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Q: How do dogs cool themselves? ( Answered 5 out of 5 stars,   2 Comments )
Question  
Subject: How do dogs cool themselves?
Category: Science > Biology
Asked by: archskeptic-ga
List Price: $20.00
Posted: 10 Nov 2005 19:22 PST
Expires: 10 Dec 2005 19:22 PST
Question ID: 591736
I spent some time down South recently.  The weather was oppressively
hot and humid.  I was sweating profusely the whole time I was
outdoors.  The warm temperatures didn't seem to bother the dogs,
though.  It occurred to me that I don't understand how dogs get rid of
excess body heat.

Oh, yeah, I know they do it through panting, but this still doesn't
make sense.  Humans can use the whole surface of their bodies as a
cooling system, and sometimes even _that_ isn't enough (as in heat
stroke).  A dog just has the area of his mouth and throat. And, on top
of that, the dog is covered with fur that traps a layer of warm air
next to the skin.

What gives?  Dogs seem to break some kind of law of thermodynamics.

I'm hoping that someone can explain the physiology of temperature
control in dogs in a way that I understand the resolution to the
paradox I've outlined above.  Don't worry about being too technical --
I have a background in science.

Incidentally, I realize that my question can apply to many other
animals than just dogs, so it's fine if you answer in general terms.

Thanks.
Answer  
Subject: Re: How do dogs cool themselves?
Answered By: crabcakes-ga on 10 Nov 2005 21:07 PST
Rated:5 out of 5 stars
 
Hello Archskeptic,

    A dog?s tongue is not the only body part that helps dissipate
heat: you may be surprised to lean the spleen is plays a role!

   ?Environmental scientists call dogs and humans homeotherms because
they can maintain constant body temperatures, even in the face of
extreme outside temperatures.  Snakes and lizards' body temperature
changes with the outside temperature - they are called poikilotherms. 
In people, normal body temperature is approximately 98.6 F.  While
this temperature varies during the day; it stays relatively constant,
regardless of whether it is 50 or 90 outside.
The body's regulates temperature in a part of the brain called the
hypothalamus.  This brain center has several sensors that are critical
for temperature regulation.  In humans, the most important center for
controlling heat is the sweating center.  This center, which
stimulates the sweat glands to release sweat, is less important in
dogs.  Each gram of sweat that evaporates gets rid of half a calorie
of heat.

In dogs, the panting center, which stimulates panting, helps the
animal get rid of heat.  Panting involves rapid shallow breathing that
causes evaporation of fluid on the tongue.  Dogs have a rich blood
supply in their tongues.  Heat produced in the dog's muscles and
tissues enter the blood and is transported to the tongue, where it is
eliminated through panting.

The blood stream is critically important for regulating temperature in
both dogs and humans.  Water is the most plentiful substance in our
bodies.  Water is important for temperature control because it can
hold a lot of heat; it can also carry heat to the surface via the
blood stream.

The spleen is a blood storage organ that helps control heat,
particularly during physical activity.  When you or your dog exercise,
the spleen contracts, releasing blood into the blood stream.  This
blood is then available to help carry excess heat to the surface. 
Relative to body size, dogs have larger spleens than humans do.  The
relatively greater release of blood from the spleen during exercise is
one reason why dogs usually have better endurance than people.?
http://k9power.com/don'tget.htm


   Even though dogs have sweat glands in their feet and ears, they are
not very efficient, and play a small role in thermoregulation. A dog?s
fur protects not only against cold, but heat too!
?A dog's skin is made up of the epidermis and the dermis.  The
epidermis is an outer layer of cells that constantly gets replaced. 
The dermis is found under the epidermis and supplies it with nerves
and blood.  Sweat glands are found in the dermis along with sebaceous
glands which feed oil into hair follicles in the epidermis to
lubricate the skin.  Hair grows up through follicles from papillae in
the lower epidermis.  More than one hair grows through the same
follicle in dogs and one hair from each follicle, the guard hair, is
usually longer than the others.  Dogs can achieve piloerection,
raising of the guard hairs, by contracting or lengthening muscle
fibers in the dermis (Whitehead et al. 1999).  A dog's fur insulates
it against cold and heat (Johnson 1977) with the degree of insulation
increasing with fur thickness (Schmidt-Nielson 1997).  Dogs normally
shed twice a year, in the spring and in the autumn.  Shedding in the
spring gets rid of hair that is not needed for the summer and shedding
in the autumn is in preparation for growing a thicker coat for the
winter.?
http://www.bio.davidson.edu/Courses/anphys/2000/Hatfield/Hatfield3.htm



This site begins by discussing the opposite, heat maintenance, but
moves onto heat dissipation in dogs:

?Anyone who has dealt with dogs knows that the dog pants when he is
hot. It is his panting mechanism that allows the dog to lose heat he
does not need when the weather is hot. The mechanism functions on the
principle of evaporation. As I have said before, evaporation is
extremely effective because you can lose almost five-hundred and forty
times as much heat without changing the temperature of the air at all.
This evaporation takes place in the dog's nose and is enhanced by the
fact that there are many folds of tissue in the nose which increase
the surface area that the air is in contact with. In addition, this
tissue actually perspires and has a large blood supply, thereby acting
much like the radiator in the car. This makes a good mechanism of heat
loss, but one needs some way in which to shut it off when one does not
want to lose heat. Since the animal must breathe and must take in a
pretty constant volume of air at all times, because varying this
volume of air would interfere with other bodily functions, people have
often been concerned as to how this is controlled.

