It seem that humans and other warmblooded animals will suffer organ damage and eventually death if their body temperature falls below a certain point, but cold-blooded animals don’t have this problem, they just hibernate. What protects cold-blooded animals from organ damage due to cold temperatures?
no patience to find the link, but there is a frog(?) that can survive sub-zero because of a natural “anti-freeze” in its blood.
I’m sure a Doper will soon link to this species.
i think a better question is why do humans need to stay above 35 degrees and below 39 to stay alive in the first place?
Heh, that’s the temp recommended to put my tortoise into hibernation
The problem is that human enzyme systems, and by extension organ systems, are optimised to run at 37oC. The whole advantage of being warm blooded si that it allows such optimisation, so that although we use more energy than cold blooded critters we are just much better at doing everything. In contrast cold blooded animals use less energy, but their enzyme systems need to be able to operate over a wide range of temps. That means they are much less efficient at any given temperature than our enzymes are at 37oC.
The difference between the system of a warm blooded animals and the system of a cold-blooded is like the difference between a performance sports car and an old stationary diesel. The sports car performs much better on high-octane premium gas but won’t run at all on anything else. The stationary diesel runs like crap on high octane premium, but it will also run like crap on diesel or kerosene or even sunflower oil. It may run like crap, but it runs no matter what the conditions.
Bare with me, I am answering the question. 
The downside of such specialised enzyme systems is that they really do need to be kept operating within their range. Once they get outside that range they start to shut down. The problem is that they don’t shut down uniformly. They have no mechanism for doing so because they have never needed to evolve such a mechanism. What happens instead is that different systems and even different enzymes within the same system shut down at wildly different temperatures.
This is literally like throwing a spanner into the works. The machine tries to keep going but one of the parts just isn’t working. One system is still producing a product that is toxic but the system that is supposed to detoxify it stopped working 3 degrees ago. The skeletal muscles are still burning energy non-stop, but the smooth muscle has shut down, meaning they don’t have enough blood flow to supply them with food or to remove waste. They start to starve and suffocate simultaneously. The blood stream tries to remove excess wastes as best it can, but the blood pressure is too low for the kidneys to work, so the waste just gets moves form the muscles to the brain. The list of problems just goes on and on.
That’s the basic reason why warm blooded animals have trouble with low temperatures. In contrast cold blooded animals have systems that have evolved to operate over a wide range. As their temperature falls the systems simply shift down a gear to cope. The muscles use less energy and produce less waste at the same rate or a slightly higher rate than the blood pressure falls. Enzyme systems shut down in progression, so that wastes don’t build up, or else the animals can happily cope with the waste etc. It’s not that something protects the organs so much as there is nothing damaging the organs
It is worth noting that many small mammals and birds can tolerate lowered temperatures and go into a state of torpor annually or even daily where there temperature falls down to 15oC or so. They can do this because they have the same ability to sequentially shut down and the same wide spectrum responses as cold blooded animals. But there is still a limit to how cold they can get. Large mammals have lost this capacity because they could simply never store enough energy to be able to recover from such an event.
As Duffer points out, there are animals, including frogs, that can literally freeze solid. They achieve this by a range of tricks such as antifreeze, compounds to inhibit crystal growth and even deliberately dehydrating their own cells. All of that protects them form damage. Warm blooded animals could never manage such a feat because of the energy issue. They could never store enough energy to enable them to come out of such a state and regain their operating temperature before the partially operating systems managed to kill them as they fired up out of sequence.