Area, Metabolic Rate, and Evaporation
13.1 Area, Metabolic Rate, and Evaporation
The total body area in square meters (often called the DuBois area in honor of DuBois and DuBois (1915) who first proposed the formula) can
be calculated from:
where m is the body mass in and h is height in meters. As a rough rule of thumb, body area of adults can be estimated from:
(13.2) Metabolic rates can be calculated using
A = 0.026 m.
but a better guide
can be obtained from measurements. Table 13.1 gives values of M for various activity levels. These activity levels conform quite well to our rules of thumb of
= 30 50 and
The published values for caloric content of foods are normally in units
of (called calories in the food literature) rather than joules. Assuming a 2 m 2 surface area for a person, W
in Table 13.1 can be converted to kcal of food intake per hour. The conversion factor is:
kcal J
Therefore, according to the numbers in Table 13.1, desk work would consume 160
and sleeping
85 kcdhr. For eight hours of sleep
Humans and their Environment
T A BLE 13.1. Rates of metabolic heat production for humans
Activity
Sleeping Awake, resting Standing Working at a desk or driving Standing-light work Level walking at 4
or moderate work Level
at 5.5 or moderately hard work Level
at 5.5 with a 20-kg pack or sustained hard work Short spurts of very heavy activity such as in
or sports Data from
and 16 hours standing, the daily caloric requirement would be around 3100 kcal. If a person performed hard physical labor for 12
and
rested for the remaining 12 hours, the caloric intake would need to increase to 6000
For those who exercise for weight control, one hour of
strenuous exercise is worth about 600 kcal in excess food intake. The caloric content of fat is 40
so strenuous exercise for 1 would use
63 g of fat. One might conclude that regulation of caloric intake is an easier mode of weight control that exercise. As a note of caution, remember that the values in Table 13.1 are for thermoneutral temperatures. If additional metabolic energy is required for thermoregulation (Eq. (12.1 1)) this must
be added to the values in Table 13.1. Latent heat is lost through respiration and through water loss directly
from the In Ch. 12 we derive an expression for respiratory latent
heat loss, and find it to be around 0.1 M in relatively dry environments
(12.15)). In more moist environments, it is smaller. Evaporation from the
in the absence of thermal sweating is called insensible perspiration, and can
(12.16) using the appropriate value for
be calculated from
conductance from Table 12.1. Under typical conditions =
and
12 This is a little over twice the respiratory latent heat loss at M = The core temperature of the body depends mainly on metabolic heat production until environmental conditions become too severe for moregulation. A convenient equation expressing the relationship between
metabolic rate and core temperature is (Kerslake, 1972):
where M is in Resistance to heat transfer in the human body is, as with other homeotherms, subject to vasomotor control. The tissue conductance varies, within limits, to balance the energy budget. The limits given in Table 12.2 are
0.46 mol
for vasoconstriction and
Survival in Cold Environments
2.8 rnol for vasodilation. These values were calculated slake (1972, Fig. 7.22). Monteith gives a range of
to 1.4 rnol
The difference is probably due to acclimatization of subjects or possi- ble subject-to-subject variation. In any case, we use the range 0.46 to
2.8 rnol
for our calculations.
Clothing conductance for humans is more difficult to treat than coat conductance for animals because of the extremely wide possible range of clothing available (down parkas to bathing suits). Normal indoor clothing has a conductance of around 0.4 rnol
in still air. In moving air, this is drastically increased, as common experience will verify. In the absence of conductance measurements for a given assemblage of clothing, one can use estimates based on windspeed, permeability, thickness, and ventilation of the clothing.