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CHAPTER 15 SUMMARY
Energy Balance
•Energy input to the body in the form of food energy must equal energy output, because energy
cannot be created or destroyed (the First Law of thermodynamics). Furthermore, useful energy
degrades with all reactions (Second Law; entropy).
•Energy output or expenditure includes (1) external work, performed by skeletal muscles to
accomplish movement of an external object or movement of the body through the external
environment; and (2) internal work, which consists of all other energy-dependent activities that
do not accomplish external work, including active transport, smooth and cardiac muscle
contraction, glandular secretion, and protein synthesis.
•Only about 25% of the chemical energy in food is harnessed to do biological work. The rest is
immediately converted to heat (entropy). Furthermore, all the energy expended to accomplish
internal work is eventually converted into heat, and 75% of the energy expended by working
skeletal muscles is lost as heat. Therefore, most of the energy in food ultimately appears as body
heat.
•The metabolic rate, which is energy expenditure per unit of time, is measured in kilojoules or
kilocalories of heat produced per hour.
•The basal metabolic rate (BMR, for endotherms) or standard metabolic rate (SMR, for
ectotherms) is a measure of a body’s minimal rate of internal energy expenditure. The values
must be measured under strict conditions to minimize other costs.
•BMR has been found to scale to a power of 0.75 in endotherms. That is, larger birds and
mammals use less energy per unit weight than do smaller ones. This is attributed to the surface
area-volume ratios and rate of heat loss vs. internal production. However, many ectotherms scale
to powers of less than 1 even though heat loss is not an issue; thus the scaling pattern is not
fully explained.
•For a neutral energy balance, the energy in ingested food must equal energy expended in
performing external work and transformed into heat. The overal :animal energy equation is:
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•If more energy is consumed than is expended, the extra energy is stored in the body, primarily
as adipose tissue in mammals, so body weight increases. Some animals, such as many fish, can
continue to grow throughout adulthood in this way, as do hibernating mammals in the autumn.
By contrast, if more energy is expended than is available in the food, body energy stores are
used support energy expenditure, so body weight decreases.
•Usually, body weight of adult mammals remains fairly constant over a prolonged period of time
(except during growth, and seasonal changes in hibernators) because food intake is adjusted to
match energy expenditure on a long-term basis. Compensating processes include
•Food intake in mammals is controlled primarily by the hypothalamus by means of complex,
incompletely understood regulatory mechanisms in which hunger and satiety are important
components. The key factors known to be important are as follows: (1) Adipocytes secrete the
hormone leptin. (2) Leptin reduces appetite and decreases food consumption by inhibiting
neuropeptide Y-secreting neurons and stimulating melanocortin-secreting neurons in the arcuate
nucleus of the hypothalamus. (3) Neuropeptide Y (NPY) increases appetite and food intake,
whereas melanocortins suppress appetite and food intake. (4) NPY and melanocortins in turn are
believed to bring about their effects by acting on the lateral hypothalamic area (LHA) and
paraventricular nucleus (PVN) to alter the release of chemical messengers from these areas. (5)
The LHA secretes neuropeptides such as orexins that are potent stimulators of food intake,
whereas the PVN releases neuropeptides that decrease food intake.
•Whereas control of energy balance or energy homeostasis to match energy intake with energy
output is largely controlled by these hypothalamic mechanisms, short-term control of the timing
and size of meals is mediated primarily by the nucleus tractus solitarius (NTS) in the brain stem.
•Satiety signals that act through the NTS to terminate a meal include (1) gastrointestinal
distention, (2) increased glucose use, (3) increased insulin, and (2) increased cholecystokinin.
•Psychosocial and environmental factors can also influence food intake above and beyond the
internal signals that govern feeding behavior.
Thermal Physiology
Body temperature is one of the most pervasive influences on body functions, which slow down if
too cold, and suffer from denaturing macromolecules if too warm. Membranes can be
acclimatized or evolutionarily adapted through changes in lipid saturation: unsaturated lipids are
properly flexible in the cold, but too loose in the warm; saturated lipids work well in the warm
but are too rigid in the cold. Protein flexibility similar evolves to maintain a proper compromise
between flexibility and thermostability.
