691
TEMPERATURE
REGULATION
IN EXERCISE
Sid Robinson, Ph.D.
Department of Anatomy-Physiology, Indiana University, Bloomington, Indiana
I
N WORK, about three-fourths of the energy expended is converted to heatand heat dissipation from the body must be increased in order to avoid an excessive elevation of body temperature. In cool or
cold environments the regulation of body temperature is not a limiting factor in the performance of work by a normal man. Under these conditions and in constant work which can be endured for an hour or
more, a man can dissipate the increased metabolic heat of the work by circulatory
adjustments to increase heat conductance
from deep tissues to skin and by the evap-oration of sweat to cool the skin. In severe
work, where the energy requirement ex-ceeds the maximal rate of aerobic energy release, heat production exceeds heat
dissi-pation. However, when the environment is cool, exhaustion will usually result from ac-cumulation of anaerobic metabolites before heat storage reaches a limiting level. Only
in hot environments does inadequate heat dissipation and the resulting excessive ele-vation of body temperature become a limit-ing factor in the performance of work.
Body Temperature Changes in Work
Even in cool or cold environments where heat dissipation is not a limiting factor the internal or rectal temperature of a man
rises during work, the elevation of tempera-ture being directly proportional to the in-tensity of the work and therefore to the
metabolic rate of the man. Nielsen1 who clearly demonstrated this relationship in
1938 and Robinson2 have found that the rectal temperature of a man working at a
constant rate rises gradually during the
first half hour to a new level characteristic of the work rate, levels off, and remains remarkably constant until the work is stopped (Fig. 1). During recovery follow-ing exercise, rectal temperature slowly
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Fic. 1. Rectal temperature of a man during 60
minutes of work at each of five different rates: I, 360 kg-m/min; II, 540 kg-m/min; III, 900 kg-m/
min; IV, 1080 kg-rn/mm; V, 1260 kg-rn/mm. Room temperature 22 to 23#{176}Cin all experiments.
Data of Nielsen’.
dines to the resting level. During work a man’s thermostat, represented by the tem-perature regulatory center in the
hypothala-mus, appears to be set at a higher tem-perature and under ordinary environmental conditions is more precisely regulated in
constant steady state work than in rest. These adjustments of internal temperature in light to moderately hard work are
inde-pendent of variations in environmental temperature ranging from cold to moderate heat (see ET 9.5 and ET 31.1#{176}C of Fig.
2). On the other hand in extremely hot en-vironments (see ET 35.0#{176}C of Fig. 2) the resistance to heat dissipation may be so great that even in moderate work the physiological adjustments of circulation and sweating are not capable of dissipating
heat rapidly enough and body temperature will rise excessively.3 ET, the effective tem-perature scale of Yaglou4 is a physiological scale based on direct observations of
varia-tions of air temperature, humidity and air movement on the comfort of human sub-jects.
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Fic. 2. Rectal and mean skin temperature of a man during exposures to three different effective tem-peratures. In each experiment he walked on
tread-mill for 90 min. at 5.6 km/hr up a 2.5% grade. Data of Robinson3’
Such a failure of temperature regulation
is dependent on both the intensity of the
environmental heat stress and the rate of
work. In very hard work, such as competi-tive distance running, a man may become
overheated in 14 to 30 minutes on a mod-erately hot summer day (Fig. 3). In per-forming moderate work on such a day he
can easily dissipate his heat and maintain thermal equilibrium (Fig. 2). The data in
figure 3 represent the changes in rectal temperature of champion athletes in ex-hausting races of 3 miles, 5 km and 10 km.
On cool or cold spring days with air tem-peratures in the stadium varying from 5#{176}to
16.1#{176}CLash and Deckard ran several 5 km races. The warm-up before the race each time raised the rectal temperatures above
37.8#{176}Cand in the races, lasting about 15 minutes, the men’s temperatures were
further elevated to about 39.7#{176}Cin every race. On one occasion Lash ran 5 km in 15 minutes, stopped long enough to take his temperature, and immediately ran another
5 km. The second run raised his
tempera-ture only slightly above the 39.7#{176}C which
appears to be characteristic of this high level of metabolic activity, and is inde-pendent of variations of environmental
temperature ranging from cold to cool. In
contrast, on a warm (30#{176}C) humid day with
rather intense solar radiation, Lash ran 10 km continuously in 31 minutes and
fin-ished with a rectal temperature of 41.1#{176}C.