 A recent study, just published, has looked into this problem and has
come to this conclusion. The air is brought in through the dog's nose
and in the case of panting is exhaled through the mouth. In doing
this, the air picks up much moisture and heat from the nose, this
cooling the nose and drying it. When it is exhaled, it is exhaled
through the mouth and since there is much less blood supply to the
area around the mouth and the surface area is much less, almost all of
the moisture is absorbed and the heat will leave the dog's mouth and
be lost. When the dog does not wish to lose body heat, he simply
exhales the air that he has brought in through the nose back out the
nose, in which case a great proportion of the moisture and heat will
be returned to the large surface area and the membranes in the nose
thus minimizing the loss of heat.

 Therefore, when an animal is hot, the hotter the animal is the more
of the air he brings in through his nose will be exhaled through the
mouth, thus the wide open, tongue out position. If he does not wish to
lose heat when it is cold out, he will breathe in and out through his
nose and keep his mouth closed. The provides a quite sensitive means
of temperature control, particularly in the case of the animal who has
other means of protecting himself from the cold and from the outside
temperature as do the long-haired Northern breeds. The animal,
therefore, in general has developed enough capacity of heat loss
through his panting mechanism to compensate for the very wide swings
in temperature.?
http://www.samoyed.org/heat.html


?Major changes in body fluids and tissues associated with over heating is
known as Hyperthermin. This condition may be influenced by the dog?s muscular
exertion, physical condition, hair coat, and surrounding factors like
relative humidity and ventilation. The severity and duration of
hyperthermia is influenced by 3 separate but often interrelated
syndromes.
http://www.webheads.co.uk/sleddog/ezine/pdf/Heat_Stress.pdf



   ?A special circulatory adaptation called countercurrent heat
exchange enables both species to maintain the appropriate body
temperature in their extremities. Countercurrent heat exchange is only
one of many clever adaptations mammals have to help them to deal with
variable temperatures.

All mammals are endothermic: they maintain and regulate their own body
temperature. Living in widespread environments around the world,
mammals face daily and seasonal fluctuations in temperatures and
some--for example those living in harsh arctic or tropical
habitats--face extreme cold or heat. To maintain their correct body
temperature, mammals must be able to produce and conserve body heat in
colder temperatures as well as dissipate excess body heat in warmer
temperatures.? Please see this site (and all the others posted) for
more information.
http://animals.about.com/cs/mammals/a/aa061601a.htm

Do humans count as another species?
??the temperature of warm-blooded animals is maintained with but
slight variation. In health under normal conditions the temperature of
man varies between 36 C and 38 C, or if the thermometer be placed in
the axilla, between 36.25 C and 37.5 C In the mouth the reading
would be from 0.25 C to 1.5 C higher than this; and in the rectum
some 0.9 C higher still. The temperature of infants and young
children has a much greater range than this, and is susceptible of
wide divergencies from comparatively slight causes.

Of the lower warm-blooded animals, there are some that appear to be
cold-blooded at birth. Kittens, rabbits and puppies, if removed from
their surroundings shortly after birth, lose their body heat until
their temperature has fallen to within a few degrees of that of the
surrounding air. But such animals are at birth blind, helpless and in
some cases naked. Animals who are born when in a condition of greater
development can maintain their temperature fairly constant. In strong,
healthy infants a day or two old the temperature rises slightly, but
in that of weakly, ill-developed children it either remains stationary
or falls. The cause of the variable temperature in infants and young
immature animals is the imperfect development of the nervous
regulating mechanism.?
http://en.wikipedia.org/wiki/Temperature_regulation#Variations_in_the_temperature_of_man_and_some_other_animals

http://en.wikipedia.org/wiki/Temperature_regulation#Variations_in_the_temperature_of_man_and_some_other_animals



Dogs CAN suffer from hyperthermia:

?Heatstroke occurs when normal body mechanisms cannot keep the body's
temperature in a safe range. Animals do not have efficient cooling
systems (like humans who sweat) and get overheated easily. A dog with
moderate heatstroke (body temperature from 104 to 106F) can recover
within an hour if given prompt first aid and veterinary care (normal
body temperature is 100-102.5F). Severe heatstroke (body temperature
over 106F) can be deadly and immediate veterinary assistance is
needed.?
http://www.peteducation.com/article.cfm?articleid=1683

