Ever wonder why birds can land on freezing metal in winter and their feet don't get stuck to it?
Our fingers will get stuck on cold metal ice cube trays when pulling them out of the freezer. Our tongues freeze to cold metal ala A Christmas Story. That's because the moisture on our skin freezes in contact with the icy metal.
Birds' feet are covered with dry scales so there is no moisture to freeze to frigid metal. Birds have no sweat glands and essentially no secretory glands (not zero) so the skin does not secrete moisture through the skin on their feet.
The photo below of an Osprey's foot shows these scales in the extreme.
Frigid temps can be hard on wildlife. How do they keep those bare feet from freezing? Countercurrent heat exchange.
Basically, the arteries carrying warm blood down to the feet are very close to, if not intertwined with, the veins carrying cooled blood back up to the body and the heart. So, the warm blood in the artery essentially rewarms the blood coming back up the leg's vein so it does not cool the body's core temperature.
And birds aren't the only ones that use countercurrent heat exchange in their extremities to conserve body temperature. Other animals like arctic foxes and wolves use it, too. Deer species, as well. Also, beaver, muskrat, otters, and sea mammals.
I want that. Maybe my toes and fingers wouldn't be so cold so often.
19
ironhydroxide @sh.itjust.works - 21hr
The opposite would happen.
Fingers and toes would be constantly cold (or hot).
14
Eiri @lemmy.ca - 21hr
Wait wouldn't more heat exchanged mean that the fingers would be closer to the average temperature of the rest of the body? That sounds comfortable to me, unless I'm missing something
3
ironhydroxide @sh.itjust.works - 20hr
It means that the body keeps the heat it has. And the fingers and toes are closer to ambient temperature.
The researchers made detailed molds of the veins and arteries in canine footpads. They also examined histological specimens in which these vessels were stained with India ink. What they found were complex patterns of blood flow that were adaptive to cold weather in two ways. The first adaptation noted was that each footpad artery was sandwiched between two veins. In this way, heat in the feet was conserved. Got it? The warm blood coming into the feet via arteries was insulated by a surrounding network of veins flowing in the opposite direction. The heat from the blood going in one direction warms the blood going in the other direction, preventing heat loss by recirculating it back to the body core (1). It’s called countercurrent heat exchange.
A second vascular mechanism, called arteriovenous anastomoses, was also identified. This allows blood flow to shift (2) when a dog is in cold environments, draining blood to the skin surface, but retaining warm blood in the pad surface. Other studies have identified similar systems in the feet of Arctic foxes and gray wolves. These animals appear to be able to regulate blood flow and heat exchange in their feet, keeping their paws at just one degree above the temperature at which tissue damage would occur. (3)
(1) Thermodynamics states there isn't any additional heat being produced by the body. They can't increase the temp of their extremities, rather they have a means of pulling that heat back into the body instead of it radiating off into the environment as readily.
(2) Looking at pros/cons in mechanical countercurrent heat exchangers, one thing that comes up is pressure drop is inherent to the design. This sounds like what is going on here. This may be a negative for us, as we don't have a furry, insulated core or appendages the way most mammals do, so this may be detrimental to other body parts or expose our extensive bare skin to cold damage over a larger area?
I'm not sure the reasons for the pressure drop. Most articles discussed something about how gas molecules move in directions counter to gravity and other fancy thermodynamic terms I have no clue about.
(3) This sounds like a discomfort at the cost of survival thing more than a this keeps me comfy thing, so the other people are probably onto something saying your parts would be overall colder, but they're less likely to fall off or go gangrene on you. A narrow heat gradient seems to be the major plus of this system, not maximizing overall temp, as you're sharing the "surplus" heat on one side to offset the hear deficit on the other side of the system.
The upside of this though, is that it can also run in reverse, like a home heat pump!
