What makes animals warm blooded?

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What makes animals warm blooded?
« on: 01/06/2007 07:47:49 »
Humans are said to be warm blooded, but what makes our blood warm?
« Last Edit: 02/06/2007 22:51:12 by chris »



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Re: What makes animals warm blooded?
« Reply #1 on: 01/06/2007 12:21:21 »
The term warm blooded is a bit misleading.

All animals are to some extent warm blooded, but some animals (such as mammals and birds) will ordinarily (this is violated when an animal hibernates) maintains its internal temperature within a very much narrower range than than others.

An animal is considered warm blooded if it has the capability (and necessity) to internally regulate its temperature within a very narrow range, whereas 'cold blooded' animals can tolerate a wider range of internal body temperature, and make less precise regulation of their internal body temperature.

There is no absolute sharp divide between warm blooded and cold blooded animals (a bee, which is technically cold blooded, cannot fly until it has warmed its body temperature sufficiently, and it does this by flapping its wings until it has warmed its body up sufficiently; and termites, although they are again cold blooded, will actually act to maintain a fairly constant temperature within their nests).

So, all animals will seek to regulate internal temperature to some degree, but warm blooded animals are far more intolerant of even slight changes in internal body temperature, so have far more precise mechanisms to maintain their internal temperature (e.g. more internal muscle heat, subcutaneous fat to contain heat, seat to cool down when overheated, dilation of blood vessels to manage the amount of heat loss).


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What makes animals warm blooded?
« Reply #2 on: 02/06/2007 23:31:06 »
Warm blooded animals are also called homeotherms, which means "keeping the same temperature". The source of body heat are the chemical reactions occurring in every one of our 10,000 trillion cells. Cells collect sugars (glucose), fats and proteins from the bloodstream and shovel them into a metabolic boiler which yields the chemical currency ATP (adenosine triphosphate).

This molecule has three high-energy phosphate bonds, and energy is released every time one of them is severed. Cells use this process to drive other chemical reactions, including anabolic (molecule-building) and catabolic (breaking down) processes; in both cases these chemical pathways yield heat. This leaves the body by radiation, conduction and convection and also by voiding warm urine and faeces.

A warm blooded animal maintains a regular temperature by both altering the rate at which heat is both produced and lost from the body. These processes are coordinated by the brain's hypothalamus which continuously monitors body temperature and effects changes necessary to keep it at a steady level; in us that's 37C.

An animal that's too cold activates shivering, in which antagonistic muscle groups are simultaenously recruited, meaning that muscles that move the body in opposing directions are switched on together. As muscles are only about 20% efficient (with 20% of the energy they burn turning into useful work and 80% turning into heat), this is an excellent way to warm up. Behavioural changes also kick in; we put on warmer clothes, seek shelter, drink a warm drink or increase voluntary activity.

At the same time blood flow to the skin is dramatically reduced by vasoconstriction (closing arteries). This shunts blood away from the body surface and keeps it beneath the insulating layer of fat that sits below the skin. This slows down heat loss.

The sympathetic nervous system also activates pilo-erector muscles. These cause hair to stand on end. In furrier species than us this has the effect of trapping a layer of air within the hair, slsowing down air movement next to the skin and hence reducing wind-chill. In humans it just provokes "goose pimples".

The body also produces adrenaline and adrenaline-like hormones, which boost metabolic rate and increase heat production. One site at which this happens is "brown fat", which is a form of adipose specialised for heat production. It's mainly found between the shoulder blades and also around the aorta, the body's largest artery.

Conversely, if we are too hot (following severe exercise for instance), these processes are reversed. The body opens up blood flow to the skin to a maximum. Sweat glands are activated to boost cooling; as the water evaporates from the skin surface it removes additional heat (latent heat of evaporation). There are also behavioural changes (clothing is removed, we seek shade and a cold beer) that kick in to help the process.

The benefit to us of this warm state is that we can maintain activity over a much broader range of temperatures and thus tolerate a greater range of environments and climatic conditions.

Non-homeotherms (poikilotherms) on the other hand, like lizards, snakes and crocodiles, have a broad range of body temperatures and are much more slaves to the environment. They need to warm up to increase the rate of their metabolic reactions in order to boost their activity. This can restrict them to being active only in certain seasons, certain environments or certain climates.

But there is another way of maintaining your temperature, which is referred to as gigantothermy. Put simply, once you get to a certain size (like a massive salt-water crocodile, or a dinosaur), you're producing so much heat that staying warm ceases to be a problem. Instead, staying cool enough is the priority. That's why some scientists think that certain dinosaurs were feathered when juvenile (to keep them warm), but once they grew large enough and gigantothermy kicked in, they ditched the feathers.

This is why the really really big dinosaurs, like the brachiosaurs (e.g. what people refer to as diplodocus) developed such enormous necks and tails, as a heat loss mechanism.

« Last Edit: 02/06/2007 23:35:16 by chris »
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