Primary Factors
Humans and the other warm-blooded animals have developed thermoregulatory systems to carefully control body temperature to levels that enable them to function and survive effectively. In general, thermal comfort occurs when the physiological effort to control body temperature is minimized for the activity. Table 5.1
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TABLE 5.1 Thermal Environment and Physiological Responses of Thermoregu lation
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Illustrates that as conditions deviate from neutral the body activates mechanisms to stabilize body temperature. These efforts all result in small but noticeable and measurable increases in metabolism and physiological effort.
Body Temperature
To maintain proper body temperatures (T), metabolic energy (M) must be continuously transferred to the environment.
Energy balance:
Metabolism — energy losses — work = rate of energy storage in body M — L — W = C dT/dt ‘
Where
Energy losses (L) = dry heat loss + evaporative heat loss
If M — W > L, then body T T — feel warmer
If M-W < L, then body T i -—■ feel cooler
The body temperature limits for health in terms of internal or core temperature are fairly limited. The limits are basically related to the function of nervous tissue. Body temperatures around 28 °C or less can result in cardiac fibrillation and arrest. Temperatures of 43 °C and greater can result in heat stroke, brain damage, and death. Often, too high a temperature causes irreversible shape changes to the protein molecules of nervous tissue. That is, cooling overheated tissue to normal temperatures may not restore its original function.
Metabolism
Metabolism is often characterized by a convenient, relative, and dimen — sionless quantity called the met unit:
Met = metabolism/resting metabolism (5.1)
The metabolism of a resting person is then 1 met. Some met levels of various activities are listed in Table 5.2.
Metabolic energy is often normalized in terms of energy per unit area of skin (1 met = 58.2 W/m2):
M/Ad = 58.2 ■ met watts/m2 (5.2)
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TABLE 5.2 Met Level of Various Activities
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Or
TOC o "1-5" h z M = Ad • 58.2 ■ met watts, (5.3)
Where Ad is body surface area in m2 . The subscript D refers to the DuBois
Equation6 commonly used for calculating the area of the skin:
A n 0.425/ 0.725 2 ,
where m = mass (kg), b = height (m). Surface areas are generally in the range of 1.4 to 2.2 m2 .
In some activities metabolic energy may be converted to useful work (force ■ distance). At steady state the rate of doing work F = force — distance/’ time and the thermal losses must balance with metabolism:
M = P+L (5.5)
And if the rate of work is expressed as a thermal efficiency, rj — P/M, then Eq. (5.5) simplifies to
M(1 — 17) = L. (5.6)
Example
Determine the met level of a person who bicycles up a 150 m-high hill in 10 minutes. The person weighs 75 kg and is 182 cm tall. The bicycle weighs 10 kg.
Work of cycling up the hill = force ■ distance
= (75 + 10) • 9.8 • 150 = 124 950 N m.
The work is accomplished over a period of 10 minutes, so
P = 124 950/(10 • 60) = 208 Nm/s = 208 watts.
Cycling with the legs is rather efficient and it can be reasonably assumed that: the thermal efficiency (77) is about 20%. Thus
M = P/7] = 208/0.2 — 1040 W.
This energy, normalized per unit of body surface area (M/AD) where a a tai 0.425» 0.725 /% — t^0.425 ^ 0.725 — i o c
Aj) = 0.202m h = 0.202 • 75 • 1.82 = 1.95 m,
Is
M/Ad = 1040/1.95 = 533.3 W/m2.
Expressed in terms of met:
M/Ad = 533.3/58.2 = 9.2 met.
Since this activity is greater than about 7 met, the effort of breathing may make it difficult to talk during the climb.
Physiological Temperature Regulation
For most situations and conditions in daily life, the human can be represented adequately by a simple model that is helpful for understanding human thermal regulation.7 The model has two thermal compartments (Fig 5.1). The
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Skin bloodflow (acti’ |
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Respiratior |
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Water diffusion (passive) |
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FIGURE S. I Simple representation of physiological temperature regulation in man. |
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Compartments are characterized as having relatively uniform temperatures throughout. The bigger compartment (85 to 95% of body weight) represents
The body’s core and contains all of the muscles and other significant heat — and energy-generating tissue. Blood profusion of the muscles and internal organs
Distributes the heat fairly well so the core can be represented as having an approximately uniform temperature (Tc). The smaller compartment represents the skin with uniform temperature The temperature uniformity of this simple lumped parameter model is reasonable for people at sedentary to medium activities (0.7-5 met) in conditions where healthy people feel slightly cool to very hot.
