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Convection
is when the body shell transfers heat to the surrounding air. Because
warm air tends to expand and rise, and cool air (being denser) falls,
cooler air molecules continually replace the warmed air that surrounds
the body. This process, called convection, substantially enhances
heat exchange from the body surface to the air, because the cooler
air absorbs heat by conduction more rapidly than the already-warmed
air. Together, conduction and convection account for 15% to 20%
of heat loss to the environment. These processes are enhanced by
anything that moves air more rapidly across the body surface, such
as wind or a fan.
Evaporation
is when water molecules absorb heat from the environment and become
energetic enough (that is, vibrate fast enough) to escape as gas
(water vapour). The heat absorbed by water during evaporation is
called heat of vaporisation. Because water absorbs a great deal
of heat before vaporising, its evaporation from body surfaces removes
large amounts of body heat.
There is a basal
level of body heat loss due to the continuous evaporation of water
from the lungs, from the mucus of the mouth, and through the skin.
The unnoticeable water loss occurring via these routes is called
insensible water loss and the accompanying heat loss is called insensible
heat loss.
Evaporative heat loss becomes an active (sensible) process when
body temperature rises and sweating provides increased amounts of
water for vaporisation. Intense exercise can thrust body temperature
upward as much as 2-3°C. During vigorous muscular activity,
when sweating is profuse, 1-2 L/hour of perspiration can be produced
and evaporated, causing 2000 kcal of heat to be removed from the
body each hour.
Role
of the Hypothalamus in Heat Regulation
The hypothalamus
is the major integrating centre for thermoregulation and it receives
afferent temperature information from:
- Peripheral
thermoreceptors, which are located in the skin.
- Central thermoreceptors
(receptors sensitive to the temperature of blood), which are located
in the body core including the anterior portion of the hypothalamus
itself.
Much like a
thermostat, the hypothalamus responds to this input by initiating
the appropriate heat promoting or heat-loss reflex mechanisms.
Although the
central thermoreceptors are more critically located than the peripheral
ones, changing inputs from the shell probably alert the hypothalamus
that modifications must be made to prevent temperature changes in
the core, i.e., they allow the hypothalamus to anticipate possible
changes to be made.
Heat-Promoting
Mechanisms
- Vasoconstriction
of cutaneous blood vessels
- Increase
in metabolic rate
- Shivering
- Behaviour
modification (posture, activity, clothing)
Heat
Loss Mechanisms
- Vasodilation
of cutaneous blood vessels
- Increased
sweating
- Behaviour
modification (clothing, shade)
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