The Heat from Metabolism Must Be Regulated
how body heat is produced and how it
how body temperature is regulated.
the responses of your body to fever
s you learned earlier in the chapter, catabo-
lism of food transfers only about 40% of the
energy to ATP for cellular functions. The
rest of the energy is lost as heat, which is
what makes us warm-blooded animals. The rate at which
this heat is produced is called the basal metabolic
rate (BMR). BMR is about
for adults and can be affected by many factors, includ-
ing physical activity, hormones, nervous activity, age, and
gender (Figure 14.17).
Because heat is continuously produced, our bodies
must have a way to remove heat so that body temperature
does not rise constantly. The processes of heat loss include
radiation, conduction, convection, and evaporation (see
Figure 14.17). So, overall body temperature is a balance
between heat production and heat loss. Your body has a
system of temperature regulation in place to maintain this
balance, especially in your head and trunk. Let’s take a
B o d y t e m p e r a t u r e is a b a l a n c e b e t w e e n h e a t p r o d u c t i o n a n d h e a t l o s s
F ig u r e 1 4 . 1 7
Cellular metabolism constantly generates heat. The rate of heat production, called the
basal metabolic rate (BMR), is influenced by many factors. The heat produced must be
removed by several processes to maintain a constant body temperature.
Factors in heat production
Males have higher BMR than females.
The sympathetic nervous system secretes
epinephrine and norepinephrine, which increases BMR.
Thyroid hormones, testosterone, insulin, and
hGH increase BMR.
Physical activity increases BMR.
The thermic effect of food increases BMR.
BMR is proportional to body temperature.
BMR decreases with age.
BMR is lower during sleep.
Malnutrition lowers BMR.
BMR is lower in the tropics.
Methods of heat loss
Conversion of liquid to vapor as in sweating;
removes about 22% of heat at rest.
Heat loss between two materials in contact,
like water poured on a runner or a swimmer in a pool.
Transfer of heat (infrared rays) between warm
objects and cooler ones without contact. Your body
constantly radiates heat to its surroundings.
Transfer of heat by movement of a gas or
liquid between areas of different temperatures. A fan
blowing air over you removes heat by convection.
The Heat from Metabolism Must Be Regulated 427