On a humid summer day when the air temperature was 30.6#{176}C and solar radiation
was high Lash and Rice (both world rec-ord holders) ran a 3 mile race, time 14:15,
and finished with rectal temperatures of
40.0#{176}and 41.1#{176}Crespectively (Fig. 3). These highly trained athletes made
nor-mal recoveries from the great elevations of
body temperature observed here. However, 41.1#{176}Cis a critical rectal temperature and
we have observed heat stroke in young men
at considerably lower temperatures than this. In fact, our interest in this problem was aroused by the incidence of heat stroke
and other heat illness in prolonged athletic
performances carried out in hot weather.
Notable examples are the numerous col-lapses of distance runners in the 1924
Fic. 3. Rectal temperatures (Tr) of athletes im-mediately before and after running 3 mi (14 mm.), 5 km (15 mm.), and 10 km (31 mm.). T was
ele-vated by warm-up before races. Air temperature during each race printed under man’s name; hu-midity and solar radiation high with temperatures
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Fic. 4. Relation of rate of sweating to increments in rectal temperature of two
men (RB and RK) during work at 5 different rates (O intake 13 to 44 cc/kg/mm). Each man performed two series of 90-mm work experiments-one in a warm room
(ET 25.2#{176}C),the other in a cold room (ET 10.5#{176}C).Skin temperature and sweat-ing were uniformly higher in the warm room than in the cold room. Each value plotted represents average of observations made during the last 30 minutes of a
90-minute experiment.
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Olympic Games and in the 1931 and 1939
National AAU Outdoor Championships
held in the U. S., and the more recent mci-dences of fatal and near fatal heat stroke
among college football players during long practice sessions held in early September.
Gregory Rice, Thomas Deckard, and Ralph Schwartzkoff were victims of heat stroke in the 5 km National Championships in
1939, one year before we made the above observations on Lash, Rice and Deckard. The facts presented here show the danger
of prolonged competitive athletics in hot weather and should serve to warn coaches
and athletic administrators that this kind of competition should be limited to cool and
cold seasons of the year.
Physiological Regulation
The regulation of body temperature of
men in warm environments and during the increased metabolic heat production of work is dependent on sensitive control of
sweating to provide evaporative cooling of the skin and on variation of cutaneous
blood flow which determines the conduct-ance of heat from the deeper tissues to the
skin. Evaporative cooling may vary from
insensible loss of 10 Cal/hr/m2 in a man
resting in a cool environment to 375 Call hr/rn2 in the same man working in thermal
equilibrium with a hot, dry environment.’ Heat conductance to the skin, which is
pro-portional to the rate of cutaneous blood flow, varies from about 10 Cal/hr/m2/#{176}C difference between central body tempera-ture and mean surface temperature in sub-jects resting in a cool environment,2’61#{176} to
78 Cal. when a man is working in thermal
equilibrium in a hot environment.2 Blood flow through the fingers has been found to
vary from 0.5 ml. to 100 ml. per 100 grams of tissue per minute when a man changes from a cold to a hot environment.” The fingers, with their high ratio of surface
area to mass, are especially adapted for heat exchange and they are capable of
making unusually great adjustments of blood flow.
The Regulation of Sweating
When a man performs moderate to hard work he begins to sweat within 10 minutes, even in a cool environment, and within 20
to 30 minutes the sweat rate will rise to a new steady state level. In a constant cool environment the rate of sweating attained
694
work and therefore to the elevation of rectal temperature and metabolic rate
with-out a corresponding rise in skin tempera-ture. This is illustrated in Figure repre-senting men’s steady state responses to 5 different intensities of treadmill work, each performed for 90 minutes in two
dif-ferent effective temperatures (ET 10.5#{176}C and 25.2#{176}C) of the room.2 Metabolic rate in each of these experiments is indicated by the men’s oxygen consumptions which
varied from 13 cc/kg/mm in the lightest
work to 44 cc/kg/mm in the hardest work.
