1. The document discusses various mechanisms of thermoregulation in animals including endotherms that generate metabolic heat and ectotherms that rely on environmental temperatures. It describes behavioral, anatomical, and physiological adaptations that help regulate body temperature. 2. It then covers three broad categories of thermoregulatory mechanisms: changing behavior, increasing metabolic heat production, and controlling heat exchange with the environment. 3. The four main mechanisms of heat exchange between the body and environment are discussed: conduction, convection, radiation, and evaporation. Certain hypothalamic neurons sense changes in preoptic temperature and control thermoregulatory responses.

1. Energetic Metabolism and
Metbolic Rate
Maryam mohammadzadehsari

2. The metabolic breakdown of food is typically depicted as a spontaneous energy-
releasing act of energy exchange (recall that the aerobic oxidation of glucose starts
with seven molecules
But the metabolic resynthesis of ATP requires energy and so is nonspontaneous. The
metabolic biochemical pathways couple these two exchanges As an energy-exchange
device, metabolism represents a biological machine of sorts, a marvel of engineering
with moving parts and all.

3. Mechanisms of thermoregulation
As a refresher, animals can be divided into endotherms and ectotherms based on their
temperature regulation
• Endotherms, such as birds and mammals, use metabolic heat to maintain a stable
internal temperature, often one different from the environment.
• Ectotherms, like lizards and snakes, do not use metabolic heat to maintain their
body temperature but take on the temperature of the environment.

5. Both endotherms and ectotherms have adaptations—features that arose by natural
selection—that help them maintain a healthy body temperature. These adaptations
can be behavioral, anatomical, or physiological. Some adaptations increase heat
production in endotherms when it’s cold. Others, in both endotherms and ectotherms,
increase or decrease exchange of heat with the environment.

6. We will look at three broad categories of thermoregulatory mechanisms:
• Changing behavior
• Increasing metabolic heat production
• Controlling the exchange of heat with the environment

7. Basal metabolic rate
The basal metabolic rate (BMR) is the metabolic rate of a person measured under
basal conditions, i.e. when a person is awake and in absolute physical and mental rest
after 12 hours of absolute fasting, and when the environmental temperature is 20–25
°C. As long as the person remains healthy, his/her BMR does not vary more than 5–
10% except for the age related change, and 85% of normal people have a BMR within
10% of the mean. BMR increases with the increase in body surface area, so to compare
BMR between different people, it is expressed as calories per hour per square metre of
body surface area.

9. Classification of Physical Activity and Level of
Intensity
Physical activity refers to any bodily movement produced by skeletal muscles that
increase energy expenditure above a basal level. It can be divided into two main
categories. One is exercise that involves structured and repetitive bodily movements.
The other is non-exercise physical activity, such as standing, commuting to and from
school or work, or participating in household chores or occupational work. Both
exercise and non-exercise physical activity can further be classified by the level of
intensity: light, moderate and vigorous. While vigorous-intensity physical activity (e.g.,
jogging) can provide greater benefits for physical fitness and burn more calories per
unit of time than moderate-intensity physical activity (such as brisk walking), engaging
in low-intensity physical activity (such as light walking) is better than no physical
activity at all.

10. Direct Calorimetry Vs Indirect Calorimetry
When you are going to calculate the heat involved in certain physical changes and
chemical reactions, then you are studying the field of calorimetry. Taken from the word
‘calor,’ a Latin word that literally translates as heat, calorimetry was pioneered by a
Scottish scientist named Joseph Black, who first noted the difference between
temperature and heat. Using a calorimeter, he also classified two different forms of
calorimetry namely direct and indirect calorimetry.

11. The subject may sound too technical but the underlying principle is pretty basic.
Indirect calorimetry involves measuring the heat that living things create from
manufacturing carbon dioxide (CO2) and nitrogenous wastes, that are usually coming
from the ammonia in aquatic creatures and so as the urea from terrestrial organisms. Â
Indirect calorimetry also deals with calculating the heat from O2 (oxygen)
consumption.

13. Direct calorimetry also has the same goal of measuring the heat but it uses another
approach ‘“ the organism under study is contained inside a calorimeter for direct
observation and calculation of values.
Heat is basically calculated using the formula q = ms∆T where ‘m’ stands for the
mass, ‘s’ for the specific heat, and ‘∆T’ for temperature change. Their product leads to
the ‘q’ which is the heat or energy. Take note, this is just one of the many formulas
used to estimate energy expenditures.
Of course, because of the advancement in human technology, humans are able to
compute for the heat energy, while incorporating other variables in the formula. This is
achieved by using modern calorimeters like the constant volume calorimeter, in the
concept of Bomb calorimetry.

