Obviously not temperatures above the boiling point, but could you theoretically survive in an environment hotter than a normal human could thermoregulate in if you attached a blood-cooler and if so how much hotter?
The way you see blood cooling the body is backwards of what it actually does
Blood doesn't pull external heat away from the skin, it pushes internal heat to the skin where it can be removed from the body through sweat evaporation. Sweat evaporation protects the body from both internal and external heat.
Your blood cooling system would benefit people by allowing them to work harder and longer in a controlled environment because the heat generated by the body will be removed much more efficiently than it is through sweat. Athletes around the world will praise you!
But it won't improve the body's ability to withstand external heat. Not only does the blood not pull heat from the skin,1 but the skin itself is a considerable insulator against heat.
Now, you might think that the skin's insulative abilities would benefit the body. They do! They keep the outside... outside. It's why you can walk around on a bright summer day and not boil in your own juices. But those insulative abilities have a consequence.
If you use your blood cooler to keep the interior of the body at normal body temperature (you can't lower that without serious consequences... like death...) then your body won't release sweat as early as it normally would, causing skin to burn faster, and you're only way of knowing that your skin is burning is through the pain caused by damaged nerves. Yup, you won't even feel warm. You'll just suddenly start howling because you have 1st-to-3rd degree sunburns and never knew they were happening. Everything felt normal until that moment.
And just to underscore those concepts: your body won't release sweat as soon as it normally would... so you will burn faster.
While I generally like the idea, and can see some clever uses of it for stories, such a system would not significantly (if at all) improve a person's ability to withstand hotter environments. On the contrary, the human would suffer more for not knowing how those environments were affecting them.
1 Think about it, where would the heat go? Since the body evolved without anywhere for heat drawn from the skin to go, the system can't be significantly benefited by giving the heat somewhere to go without changing the entire system. That means changing the distribution of arteries, veins, and capillaries in the body to better capture the heat from the skin, which means pushing them closer to the surface of the skin, which has its own ugly set of consequences. The body is absolutely amazing. Which is why artificially changing it rarely improves it.
I think you have just tried to reinvent sweating.
Sweating achieves exactly what you intend to do:
- sweat secreted by the sweat glands reaches the surface of the skin
- it evaporates, subtracting heat to the surface
- this cools down the blood flowing in the capillars irrorating the body surface
- as a result the entire body is cooled down
It is a very clever way of managing excess heat, because the skin is one of the largest surfaces available to the body.
When you can't sweat, or if the core body temperature is high, having some extra cooling could be useful. A few years ago Stanford University investigated cooling gloves with the idea of being able to lower the core body temp. They also found it useful to heal warm people up who had been cooled down for certain surgeries. I think it has been commercialized to help athletes in post recovery after exercise.
They apparently were inspired by bears coming out of hibernation and wondering how they regulated heat when they had a thick layer of insulating fat.
Heller and Grahn discovered that bears and, in fact, nearly all mammals have built-in radiators: hairless areas of the body that feature extensive networks of veins very close to the surface of the skin.
Rabbits have them in their ears, rats have them in their tails, dogs have them in their tongues. Heat transfer with the environment overwhelmingly occurs on these relatively small patches of skin. When you look at a thermal scan of a bear, the animal is mostly indistinguishable from the background. But the pads of the bear’s feet and the tip of the nose look like they’re on fire.
These networks of veins, known as AVAs (arteriovenous anastomoses) seem exclusively devoted to rapid temperature management. They don’t supply nutrition to the skin, and they have highly variable blood flow, ranging from negligible in cold weather to as much as 60 percent of total cardiac output during hot weather or exercise.
Their device basically a glove that cools the blood going through the hand had two features:
- They understood where the body lost heat by having larger blood vessels near the surface of the skin. In this case they chose the palm of the hand.
- They understood vasoconstriction as a function of temperature , if they cooled the hand too much, then the blood vessels close up too much and you just get a cold hand.
So depending on how far you pushed the concept in effect your blood cooler could be some kind of device that could help radiate heat away from the body. Although you would have concerns like JBH mentioned, and it seems like cooling suits are a little simpler. But it is your world to build and if you can solve things like blood clotting issues etc. why not?