Sweating the issue...

[Boerseun]

I saw a dog panting yesterday, and wondered how much heat is actually regulated by its tongue. We've been told for ever that dogs pant in order to increase the airflow over their tongues, whilst at the same time slobbering all over the place to give the tongue some saliva to evaporate in the increased airflow. I think this explanation is a bit lacking.

 

My theory goes as follows:

 

A dog gets hot. It must shake heat somehow. So it increases its breathing not primarily for evaporation over the tongue, but to increase air to its lungs. The lungs have a much greater surface area than the tongue, and are probably the richest concentration point of fast-moving warm blood in the body. The internal folds and crevasses of the air pipes and sacs in the lungs also function very much like a car's radiator - it's got a heck of a big surface area for its size, not only to ease gas diffusion into and out of the blood stream, but also, quite handily, as a very efficient radiator of body heat.

 

So why does the dog drool whilst panting? Quite easy. The tongue is a very sensitive organ, and the increased airflow to the lungs going over the tongue will make it crack and dry out. So, it gets moisturised continuously, much to the dog-owner's detriment. Sure, some heat is lost through the tongue, but I think the bulk is radiated through the lungs.

 

And I think the same applies for us, as well. Sure - we sweat, and our skins do cover a big area. Primarily, sweating's the way to go. But we must also lose quite a bit of heat through our lungs. If you breathe onto a piece of glass, you'll notice two things:

 

1) The piece of glass heats up (your breath is warm, taking heat from right inside your body core).

2) The glass fogs up. This means that some evaporation of some sort is happening right inside your lungs. This moisture might be primarily to protect your delicate longue tissue from drying up - same as the dog's tongue; but evaporation also cools down.

 

There should be a way to test this. Would a hot person's breath be more moist than a cold person's, or would the moisture content in your breath be primarily determined by the dryness of the air, i.e. how much protection your longue tissue needs?

 

But still - we must be losing a certain percentage of body heat simply through breathing. Anybody has any idea how much this percentage would stack up to how much we lose through sweating?

 

Thoughts?

[InfiniteNow]

While an interesting thought, I’d suggest that we lose more heat through our skull than through our lungs and out through the chest. However, sweat and panting appear to participate as primary mechanisms.

 

Your question prompted me to do some searching and I found a rather enlightening article on wiki:

 

Thermoregulation - Wikipedia, the free encyclopedia

 

 

However, after becoming more comfortable with the basics of the topic from the wiki article above, I found this gem of a .pdf on the web:

 

Mechanisms for the control of respiratory evaporative heat loss in panting animals

 

Since respiratory gas exchange requires the humidification of inspired air, an increase in respiratory ventilation will also elevate respiratory evaporation as long as respiratory dehumidification and cooling does not occur. Thus, evaporative heat loss by panting would be a relatively simple function to accommodate. In terms of evaporative heat loss of terrestrial endotherms it is possible that, from an evolutionary standpoint, either panting or saliva spreading may have been the first heat loss mechanisms to emerge that utilized the latent heat of vaporization of water to increase heat loss. However, alveolar exchange of oxygen and carbon dioxide would also need to be controlled in order to preserve the requirements for gas exchange and pH homeostasis. Ideally the increase in ventilation should be confined therefore to the dead space where humidification takes place and should not compromise alveolar gas exchange, a requirement which can be achieved by an increase in respiratory frequency with a proportionate decrease in tidal volume which, in fact, becomes a definition of panting.

 

The most notable non-panting species in mammalian terrestrial species (marine mammals for obvious reasons relating to the physics of heat transfer in water need no such mechanism) are the elephant and man. The elephant has no identified evaporative heat loss mechanisms that can be activated as part of thermal homeostasis; the elephant simply stores heat during the day and dissipates it at night. Primates, other than man, demonstrate panting to a limited extent. In man no respiratory response to heat exposure falls within the definition of panting.

 

When a comparison is made of the relative efficacy of the two main modes of evaporative heat loss, i.e. panting and sweating, it is often concluded that, although the movement of air across the moist surfaces of the turbinate bones in the nasal cavity assists in the evaporation of water in a way not generally available to the skin surface of sweating species, the heat loss of panting is limited by the increase in heat production of the muscles of respiration. However, the energy cost of panting, when measured as the change in total oxygen consumption between the thermoneutral and thermolytic zones is found to be zero; truly an efficiency of 100%!

 

The solution to this paradox was revealed by Hales who compared the distribution of cardiac output before and during heat exposure and demonstrated that an increase in blood flow to the respiratory muscles during panting was compensated by a reduction in flow to some of the non-respiratory muscles leading to the conclusion that, if blood flow and oxygen consumption are matched, the metabolism of respiratory muscle may indeed be elevated during panting but that of other muscles would be equally depressed. An additional feature which contributes to the high energetic efficiency of panting is that the maximum panting frequency occurs at the resonant frequency of the respiratory system.

 

Since the maximal panting frequency will therefore be inversely related to body size this may explain the observation that in a range of bovid animals of different adult body size that use both panting and sweating, the larger species utilize sweating more than panting as a strategy for increasing evaporative heat loss. One may speculate, therefore, that if panting represents a primitive form of evaporative heat loss of the early mammals, which were small, the subsequent evolution of larger species necessitated the development of a supplementary form of evaporative heat loss, namely sweating. Evaporative heat loss of the kangaroo is unique in that all three strategies for increasing evaporation are used; saliva spreading and panting at rest and sweating during exercise.

 

They have a pretty nice set of references listed as well if you care to drill deeper still. :shrug: :naughty:

[Boerseun]

Thanks! Good info, there.