blood flow (5, 7). In the example of the runner, if a person with a
high core temperature experiences surface cooling, the integration
of core and skin temperature variables results in lower values for
sweating and skin blood flow compared with if skin temperature
had been higher. The cooling of the skin allows the body to
“conserve” blood flow or plasma volume by decreasing the need
for these effector outputs.
Afferent information regarding core and skin temperature is
integrated primarily at the preoptic/anterior hypothalamus,
which then regulates the appropriate outputs in sweating and
skin blood flow (1). Interestingly, the influence of skin temperature can be even more complex, since the local temperature of a small area of skin can influence sweating and skin
blood flow, independent of reflex control from the hypothalamus (6 – 8). This allows for an additional dimension of integration for the purpose of optimizing whole body thermoregulation. Not only can the hypothalamus integrate signals related to whole body temperatures, but each small area of skin
has its own capacity for modulating heat dissipation to meet, in
a sense, its own “local” thermoregulatory needs. Thus local
warming of the skin increases skin blood flow and increases
sweating rate for a given hyperthermic state, and local cooling
decreases both skin blood flow and sweating rate (6 – 8). An
analogy could be drawn with muscle blood flow, which can
regulate its local blood flow based on its metabolic needs
during exercise, and also relies on reflex mechanisms, such as
the exercise pressor reflex, to more perfectly match its blood
flow with metabolic requirements (4).
THE PHYSIOLOGICAL CHALLENGE
Invited Editorial
1289
signals, other local signals, and modulation by central command,
in addition to the few points touched upon here (7). The findings
of Wingo and colleagues (10) give us novel mechanistic insight
into the interplay of reflex and local thermal factors determining
sweating rate in a given area of skin. Their results emphasize both
the complexity and redundancy of a thermoregulatory system that
has the capacity to keep human core temperature within a few
tenths of a degree of 37°C during a wide range of daily activities
and environments. The capabilities of the system are even more
impressive when it is considered that thermal responsiveness is
balanced with hemodynamic, autonomic, metabolic, and other
simultaneous physiological requirements, which may need to
“share resources” with thermoregulation.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
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J Appl Physiol • VOL
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