Essentially all the energy produced in the body by the various metabolic activities is generated in the core. The skin functions as a protective and heat transfer surface for the core. As such, the skin, which is about 1.6 mm thick on average, has tissue with very small oxygen needs and heat-producing capabilities. The energy (M) produced by the core includes the extra heat generated by muscles in tensioning and shivering (Table 5.3) under active control for thermoregulation. In humans, shivering thermogenesis potentials are small with a maximum incremental heat increase capability of about 1 met.
The metabolic energy generated by the core (M) is lost by (1) doing work, (2) respiration, (3) passive heat conduction to the skin, and (4) active blood flow to the skin. Any heat not transferred from the core is stored, with a resulting increase in core temperature. Work is energy that leaves the body as in
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TABLE 5.3 Active Physiological Controls: Shivering, Sweating and Skin Blood Flow
Shivering = K(hlv (T^K, — T, k ) • (TlSCT- TJ W/m~, TAftl = 33.7° C T„, s 36.8* C skin blood flow = (BFN + Q.,- (Tc — T^,)/( 1 + Str — (Tskicl — T*» liters/(h m:j
Where BFN is normal blood flow to skin for its metabolic needs. It is small
‘6.3 I7h/irr. SKBl о as 7T >*6.8° C, SKBI. i as T^l <.33.7° C
Sweat = Ksw • (T^-T^Je — (Tsk -7kM,)/10.7g/min/m*, where Thm = aT* + |l — a|T. and a = 0.1.
Rbe raising of a weight or other thermodynamic work (force • distance) activities. Respiratory heat loss occurs from bringing ambient air into the core, raising its temperature to near core temperature, humidifying it to near saturation at core temperature, and exhaling it. The resulting heat loss is proportional to breathing rate and to the temperature and humidity differences. The breathing rate or air flow through the lungs is regulated mainly by C02 levels in the blood and as a result is proportional to metabolic rate.
The skin receives heat from the core by passive conduction and active skin blood flow (Table 5.3). It transfers this heat to the surroundings by convection, radiation, and evaporative (perspiration and diffusion) mechanisms. All of these mechanisms are unregulated or passive except evaporation from sweating. The sweating process is actively controlled by the human’s thermoregulatory center where the rate of sweat secretion is proportional to eleva tions in core and skin temperature from respective set point temperatures (Table 5.3).
The physiologically active elements in body temperature regulation, summarized in Table 5.3, function and regulate in part on deviations in body temperatures from set points. In humans thermogenesis by shivering is small and inefficient in comparison to other animals. Thus the very precise regulation of body temperature in man is primarily due to only two active mechanisms associated with the skin: blood flow and sweating. Under normal comfort conditions, blood flow to skin is about 6 liters per hour per m2 of skin. Of this about
1.5 I /(h m2) is for the relatively constant minimal metabolic needs of the skin. In hot environments and during exercise skin blood flow can be increased by 15 times to about 90 L/(h m2).7 When necessary to reduce heat loss in cold environments, the vessels can restrict blood flow to as little as 1 L/(h m2). With continued heat exposure, the thermoregulatory system increases its sensitivity so that blood flow increases with smaller and smaller changes in body temperature as the body acclimates to the hot environment.
Sweating, the other powerful heat loss mechanism actively regulated by the thermoregulatory center, is most developed in humans. With about 2,6 million sweat glands distributed over the skin and neurally controlled, sweat secretion can vary from 0 to 1 L/(h m2). The other, lesser, passive evaporative process of the skin is from the diffusion of water. The primary resistance to this flow is the stratum corneum or outermost 15 (im of the skin. The diffusion resistance of the skin is high in comparison to that of clothing and the boundary layer resistance and as a result makes water loss by diffusion fairly stable at about 500 grams/day.
When the energy flows in and out of a compartment do not balance, the energy difference accumulates and the temperature increases or decreases. The changes in core and skin temperature then in turn alter the physiological control signals to restore balance and thermal stability.
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