Animals and their Environment
Metabolic rate increases with animal activity. This can be accounted
for in the energy budget in one of two ways. If about 30 percent efficiency
is assumed for conversion of chemical energy to work in animals, then
for each unit of work done there will be about two units of heat produced.
If it is known how much work is done, the metabolic production of heat
can be calculated. The other method is somewhat simpler, As a rule of
thumb, the maximum aerobic metabolic rate an animal can sustain can be
assumed to be about ten times the basal rate (Fig. 12.2). If the animal's
activity can be estimated as a percent of maximum (say from oxygen
consumption measurements or running speed compared to maximum)
the metabolic contribution can be estimated from:
where a! is the animal's activity and a ! ~
is the maximum sustainable
activity. If Mb is 50 W m-2, M will vary between 50 and 500 W m-2.
12.3 Latent Heat Exchange
Evaporation of water from the respiratory tract and from the skin result in
latent heat loss from the animal. The total latent heat loss, needed for the
energy budget equations, is the sum of the respiratory and skin latent heat
losses. Respiratory loss is a direct result of the air exchange for breathing.
Skin water loss was already treated in detail in Chs. 6 and 7.
In respiratory evaporation, air is breathed in at ambient vapor pressure
and breathed out at the saturation vapor corresponding to the temperature
of the nasal passages. In most species the nasal passages are maintained
at about body temperature. Since increased metabolic heat production
results in increased oxygen consumption, and this increases breathing
rate, it would seem reasonable to compute respiratory latent heat loss as
some fixed fraction of metabolic heat production. Taking into account the
concentrations of inhaled and exhaled oxygen and water vapor, and the
heats of combustion and evaporation we can write:
M h (e, - ei)
hEr =
rpa ( c o i - c o e )
where e, and ei are expired and inspired vapor pressure, C,, and Coi are the
corresponding oxygen concentrations, h is the latent heat of vaporization
for water (44 kJImol), p, is the atmospheric pressure, and r is the heat
produced per mole of oxygen consumed (480 Wmol). The difference
in oxygen concentration between inhaled and exhaled air is around five
percent or 0.05 mol/mol. To get an idea of the magnitude of respiratory
latent heat loss, assume air is breathed out at 34'C and has a vapor pressure
of 1 kPa when it is breathed in. From Table A.3, the exhaled vapor pressure
is 5.3 kPa. Substituting these values into Eq. (12.15) gives hEr = 0.1 M .
Some animals with small nasal passages exhale air at temperatures well
below body temperature. Figure 12.3 compares exhaled air temperatures
for several bird species with values for humans and for kangaroo rats. The
Metabolic rate increases with animal activity. This can be accounted
for in the energy budget in one of two ways. If about 30 percent efficiency
is assumed for conversion of chemical energy to work in animals, then
for each unit of work done there will be about two units of heat produced.
If it is known how much work is done, the metabolic production of heat
can be calculated. The other method is somewhat simpler, As a rule of
thumb, the maximum aerobic metabolic rate an animal can sustain can be
assumed to be about ten times the basal rate (Fig. 12.2). If the animal's
activity can be estimated as a percent of maximum (say from oxygen
consumption measurements or running speed compared to maximum)
the metabolic contribution can be estimated from:
where a! is the animal's activity and a ! ~
is the maximum sustainable
activity. If Mb is 50 W m-2, M will vary between 50 and 500 W m-2.
12.3 Latent Heat Exchange
Evaporation of water from the respiratory tract and from the skin result in
latent heat loss from the animal. The total latent heat loss, needed for the
energy budget equations, is the sum of the respiratory and skin latent heat
losses. Respiratory loss is a direct result of the air exchange for breathing.
Skin water loss was already treated in detail in Chs. 6 and 7.
In respiratory evaporation, air is breathed in at ambient vapor pressure
and breathed out at the saturation vapor corresponding to the temperature
of the nasal passages. In most species the nasal passages are maintained
at about body temperature. Since increased metabolic heat production
results in increased oxygen consumption, and this increases breathing
rate, it would seem reasonable to compute respiratory latent heat loss as
some fixed fraction of metabolic heat production. Taking into account the
concentrations of inhaled and exhaled oxygen and water vapor, and the
heats of combustion and evaporation we can write:
M h (e, - ei)
hEr =
rpa ( c o i - c o e )
where e, and ei are expired and inspired vapor pressure, C,, and Coi are the
corresponding oxygen concentrations, h is the latent heat of vaporization
for water (44 kJImol), p, is the atmospheric pressure, and r is the heat
produced per mole of oxygen consumed (480 Wmol). The difference
in oxygen concentration between inhaled and exhaled air is around five
percent or 0.05 mol/mol. To get an idea of the magnitude of respiratory
latent heat loss, assume air is breathed out at 34'C and has a vapor pressure
of 1 kPa when it is breathed in. From Table A.3, the exhaled vapor pressure
is 5.3 kPa. Substituting these values into Eq. (12.15) gives hEr = 0.1 M .
Some animals with small nasal passages exhale air at temperatures well
below body temperature. Figure 12.3 compares exhaled air temperatures
for several bird species with values for humans and for kangaroo rats. The