Each value of sweating, rectal tempera-ture, skin temperature and 02 consumption represents the average of values observed
during the last 30 minutes of a 90 minute experiment, in which the environment and work rate were constant and the man was
in a virtual steady state. The relations of sweating to skin and rectal temperature and oxygen consumption in these experi-ments, as shown in Figure 4, are as follows:
(a) skin temperature at each ET was about the same in all grades of work, (b) at each skin temperature the rectal temperature
and the rate of sweating increased linearly as the work, metabolic rate and rectal tem-perature increased, (c) the difference of
5#{176}Cin the subjects’ skin temperature in the two environments produced a uniform difference in sweating of each man at all work rates, (d) at a given effective tem-perature there were no increases of skin
temperature to increase cutaneous reflexes and cause the rise in sweating. The data at each effective temperature in Figure 4 indicate that sweating in work may be increased by a direct effect of elevated
in-ternal temperature on the heat regulatory
center itself without a rise in skin
tempera-ture. Taking into account the responses in both of the series of experiments in
Fig-ure 4 it appears that changes in both skin
and internal body temperatures may par-ticipate in the regulation of sweating.
Data in Figure 5 show that sweating in
man increases with elevations of mean skin temperature (T.) and mean body tem-perature (Tm) calculated according to
Bur-ton,’2 and with reductions in the gradient
between rectal and mean skin temperature
(TrT0) under steady state conditions of rest or work in which rectal temperature
and metabolic rate are constant. These
re-lationships are shown, in Figure SB, D and
E, for a man in a series of 90-minute work
experiments at a constant rate. The ob-served variations of temperature in the
subject were produced by varying
environ-mental temperature (ET 9.5 to 35#{176}C)from
experiment to experiment (Fig. 5, insert
2A). The plotted values of temperature and sweating represent average values ob-served during the last 60 minutes of the respective 90-minute exposures with the
man in a steady state of body temperature
in all except the most intense heat stresses.
The subject’s sweat rate shows close posi-tive relations to variations of T8 and Tm
and an inverse relation with the tempera-ture gradient (TrTs) through the entire
range of conditions. On the other hand,
sweating during the last hour of the work experiments increased up to 0.68 kg/m2/hr with no rise whatever in the steady state level of rectal temperature for the work.
The lowest rates of sweating were observed
in the coolest environments and were
asso-ciated with a rectal temperature of 38#{176}C,
and all other rates up to 0.68 kglm2/hr were associated with temperatures of 37.6
to 37.8#{176}C. In the more severe environ-ments, increments of sweating above 0.7 kg/m2/hr were accompanied by further
in-creases of skin temperature and also by elevations of rectal temperature above the control work level. Since exercise, meta-bolic rate, and internal temperature were
constant over the major part of this range of conditions, the corresponding incre-ments of sweating in the working man were
apparently dependent upon either the
in-creasing skin temperature or the decreasing gradient between internal and surface tem-peratures, or both. It seems reasonable to
assume that although rectal temperature may not have been identical with the
hypo-thalamic temperature, the two probably maintained a constant relation to each other during these periods of steady state. The
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Fic. 5. Insert 2A gives rectal temperature (Tr) and mean skin tempera-ture (T.) of subject RB in exposures to 12 different effective temperatures (ET 9.5#{176}to 35#{176}C)of the environment. The other graphs show
respec-tively the relations of sweating to mean skin (T.), rectal (Tr) T - T. and mean body temperature (Tm) of the subject in the 12 experiments. Effective temperature was constant during each exposure and work by the subject was constant and the same in all. Each value plotted repre-sents the average of measurements made in the last 60 minutes of a
90-minute exposure. Rohinson.
states of work and constant internal tem-perature, the increase in sweating was
prob-ably not dependent upon increasing hypo-thalamic temperature.
There is evidence that a working man
will sweat at a much faster rate than the
same man at rest even under conditions in
which the thermal stimulus, as indicated
by body temperature, is the same in both
rest and work.’ This is illustrated in
Fig-ure 6 by the sweating responses of a man
exposed to a wide variety of thermal
stresses. The data represent average values observed in the second hour of 12 two-hour
work experiments (MR 190 Cal/m’/hr).
The effective temperature of the room was
varied from experiment to experiment to
produce increases of body temperature and
sweating in the man. In both rest and work
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Fic. 6. Relations of sweating to skin (T.), rectal
(Tr), Tr - T., and mean body temperature (Tm) of
subject L. C. in resting state and during work on
treadmill (5.6 km/hr up a 2.5% grade). Each value
plotted represents average of measurements made during second hour of a 2-hour exposure of the
man to a constant environment. Environment was varied (ET 15#{176}to 35#{176}C)from experiment to ex-periment to produce variations in body
tempera-ture and sweating. Data of Robinson.’
increments of T, and Tm and inversely with the gradient TrTs. It is significant that the man sweated faster in work than in rest
at any given T3, Tm or TrT8 and that in the work experiments sweating varied from 0.2 to 0.7 kg/m2/hr with no corresponding
in-crease in Tr. In the second hour of the resting experiments the subject’s Tr was lowered by the cooler environments and raised by the warmer environments and
therefore it varied with both T8 and sweat-ing.