•Ectotherms are dependent on external heat, while endotherms rely primarily on internal heat.
Poikilotherms have variable body temperatures, while homeotherms have nearly constant body
temperatures. Heterotherms are endotherms that are not fully homeothermic.
•An animal body can be thought of as a heat-generating core (internal organs, CNS, and skeletal
muscles) surrounded by a shell of variable insulating capacity (the skin). The heat generation is
only important for regulation in endotherms for the most part.
•The four physical means by which heat is exchanged between a body and the external
environment are (1) radiation (net movement of heat energy via electromagnetic waves); (2)
conduction (exchange of heat energy by direct contact); (3) convection (transfer of heat energy
by means of air currents); and (4) evaporation (extraction of heat energy by the heat-requiring
conversion of liquid H2O to H2O vapor).
•Because heat energy moves from warmer to cooler objects, radiation, conduction, and
convection can be channels for either heat loss or heat gain, depending on whether surrounding
objects are cooler or warmer, respectively, than the body surface. Evaporation resulting from
sweating, panting, and general skin loss only removes heat.
•Overall heat balance can be regulated in four general ways: (1) Gain external heat/avoid loss to
cold environs by using solar radiation, conduction from a warm surface, or other source of
environmental heat, and by avoiding cold areas. (2) Retain internal heat using behavior,
insulation, reduced blood flow to the integument; countercurrent exchangers, and evolving larger
body sizes. (3) Generate more internal heat. This is definitive feature of endothermy. (4) Lose
excess internal heat/avoid gains from hot environs, by behavior, anatomy; enhancing transfer via
the integument through increased blood flow; and increasing evaporation.
Ectotherms
•Some ectotherms (animals dependent on external heat) are fully poikilothermic. They either
have metabolisms at the mercy of the environment, or may make internal adjustments to
membrane structures, pH, and other factors to compensate somewhat for temperature effects.
•Some ectotherms regulate body temperatures—that is, become homeothermic—to a limited
degree by controlling heat exchanges with the environment, using behavior (e.g., basking,
shade-seeking) and sometimes physiological adjustments to blood flow and evaporation.
Endotherms
Endotherms (animals dependent primarily on internal heat) can avoid many temperature effects
on internal functions by becoming homeothermic much more consistently than ectotherms can.
•(1) Gaining external heat/avoiding loss to cold environs involves ectothermic behavior, and
anatomy such as dark skin.
•(2) Retaining internal heat involves behavior such as huddling, burrowing; insulation such as
hair, feathers, blubber; reduced blood flow to the integument by vasoconstriction; countercurrent
exchangers between the core and thin peripheral organs; and larger body sizes in polar species.
•3) Generating more internal heat occurs through a consistently high BMR (regulated by
thyroxine, and caused by cell membranes leaky to ions); shivering; and special tissues such as
brown adipose that convert food energy to heat via uncoupling proteins. These dissipated the
proton gradient of the mitochondria without making ATP.
•(4) Losing excess internal heat/avoiding gains from hot environs occurs through reduced
insulation, vasodilation in the skin; increasing evaporation via the skin itself (and its sweat glands
in some mammals) and via panting;
anatomy (such as reflective skin); and behavior.
•Thermoregulatory balance is controlled by the hypothalamus (and spine in birds). The
hypothalamus is apprised of the skin temperature by peripheral thermoreceptors and of the core
temperature by central thermoreceptors, the most important of which are located in the
hypothalamus itself.
•A fever occurs when endogenous pyrogen (interleukins) released from white blood cells in
response to infection raises the hypothalamic set point. An elevated core temperature develops
as the hypothalamus initiates cold-response mechanisms to raise the core temperature to the
new set point.
Heterotherms
•Regional heterotherms heat only parts of their bodies. Examples include swimming muscles of
lamnid sharks and scombrid fishes, and thoraxes of flying insects. Countercurrent retes help trap
the heat in these active regions.
•Temporal heterotherms maintain homeostasis at high temperature only part of the time due to
energy constraints. Many small birds and mammals undergo daily torpor, a cooling off and
slowing of metabolism that saves energy. Similarly, some undergo annual hibernation.
Thermoregulation is usually still in operation but with a lower set point.