takes a lot of heat energy to change water from its liquid state
into water vapor. This physical fact is commonly made use of in
cooling systems. Engines, for example, generate a lot of excess
heat energy. In order to prevent an engine from overheating, this
heat must be removed by cooling. Water is one of the most common
forms of coolant, because it is plentiful and uses up lots of heat
energy in the evaporative process.
the sun beats down upon the earth, land surfaces absorb the heat
and quickly release it back into the air again. Water, on the other
hand, has a larger capacity than land for absorbing heat. Heat is
not released into the air as quickly, which helps to keep the earth's
atmosphere from becoming too hot. Many of the other planets in our
solar system have wide ranges of temperature-at times varying by
hundreds of degrees. Water in the earth's atmosphere and on its
surface helps prevent this from happening on our planet.
human body can be thought of as a form of engine. The food we eat
is digested and absorbed into our body. Once it reaches a cell,
the chemical from the food can be released and used for the processes
that take place in the cell. This process is called respiration.
However, quite a lot of heat energy is also released. Most of the
time, this helps to keep our bodies at our normal temperature of
37°C (98.6°F). But, if we do strenuous physical exercise,
or we are in a hot environment, excess heat energy has to be dissipated
(got rid of) or the body will overheat. The body uses water to help
it keep cool by sweating. The skin contains millions of tiny sweat
glands that produce water with a little urea and some salts dissolved
in it. The liquid travels up a dust from the sweat glands, which
actually lie quite deep in the skin, to the surface of the skin.
As the sweat lies on the surface, the water takes heat energy from
the skin and evaporates, thereby cooling down the skin.
it is difficult for the sweat to evaporate. For example, in a hot
and humid climate, sweat often lies on the surface of the skin and
does not evaporate .This is because the air already contains so
much water vapor that no more water can evaporate . Instead, it
stays on the skin and makes the person feel sticky and uncomfortable.
In a hot, dry climate, however, the air contains little water vapor,
so any sweat quickly evaporates, making us feel cooler. A similar
effect is felt when it is windy. Wind carries the water vapor away
from the skin more rapidly, and replaces it with dry air capable
of absorbing more moisture .
few mammals are able to sweat. Most mammals have evolved other ways
of keeping cool in hot weather. Kangaroos have a network of tiny
capillaries very near the surface of the shin along their forearm.
They lick the fur above this area of skin, and the blood is cooled
as the saliva evaporates and heat is drawn from the capillaries
below. Dogs hang their tongues out and pant. These quick, shallow
breaths bring cool dry air into body and allow lots of water to
evaporate from the tongue and the rest of the mouth, thus cooling
the cooling the blood. Cats lick themselves in order to spread a
thin layer of saliva over their fur. As the saliva evaporates, it
cools the body.
live in dry desert areas where water is in short supply. They cannot
afford the luxury of being able to lose water by sweating. Nevertheless,
camels still need some means of keeping cool. While most mammals
keep their body temperature within a very narrow range day and night
(in humans within a fraction of a degree above or below 37°C
[98. 6°F]), camels allow their body temperature to fluctuate.
During the heat of the day, they allow their body temperature rise.
By the end of the day their body temperature may be as must as 6°C
(11°F) higher than it was in the morning. At night, deserts
are quite cold as there are few clouds to retain the heat. So as
nighttime temperatures drop, the body temperature of the camel falls
back to its lowest lever. It has been estimated that by allowing
the body temperature to rise and fall naturally in this way, the
camel saves approximately six liters (12. 5 pts.) of water that
would otherwise have been lost as sweat. A further adaptation of
the camel, designed to save even more water, is the production of
very concentrated urine.