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Biology for Secondary Schools

Very little sweat
Very little radiation

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Constricted
blood vessels

Subcutaneous
Sweat gland fat layer
produce sweat
Figure 5.5: Vasoconstriction

Hair erection (Piloerection): Small mammals is detected by the hypothalamus,
muscles in the skin contract, causing hairs which sends nerve impulses to sweat glands
to stand up. This traps a layer of warm air under the skin. The nerve impulses cause
close to the skin, providing insulation which sweat glands to increase the production
create a barrier between the skin and the of sweat. Evaporation of sweat requires
cold environment to reduce heat loss. energy in form of heat from the surface of
the skin. This heat is taken away from the
Responses to hot conditions skin there by cooling the body. This process

When temperatures rise, the body employs is reversed during cold conditions.
mechanisms to release excess heat. The
blood flowing to the brain stimulates the Relaxation of hair erector muscles: Hair
thermoregulatory centres. The centres present on the skin of the animal helps to
trigger the following actions: control body temperature. The hair tends to
lie flat when the environmental condition
Sweating: Human beings can control is warm and rises during cold weather.
body temperature through sweating. This When the hair erector muscles relax, the
is because their bodies have many sweat hairs lie flat and reduce the thickness of
glands. Sweat glands produce sweat, which insulating warm air. As a result, they give
evaporates from the skin, carrying heat chance for heat to escape from the skin
away and cooling the body. These sweat surface. Therefore, the temperature of the
glands are found beneath the skin over body decreases.
the whole body enabling them to be more
efficient at cooling through sweating. Some Vasodilation: This is the widening of
animals with fur have limited sweat glands the superficial blood vessels near the
which are confined to areas that do not body surface. Vasodilation occurs when
have fur, for example pads of the feet in the smooth muscles in the blood vessels
dogs and cats. Excess heat in the body of relax. When the body temperature rises,

112 Student’s Book - Form Three

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