Countercurrent heat exchange can also work in the opposite direction, in other words, to eliminate excessive heat. Several types of large antelopes, such as the oryx, gemsbok, and eland, live on the African plains, where environmental temperatures rise so high they pose a risk of causing brain damage. How do these animals survive on the sweltering, sunbeaten plains? Countercurrent heat exchange! Blood going from the heart toward the brain travels via the carotid arteries. In these animals, the carotid arteries break up into many much smaller vessels called the rete mirabile, which then pass through a venous sinus. The blood in the venous sinus has previously circulated along the nasal passages, where evaporative cooling from the moist membranes that line the nose has dropped the temperature of the blood. The cool venous blood then cools the arterial blood headed for the brain, dropping the temperature as much as three degrees, protecting the oryx from neurological damage and heat stroke.
5
Eiri @lemmy.ca - 20hr
What I don't understand is that, say, for a human, if you're warmly dressed everywhere but your feet are bare, like a bird.
The danger isn't really that you'll die because you didn't have enough heat overall. The danger is the your feet will not get enough heat to compensate exposure and will freeze by themselves.
So if birds have a mechanism to survive that, I have trouble understanding how except by sending more heat into the feet overall. Unless their feet can just survive being below freezing unlike ours?
I'm so confused
3
anon6789 - 19hr
Once again, not a doctor/biologist/etc, but I'll take a stab at this. I think I've got the hang of what is going on, but if I'm incorrect, feel free to let me know.
Body heat doesn't just appear. As warm blooded animals, we get most of our body heat as a byproduct of metabolism. Hypothermia occurs when heat is lost faster than a body can produce new heat, as all the body systems require a certain temperature range to happen. Get too cold, and organs and bodily functions get really screwed up to the point the heart and lungs will stop working.
Heat travels from hot to cold to try to achieve equilibrium, so bodies lose heat through radiation if skin isn't covered enough, through contact with colder matter, and convection from wind currents. You are always producing heat (if you are metabolizing!) but you are also always losing heat in a cold environment, or gaining heat if your environment is warmer than you are. Let's focus on the cold side since that's our original topic.
So let's look at our graphic again. You're on the Canada instance and seem to particpate in Canadian/EU topics rather than US, so I'll assume you understand these values as degrees C. (For anyone else like myself, 37C is healthy human body temp, around 98 degrees F. The minimum temp in the graphic, 16C is about 60F. )
In the top circulatory system, the hot blood comes from the core at a happy and healthy standard body temp, but as that warm blood is pushed away from the core, it loses temperature to the environment. The blood eventually returns to the core to get reheated. The problem occurs the longer the exposure to cold lasts, as your body can only generate so much heat. It's like running the heat in your house but leaving the front door and some windows open. You're consuming more and more fuel and getting less and less efficiency. You can keep it up until your fuels runs out or you burn out your heating system. Same in the body.
In the bottom circulatory system, since the blood vessels are so close to each other, sometimes touching, they still lose heat to the environment, but since they are in close proximity to each other, the hotter part of the blood vessel is going to warm up the colder part as that heat radiates away, capturing some of it in the process. This seems to be where you're getting lost if I'm understanding you correctly. So think if you and someone else are stuck outside in the cold together. You are both losing heat via the 3 pathways previously mentioned. Buuuuut, now you decide to cuddle up. What happens?
You both are still losing heat at the same rate as before, because you are both the same body temperature and your environment hasn't gotten any warmer, but now due to your proximity, whatever parts of you are touching, now a good portion of the heat being pulled away from you is being radiated and conducted to the other person and vice versa where it is reabsorbed instead of draining off directly into the ether. Neither of you is generating additional heat, rather now you are both helping each other conserve heat that would have been lost, reducing you actual need to generate that heat. Neither of you are what you would call comfortable, but your ability to tolerate the temperature has increased because your circulatory systems have increased efficiency in retaining what heat you are producing.
Back to the graphic now, heat comes to the extremities at body temp, but due to the proximity of the hot parts to the colder parts, that heat differential is minimized, so the birds in this case are their own snuggle buddies, it's just their blood vessels are what is doing the cuddling. And if they have feathered legs/feets, they are retaining even more of that heat, because that's like if you'd cover you and your snuggle buddy under a big blanket. The body loses less heat, so the burden on the metabolism and circulatory system is lowered, and they can tolerate harsher conditions for a much longer time than we can without this type of circulatory system.
