heat exchange with the environment and heat exchangers
heat exchange in the environment
image 4 the four ways of heat exchange
The body of an organism will exchange heat with its
environment all the time, it is determined via four physical processes (see image 4):
Conduction
Convection
Radiation
Evaporation
In a cold environment the body may lose heat via these four
processes,
Radiated heat- the body of a animals will lose
heat via electromagnetic waves, when these waves fall on to a cooler object or
surface it will absorb the waves as heat, in a temperate environments of 210C
a human being can lose 50-60% of heat via radiation, the air around the animals
is a very poor conductor of heat thus the heat will radiate to solid objects in
the environment, how much of the heat is radiated depends on the temperature
difference between the body and the solid environments and the amount of
exposed the animal has as fur will radiate less heat than bear skin, there is
only about 85% of the animals body involved in this type of heat exchange as
the inner surfaces of limbs will radiate heat between each other and not to the
environment.
Radiation is an important factor to consider when managing
a herd of sheep, the normal body temp of a sheep is around 102.3oFbut
can hover at around100.9-103.8 °F, this is classed as the thermal neutral zone
of the animal, and for the health of the animal this must be maintained.If the temperature is to high the sheep will
begin to pant, this will then increase the heat production as the muscles will
be contracting, if the rise in body temperature continues, enzymes in the
animals body will cease to function and death will quickly ensue. Conversely if
the temperature falls below the thermal neutral zone again the animals
metabolism will increase in an attempt to produce heat, now all the animals
nutritional stores will be going in to heat production rather than weight gain
or anything else, if the body temperature continues to fall then again the
enzymes of the body will cease and death will ensue, a physiological response
to the heat for the sheep is the network of small blood vessels called the
carotid rete and this is located in the hypothalamus, its function is to cool
the blood before it reaches the brain of the sheep, this carotid artery carries
hot blood from the heart to the and branches many times throughout the rete.
Cooled blood leaving the brain and the sinus cavity also passes through the
rete, heat is transferred from the hot blood to the cool blood ensuring the
correct blood temperature before entering the brain.(Jones, 2007)
Conduction
of heat- this is the transfer of heat from the hotter of any objects to the
cooler objects which are in contact with each other, the rate of heat loss
depends on the temperature difference between the environment, and on the area
of contact, this is not a major sauce of heat loss because the low thermal
conductivity of air and the limited area of contact between the body and the
solid environment .however this changes if the animals becomes emerged in water
as more of the body will be in contact with the water thus much more heat will
be lost and at a much faster rate.
This as mentioned above is not a major sauce of heat loss however
it is a major sauce of heat gain for the reptile species, their metabolism depends
directly on the environment and in the morning, after the environmental
temperature drop of the night, they have a very low body temperature and
metabolic rate thus movement is sluggish, to counteract this and warm up quickly
the animals will orientate its body in the direction of the sun to absorb the radiated heat the sun gives off,
additionally the animal will find a rock of sold surface to sit in the sun, and
as the sun has already warmed up the rocks the underneath of the animals will
conduct the heat from the rock in to its body as an additional sauce of heat.
Convection -is the movement of air resulting from local pockets of warm air being
replaced by cooler air and vice versa, this method is important as the low
specific heat of air allows air in contact with the skin to be warmed rapidly,
the warm air is much less dense than cold air so as it rises it will be
replaced by cool air, so setting up convection currents and maintain the
temperature gradient between the skin and the adjacent air determines the rate
of convection and on the free movement of air at the skin surface fur and
feathers will reduce heat loss via convection.
Animals use this form of heat loss all the time in environment
were the temperatures are very high, so in comparison to the example above
these animals want to lose heat through convection, so they will seek out a watering
hole and lay in the mud and the water until heat from their body has transferred
to the water around them, additionally they will lose heat via evaporation as
they leave the water again cooling their body temperature, in the case of
arctic marine mammals they need to prevent the water from cooling their temperature
so they reduce their connectivity via many layers of blubber, a very thick demise,
effectively creating a barrier between the cold water and the warm internal
fluid blood, thus reducing the conductive heat loss(Physiology,
2012)
Evaporation- this is liquid becoming vapour and
is always accompanied by cooling, the evaporation from the body’s surface
depends on temperature, humidity and air currents and can account for up to 25%
heat loss at 210C in the human some of this heat loss can be
regulated via sweating and panting. Sweating will not cool unless it is
evaporated, thus environmental factors like humidity and the speed in which the
air is moving are important when determining the efficiency of sweating,
furthermore bids have no sweat glands and many mammals only have them on the
pads of their feet thus heat loss comes about via the rapid movement of air
over their mouth and upper respiratory tract when they are shallow breathing or
panting.
The only example for heat loss is if an animals is wet and
it is the sun the water will evaporate from its body cooling it down this is
used by many animals including humans on a hot they will swim in the sea and
then the water left on them will evaporate reducing their temperature further.
When there is a rise in the environmental temperature heat
transfer will be proportional to temperature gradient in between the skin and
the environment in conduction, convection and radiation. As the temperature
increases the heat loss via these methods will decrease, until there is no heat
to be lost, at this point heat could be gained in the even the environmental
temperature is higher than that of the body then heat can be gained via
radiation, conduction and convection.
heat exchangers
image 5 counter current heat exchange
There are many examples of heat exchangers, from the
penguins and the dolphins who live in some of the coldest environment in the
world to the humble mallard on the local pond, in each of the examples above
the animals seem well adapted and happy in their surroundings, this is down to an
adaptations the animals have evolved to cope with the temperature fluctuations.
There is counter current heat exchange, lower critical temperatures of animals
living in the colder areas compared to those living in warmer places, animals
who are closely related (polar bear, brown bear) tend to be much larger in the closer
regions than in the warmer the larger the animal the larger the surface area
volume thus cutting down heat loss. Furthermore their extremities are much
smaller again to minimise the heat loss were as animals living in the warmer
areas have larger extremities with a comparatively richer blood supply allowing
them to act as a cooling mechanism.(-, 2011)
The counter current heat exchange system- the ducks legs, dolphins
flippers and penguins feet, are all susceptible to heat loss particularly in
cold weather so these structures, their arteries and veins are organised to
facilitate counter current exchange, meaning they are close to each other. The arteries
that carry the warm blood to the feet run alongside the veins that carry the cold
blood up from the feet. And as a result some of the heat from the warm blood in
the eateries will transfer to the cold blood in the veins, thus warming the
blood before it reaches the heart this is important as it helps to maintain
core body temperature (see image 5) . Additionally as the warm blood moves towards the feet
it becomes cooler helping to keep the temperature of the feet above that of freezing,
and as the blood has already been cooled very little heat is lost to the surroundings,
. This minimises the amount of energy needed for keeping the feet warm and
preventing frost bite. (-, 2011)
This mechanism is applied by many cold-adapted birds and
mammals in their extremities. In the cold adapted birds this mechanism will
also act as a way of out letting the heat if they become too warm, the blood
vessels in the feet will dilate (expand), enabling the increase of blood flow,
which in turn facilitates the loss if heat furthermore they may lay on the
ground with their flippers out stretched again facilitating heat loss as their
flippers also have the counter current heat exchange mechanism similar to that
of the legs. (-, 2011)