What is responsible for this difference in sweating between rest and work? Are there special thermoreceptors excited during neu-romuscular work which are not ordinarily activated in rest? In an attempt to answer
this question we have studied the time re-lations of the sweating response to tempera-ture changes of skin, rectum, gastrocnemius muscle, femoral vein, esophagus and tym-panic membrane as men change from rest to 50 minutes of hard treadmill work and then during recovery. Temperatures of the
various sites were recorded continuously by thermocouples which remained in place throughout each experiment. In these ex-periments the men worked for periods of
5, 5, 10, 10, 10, and 10 minutes, with 2 minutes off the treadmill between each two work periods to measure sweating by weight change. Weight loss in this
experi-ment represents total insensible loss at rest and is proportional to sweating in work. The values plotted have not been corrected for pulmonary evaporation or metabolic weight loss. The results of one of these
ex-periments are given in Figure 713 Muscle
temperature rose more rapidly and
recov-ered more slowly than sweating. Tympanic and esophageal (not shown) and rectal temperatures all lagged behind the changes in weight loss (sweating) both during work
and recovery. Skin temperature changed in the opposite direction from weight loss un-der the conditions of intermittent work and rest in these experiments. The time course
of changes in weight loss throughout work
and recovery seemed to follow more closely the temperature changes of blood in the
femoral vein than at any of the other sites. The increase of weight loss during the first five minutes of work was small and due almost entirely to increaesd pulmonary loss, as femoral vein temperature dropped
at first and then rose rapidly to 0.7#{176}Cabove the control temperature by the end of the five minutes. The abrupt drop in femoral
vein temperature during recovery was fol-lowed by a similar drop in sweating. This
suggests that thermoreceptors, capable of reflexly exciting the sweat glands, may be located in the veins which drain warm blood from the working muscles. A similar
abrupt drop in femoral vein temperature
occurred during each 2-minute rest period
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Fic. 7. Temperature changes of rectum, femoral vein, gastroc-nemius muscle, skin (mean), and finger pad in relation to changes in weight loss of subject CA during hard work (02 consumption 44 cc/kg/mm) and recovery in a cool room. The subject worked for periods of 5, 5, 10, 10, 10 and 10 minutes, with 2 minutes off the treadmill between each two work periods to measure sweating
by weight loss.1’
mit us to follow changes in sweating during these brief periods.
The possibility that mechanoreceptors in
the muscles and joints might initiate tem-perature regulatory reflexes during work
was also examined. During exposure of
men to a warm room temperature (DB 34#{176}; WB 20#{176}C.)sweating was increased during
passive exercise only in proportion to the small increases of metabolism and central
temperature which resulted from the
pas-sive exercise.
Circulation
The regulation of heat transfer from the
metabolically active tissues to the body
sur-face is one of the most important functions
of the circulatory system. The circulation
is ideally adapted for this function in that
the heat conductivity and the specific heat of the blood are high, and large vasomotor
adjustments may occur. Estimates have
been made showing variation of overall
blood flow to the skin from 0.16 liter/m2/ mm. in a nude resting man exposed to a neutral temperature of 28#{176}C.7’14, to 2.6
liters/m2/min. in men working in an ex-tremely hot environment.’6”7 These total blood flows to the skin are interesting as
compared with the flow to certain highly vascular peripheral tissues like the fingers, where the flow may vary from 0.5 to 100
cc/min/100 g. of finger tissue.”8”#{176} At the start of exercise the demand for
circulation to the working muscles is pre-dominant and blood flow to the skin is
reflexly reduced, resulting in a fall in skin temperature (Fig. 7). In the experiment
illustrated by Figure 7, as the subject
con-tinued to work sweating began and the in-creased evaporation produced a further
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Fic. 8. The relation of tissue heat conductance to mean skin (T), rectal (Tv), T - T,, and mean body temperature (Tm) of subject RB during the work experiments at 12 different effective temperatures (ET) de-scribed in figure 5. Effective temperature was constant during each exposure and work by the subject was constant and the same in all. Each value plotted represents the average of measurements made in the last
60 minutes of a 90-minute exposure.’#{176}
taneous blood flow combined with the in-creased metabolic heat of the work resulted in a rapid rise in internal body
tempera-ture as shown for the subject in Figure 7. After about 15 minutes of work the demand for heat dissipation began to predominate
and cutaneous blood flow was rapidly in-creased. These shifts in the cutaneous cir culation are best reflected by changes in temperature of the highly vascular finger pads, which cooled more profoundly with vasoconstriction during the early minutes of
work and later warmed to higher tempera-tures than other skin regions in response to the demand for heat dissipation (Fig. 7).