Does that make more sense?
3
Jumuta @sh.itjust.works - 8hr
not oc, but I think the question was why the feet can survive with 22c blood (from the graphic)
1
GenderNeutralBro @lemmy.sdf.org - 14hr
The warm blood going into the feet transfers its heat to the cold blood going back. That means less heat going into the feet where it would otherwise be lost to the environment.
Humans minimize heat loss by simply restricting blood flow to the extremities. It's uncomfortable, but not as uncomfortable (or dangerous) as a cold core.
3
anon6789 - 22hr
I'm curious what the evolutionary tradeoff is for having this.
Most of my life I was jealous we weren't cold blooded as it sounded like a plus to be the ambient temp all the time. I eventually learned it was a great evolutionary leap to become warm blooded so our internal systems will generally always be operating at peak conditions despite our surroundings.
It would really change life if a cold day would start shutting down your organ functions or leave you unable to digest food and you die of septic shock because you had a meal before going outside too long.
3
Eiri @lemmy.ca - 21hr
I'm better than that. My organs can shut down at any temperature!
3
anon6789 - 21hr
Congrats,, you beat the system! 😅
1
frostfox @lemmy.myserv.one - 1day
Cool.
15
amlor - 1day
They must be pros at Oxygen Not Included then.
8
anon6789 - 23hr
I hadn't heard of that game. The art style looks so fun!
At the start of a new game, three colonists (referred to as duplicants) find themselves in an asteroid with isolated pockets of breathable atmosphere, with no memory of how they got there.
Uh oh. Birds do not have a diaphragm to allow them to breath via muscle control and need atmospheric pressure to force air through their respiratory system.
anon6789 in superbowl @lemmy.world
Owl-natomy: Why Bird Feet Don't Freeze in Winter
Two Posts from Badger Run Wildlife Rehab
https://lemmy.world/pictrs/image/ebee8243-b589-41aa-813d-5426c1ea2a11.jpeg
https://lemmy.world/pictrs/image/5b42b922-9d34-43dd-9390-d4b98669135e.jpeg
Neat.
Owls never get cold feet. They're always cold. Got, it.
E: impressive talons
The arctic owlies have some stylish leg warmers.
Snowy
https://lemmy.world/pictrs/image/a091a211-18b8-4381-888d-48ff6f5aba65.jpeg
Great Grey
https://lemmy.world/pictrs/image/a33de5a7-9daa-4c54-a6e9-c8243ce9f4a4.jpeg
Even tiny Boreal Owl!
https://lemmy.world/pictrs/image/582a82df-01a8-4501-b819-e78ab945fa46.jpeg
I want that. Maybe my toes and fingers wouldn't be so cold so often.
The opposite would happen. Fingers and toes would be constantly cold (or hot).
Wait wouldn't more heat exchanged mean that the fingers would be closer to the average temperature of the rest of the body? That sounds comfortable to me, unless I'm missing something
It means that the body keeps the heat it has. And the fingers and toes are closer to ambient temperature.
Did some more digging:
(MVTimes)
(1) Thermodynamics states there isn't any additional heat being produced by the body. They can't increase the temp of their extremities, rather they have a means of pulling that heat back into the body instead of it radiating off into the environment as readily.
(2) Looking at pros/cons in mechanical countercurrent heat exchangers, one thing that comes up is pressure drop is inherent to the design. This sounds like what is going on here. This may be a negative for us, as we don't have a furry, insulated core or appendages the way most mammals do, so this may be detrimental to other body parts or expose our extensive bare skin to cold damage over a larger area?
I'm not sure the reasons for the pressure drop. Most articles discussed something about how gas molecules move in directions counter to gravity and other fancy thermodynamic terms I have no clue about.