?Some causes of hyperthermia include exercise, being left in a parked
car, a thick coat, short faces (such as with bulldogs and boxers),
inadequate outdoor shelter, prolonged seizures, lung and heart
diseases and a history of having hyperthermal episodes.    Dogs have
few ways to eliminate heat. When they pant, the tongue surface
enlarges in order to dissipate as much heat as possible. Dogs sweat on
the pads of their feet.
http://www.ehow.com/how_3025_detect-treat-hyperthermia.html

?Before we leave this general consideration of the subject, it may be
of interest to note that animals have numerous ways of keeping
themselves cool. Some small rodents are supplied with ample spittle
and wash themselves thoroughly if they are feeling too warm, the
evaporation of the spittle providing them with sufficient cooling.
Some other small animals are supplied with a specially structured tail
which acts as a heat exchanger when positioned in a certain way and
allows them to reduce body heat. The sweat glands of dogs and similar
creatures are in the mouth, and the rapid exchange of air in panting
provides them with sufficient evaporative cooling. Prehistoric
animals, like the dimetrodon, were equipped with a sail-like structure
on the back which served probably as a heat exchanger, acting in two
ways: to remove body heat in hot weather and to gain solar heat in the
cold. Some animals make the adjustment by reducing basal metabolism,
while others immerse themselves in water or burrow into the cool
ground. Birds lift their wings slightly.

Whatever the means, the fact remains that there are limitations which
generally restrict the species to a certain temperature zone. Man is
virtually free of such restrictions, partly by reason of his ability
to produce an artificial environment by clothing and shelter, but also
because of his ability to resist a temperature rise by copious
sweating and the consequent evaporative cooling. Not infrequently this
copiousness may seem to be wasteful. But a considerable body of
evidence exists to show that the few elements which do exist in the
sweat are of importance to man, since his body is thereby washed with
a slightly acid solution which serves to protect him against bacteria
and fungi. Unevaporated sweat, which seems to be an overcompensation
and a pure waste of body fluid, turns out to have a value of its own.
In the one area of the body where the acid might be dangerous to
himself, namely, in the forehead region, the sweat is prevented from
running down into his eyes by the eyebrows.?
http://custance.org/old/flood/5ch1.html



You may find these sites interesting:

This one is a great explanation of thermoregulation:
?Thermoregulation, or the maintenance of a fairly steady body
temperature even under a variety of external conditions, is important
to all animals because each species has a preferred body temperature
at which functioning is optimal. These external conditions can include
changes in temperature, vapor pressure, air velocity, and insulation
among other factors that affect the temperature of the skin.
Cold-blooded animals regulate their body temperature by selecting an
appropriate external environment. Warm-blooded animals also rely on
physiological mechanisms which can produce or dissipate heat?
http://www.deas.harvard.edu/courses/es96/spring1997/web_page/health/thermreg.htm

http://www.the-aps.org/publications/classics/mack.pdf

http://ajpregu.physiology.org/cgi/content/full/287/5/R1012

I hope this has answered your question . If not, please request an
Answer Clarification, before you rate. This will allow me to assist
you further, if possible.


Sincerely, Crabcakes 



Search Terms
============
Thermoregulation + dogs
Heat dissipation + dogs
Heat regulation + canines
Heat physiology dogs
archskeptic-ga rated this answer:5 out of 5 stars and gave an additional tip of: $5.00
Very nice answer.  I actually didn't expect the information to be
available on the Internet.  I made a half-hearted attempt to find the
answer myself before I posted the question, but I came up
empty-handed.  I was using different search terms, however.  Thank you
for your help.

Comments  
Subject: Re: How do dogs cool themselves?
From: brix24-ga on 11 Nov 2005 04:56 PST
 
Since you are have a background in science, you might be interested in
two side issues (no hyperventilation problem and excess heat generated
from muscular work in  panting). There is a short explanation at this
site:

http://www.newton.dep.anl.gov/askasci/vet00/vet00018.htm

The explanation about avoiding alkalosis and reducing muscular work
(with its heat generation) is succinct and I hate to copy that short
site. Here, though, is a short fact from that site that you might find
interesting:

"When a dog begins to pant, its
respiration tends to shift rather suddenly from a frequency of 30 to 40
respiration's per minute to a relatively constant high level of about 300 to
400. A dog subjected to a moderate heat load does not pant at intermediate
frequencies; instead, it pants for brief periods at the high frequency,
alternating with periods of normal slow respiration."
Subject: Re: How do dogs cool themselves?
From: crabcakes-ga on 11 Nov 2005 10:18 PST
 
Thank you for the 5 stars and the nice tip! Both are appreciated.

Sincerely, Crabcakes

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