Under these conditions of work and
en-vironmental temperature in which the
sub-ject could achieve thermal equilibrium, evaporation kept the average skin tempera-ture below the prework level, whereas, blood flow to the fingers became so great
that the temperature of the finger pad rose well above that of the general skin
SUPPLEMENT 699
of the palmar skin do not respond to ther-mal stimuli contributes to the warming of
the finger pads with increased blood flow in the fingers. The abrupt drop in femoral
vein temperature which occurred during
each of the 2-minute rest periods and in recovery following the final bout of work indicates that when work stopped a large volume of blood was immediately shunted
through the cutaneous venous plexuses and cooled before returning to the main trunk of the femoral vein. The thermocouple was located in the vein at the level of the
inguinal ring. Further evidence of this rapid re-routing of venous return through
the skin during recovery is the rapid warm-ing of the skin at each cessation of work.
(Fig. 7).
The coefficient of heat conductance, ex-pressed as Cal/m2/hr/#{176}C difference
be-tween central and mean skin temperature, has been used as an index of total
cu-taneous blood flow by a number of in-vestigators.’’#{176} Conductance is not
consid-ered to be an absolute measure of periph-eral blood flow because of the complex
variations of flow patterns and temperature gradients in the body, especially in the
limbs.
Both the minimal and the maximal rates
of tissue heat conductance are increased
during exercise by men.’ Figure 8 gives
conductances in the working man (MR 195
Cal/m2/hr) in the series of 90-minute
ex-posures to various environmental
tempera-tures described above for Figure 5. Con-ductance during the last 60 minutes of each 90-minute exposure is plotted against the
subject’s mean skin temperature (T,), rectal temperature (Tr), gradient between rectal and mean skin temperature (T-T.), and mean body temperature (Ta,) respectively.
In the steady states of these experiments
the stimulus which elicits the heat
regula-tory response of cutaneous vasodilation and
increases conductance could be represented
by any one or any combination of these 4 measures of body temperature. In chang-ing states the rectal temperature probably
does not represent accurately the hypothal-amic temperature (Fig. 7), but in steady
states the two should bear a close relation
to each other and to the mean core
tem-perature of the body. Heat conductance in the steady states represented in figure 8
shows a close positive correlation with T8 and Tm, an inverse relation to the gradient, TrT8, and no relation to Tr. Miiii mal and maximal conductances of this working man were 20 in the cold (ET
9.5#{176}C.)and 130 in the hot (ET 35#{176}C.)en-vironment. These values are more than twice as high as corresponding minimal and
maximal values which have been reported for resting subjects.
Figure 9 gives a direct comparison of
conductances determined in another sub-ject (LG) at rest and during work in a series of experiments performed in various effective temperatures ranging from 15 to
35#{176}C.The data represent average steady state values of conductance and body
tem-perature measurements observed in the second hour of a series of different 2-hour exposures of the subject. In the work
experi-ments on subject LG, the relations of con-ductance to each of the four measures of body temperature are similar in character
to those in subject RB (Fig. 8) over a cor-responding range of environmental stresses.
For the purposes of this discussion the most important consideration of the data in Figure 9 is the stimulating effect of work on
conductance in the subject. In these steady state experiments conductance was much greater in work than in rest at any com-parable measure of T8, T-T5 or Tm. It iS particularly significant that at any given total body heat load (Tm) the response was
greater in work than in rest. In both men
(Fig. 8, 9) during work conductance varied from 20 to 70 with no elevation of Tr above the control values observed in the cool environments. In the resting state, during
the 2-hour exposure to the cooler environ-ments, the lightly-clad subject’s rectal tem-perature tended to fall below control values observed in comfortable environments, and the data show a positive correlation
be-tween Tr and conductance.