14. Many say that indirect calorimetry is the more accurate measurement tool. It gives a
precise calculation of heat, because it involves actual oxygen uptake to give the caloric
burn rate. It uses the principle of requiring 208.06 mL of O2 per burning of 1 calorie.
Thus, there is a more direct relationship between calorie burning and consumed O2.
To obtain such values, indirect calorimetry usually uses other devices, such as the
incentive spirometer and other equipments that measure values of inspiration and
expiration.

15. Direct calorimetry aims to measure the actual heat getting out from the body. If you think
about the concept itself, it is somewhat impractical and difficult to monitor, unless of
course, you contain the entire subject inside a calorimeter for a certain period of time. This
is easy if the test subject is little but what if it’s man-size?
All in all,
1. Direct calorimetry measures the heat output by the subject, through direct observation
inside a calorimeter.
2. Indirect calorimetry measure heat by using the variable of O2 consumption and
manufactured CO2.
3. Indirect calorimetry gives a more feasible and accurate measure of heat or energy,
compared to direct calorimetry.

16. MECHANISMS OF HEAT EXCHANGE
 When the environment is not thermoneutral, the body uses four mechanisms of
heat exchange to maintain homeostasis: conduction, convection, radiation, and
evaporation. Each of these mechanisms relies on the property of heat to flow from
a higher concentration to a lower concentration; therefore, each of the mechanisms
of heat exchange varies in rate according to the temperature and conditions of the
environment.
 Conduction is the transfer of heat by two objects that are in direct contact with one
another. It occurs when the skin comes in contact with a cold or warm object. For
example, when holding a glass of ice water, the heat from your skin will warm the
glass and in turn melt the ice. Alternatively, on a cold day, you might warm up by
wrapping your cold hands around a hot mug of coffee. Only about 3 percent of the
body’s heat is lost through conduction.

17.  Convection is the transfer of heat to the air surrounding the skin. The warmed air
rises away from the body and is replaced by cooler air that is subsequently heated.
Convection can also occur in water. When the water temperature is lower than the
body’s temperature, the body loses heat by warming the water closest to the skin,
which moves away to be replaced by cooler water. The convection currents created
by the temperature changes continue to draw heat away from the body more
quickly than the body can replace it, resulting in hyperthermia. About 15 percent of
the body’s heat is lost through convection.
 Radiation is the transfer of heat via infrared waves. This occurs between any two
objects when their temperatures differ. A radiator can warm a room via radiant
heat. On a sunny day, the radiation from the sun warms the skin. The same
principle works from the body to the environment. About 60 percent of the heat
lost by the body is lost through radiation.

18. Evaporation is the transfer of heat by the evaporation of water.
Because it takes a great deal of energy for a water molecule to change from a liquid to
a gas, evaporating water (in the form of sweat) takes with it a great deal of energy
from the skin. However, the rate at which evaporation occurs depends on relative
humidity—more sweat evaporates in lower humidity environments. Sweating is the
primary means of cooling the body during exercise, whereas at rest, about 20 percent
of the heat lost by the body occurs through evaporation.

20. Certain preoptic and rostral hypothalamic neurons are sensitive to changes in local
preoptic temperature (Tpo). These neurons also receive much afferent input from
peripheral thermoreceptors and control a variety of thermoregulatory responses. In
thermode-implanted animals, preoptic warming increases the firing rate in warm-
sensitive neurons and elicits heat loss responses such as panting and sweating.
Preoptic cooling increases the firing rate in cold-sensitive neurons and elicits, first,
heat retention responses then heat production responses It is likely that the preoptic
thermosensitive neurons control these thermoregulatory responses because both
respond similarly to changes in Tpo and skin temperature.

21. Specifically, skin warming not only increases panting, skin blood flow, and the firing
rate of warm-sensitive neurons, but also decreases the sensitivity of all these responses
to Tpo changes. Skin cooling not only increases metabolic heat production, heat
retention behavior, and the firing rate of cold-sensitive neurons, but also increases the
hypothalamic thermosensitivity of all these responses. Low-firing warm-sensitive
neurons receive little afferent input and are most sensitive to high Tpo. Many of these
low-firing neurons probably serve in controlling heat loss responses. High-firing warm-
sensitive neurons receive much excitatory afferent input and are usually sensitive only
to low Tpo. These neurons probably exert their greatest influence on heat production
responses, possibly by inhibiting and, thus, determining the thermosensitive
characteristics of nearby cold-sensitive neurons