Circulatory stability could not be
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Ftc. 9. Tissue heat conductance (Cal/sq.M/#{176}C. Tr - TJhr of subject LG in same series of experiments described in Fig. 6. In the steady states of these
ex-periments, conductance was similar to sweating in its relations to each of the four
measures of body temperature (T., Tr, T - T., and Tm). Data of Robinson.’6
flow to skin and muscles of men in response to prolonged work, especially when the work is done under heat stress, without large increases in heart output and efficient
compensatory adjustments in the circula-tion. The principal compensatory
adjust-ments which have been found to take place in these situations are: (a) compensatory vasoconstriction in other vascular beds, shunting a greater fraction of the total cardiac output to the muscles and skin’0; (b) in hot environments a 5 to 20%
expan-sion of the circulating blood volume2l, 22;
and (c) a linear increase in heart output with increments of aerobic work rate.”t
Evidence of compensatory vasoconstric-tion during work is provided by
measure-ments of renal blood flow. Radigan and Robinson’#{176} found that prolonged moderate work (MR 190 Cal/m2/hr) in a cool room
reduced renal plasma flow from a mean of 700 to 425 ml/min. When the same rate of work was performed in a hot environ-ment (DB 50#{176}C;WB 27#{176})renal plasma
flow was further reduced to an average of
SUPPLEMENT 701
work in this hot environment for six hours and maintain thermal equilibrium with
cu-taneous blood flows estimated from con-ductance values to be 1.2 liters/m2/min.
The reduction in renal blood flow was more than 50% of the total increase in blood flow to the skin under these conditions.
Com-pensatory vasoconstriction in other vascular beds also contributes to the great incre-ments of blood flow required in the muscles
and skin of a working man, especially in hot environments.
Robinson and Kincaid” found that dur-ing two hours of moderate work in a hot environment the circulating blood volumes
of men increased 4 to 20% above control
values observed in a cool environment.
This is principally a response to the heat
since numerous other investigators have
found similar increments of blood volume
in men at rest in the heat.””
Asmussen’#{176} found that cardiac outputs
of men during the first half hour of hard work (MR 290 Cal/m’/hr) were the same in a hot humid environment (32#{176}C; 80% RH) as when they performed the same work in a cool environment. However, as
the work in the heat was continued and the men became fatigued stroke volume and minute volume declined and heart rate
in-creased as compared with values observed
in a cool environment.
SUMMARY
The central body temperature of a man
rises gradually during the first half hour of a period of work to a higher level and this level is precisely maintained until the work is stopped; body temperature then
slowly declines to the usual resting level. During prolonged work the temperature
regulatory center in the hypothalamus
ap-pears to be reset at a level which is
pro-portional to the intensity of the work and
this setting is independent of
environ-mental temperature changes ranging from
cold to moderately warm. In hot environ-ments the resistance to heat loss may be so great that all of the increased metabolic
heat of work cannot be dissipated and the man’s central temperature will rise above
the thermostatic setting. If this condition of imbalance is continued long enough heat
stroke will ensue. We have found that in a 3 mile race lasting only 14 minutes on a hot summer day a runner’s rectal tempera-ture may rise to 41.1#{176}C., with heat stroke
imminent. The physiological regulation of body temperature of men in warm environ-ments and during the increased metabolic
heat production of work is dependent on sweating to provide evaporative cooling of
the skin, and on adjustments of cutaneous blood flow which determine the conduct-ance of heat from the deeper tissues to the skin. The mechanisms of regulating these responses during work are complex and not entirely understood. Recent experiments
carried out in this laboratory indicate that during work, sweating may be regulated by reflexes originating from thermal receptors
in the veins draining warm blood from the
muscles, summated with reflexes from the cutaneous thermal receptors, both acting through the hypothalamic center, the activ-ity of which is increased in proportion to its
own temperature. At the beginning of work the demand for blood flow to the muscles
results in reflex vasoconstriction in the skin. As the body temperature rises the thermal demand predominates and the cutaneous
vessels dilate, increasing heat conductance to the skin. Large increments in cardiac output and compensatory vasoconstriction in the abdominal viscera make these vascu-lar adjustments in work possible without
circulatory embarrassment.
REFERENCES
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DISCUSSION
In the discussion following Dr.
Robin-son’s paper, he stressed that during the initial phases of exercise, skin blood flow is
reduced and most of the blood is shunted to the muscles. As the temperature in-creases in the muscle and vessels draining
the muscle, a point is reached when
vaso-dilatation of the skin occurs in order to
reg-ulate heat loss. He stated that cutaneous