(3) This sounds like a discomfort at the cost of survival thing more than a this keeps me comfy thing, so the other people are probably onto something saying your parts would be overall colder, but they're less likely to fall off or go gangrene on you. A narrow heat gradient seems to be the major plus of this system, not maximizing overall temp, as you're sharing the "surplus" heat on one side to offset the hear deficit on the other side of the system.
The upside of this though, is that it can also run in reverse, like a home heat pump!
What I don't understand is that, say, for a human, if you're warmly dressed everywhere but your feet are bare, like a bird.
The danger isn't really that you'll die because you didn't have enough heat overall. The danger is the your feet will not get enough heat to compensate exposure and will freeze by themselves.
So if birds have a mechanism to survive that, I have trouble understanding how except by sending more heat into the feet overall. Unless their feet can just survive being below freezing unlike ours?
I'm so confused
Once again, not a doctor/biologist/etc, but I'll take a stab at this. I think I've got the hang of what is going on, but if I'm incorrect, feel free to let me know.
https://lemmy.world/pictrs/image/5b42b922-9d34-43dd-9390-d4b98669135e.jpeg
Body heat doesn't just appear. As warm blooded animals, we get most of our body heat as a byproduct of metabolism. Hypothermia occurs when heat is lost faster than a body can produce new heat, as all the body systems require a certain temperature range to happen. Get too cold, and organs and bodily functions get really screwed up to the point the heart and lungs will stop working.
Heat travels from hot to cold to try to achieve equilibrium, so bodies lose heat through radiation if skin isn't covered enough, through contact with colder matter, and convection from wind currents. You are always producing heat (if you are metabolizing!) but you are also always losing heat in a cold environment, or gaining heat if your environment is warmer than you are. Let's focus on the cold side since that's our original topic.
So let's look at our graphic again. You're on the Canada instance and seem to particpate in Canadian/EU topics rather than US, so I'll assume you understand these values as degrees C. (For anyone else like myself, 37C is healthy human body temp, around 98 degrees F. The minimum temp in the graphic, 16C is about 60F. )
In the top circulatory system, the hot blood comes from the core at a happy and healthy standard body temp, but as that warm blood is pushed away from the core, it loses temperature to the environment. The blood eventually returns to the core to get reheated. The problem occurs the longer the exposure to cold lasts, as your body can only generate so much heat. It's like running the heat in your house but leaving the front door and some windows open. You're consuming more and more fuel and getting less and less efficiency. You can keep it up until your fuels runs out or you burn out your heating system. Same in the body.
In the bottom circulatory system, since the blood vessels are so close to each other, sometimes touching, they still lose heat to the environment, but since they are in close proximity to each other, the hotter part of the blood vessel is going to warm up the colder part as that heat radiates away, capturing some of it in the process. This seems to be where you're getting lost if I'm understanding you correctly. So think if you and someone else are stuck outside in the cold together. You are both losing heat via the 3 pathways previously mentioned. Buuuuut, now you decide to cuddle up. What happens?
You both are still losing heat at the same rate as before, because you are both the same body temperature and your environment hasn't gotten any warmer, but now due to your proximity, whatever parts of you are touching, now a good portion of the heat being pulled away from you is being radiated and conducted to the other person and vice versa where it is reabsorbed instead of draining off directly into the ether. Neither of you is generating additional heat, rather now you are both helping each other conserve heat that would have been lost, reducing you actual need to generate that heat. Neither of you are what you would call comfortable, but your ability to tolerate the temperature has increased because your circulatory systems have increased efficiency in retaining what heat you are producing.
Back to the graphic now, heat comes to the extremities at body temp, but due to the proximity of the hot parts to the colder parts, that heat differential is minimized, so the birds in this case are their own snuggle buddies, it's just their blood vessels are what is doing the cuddling. And if they have feathered legs/feets, they are retaining even more of that heat, because that's like if you'd cover you and your snuggle buddy under a big blanket. The body loses less heat, so the burden on the metabolism and circulatory system is lowered, and they can tolerate harsher conditions for a much longer time than we can without this type of circulatory system.
Does that make more sense?
not oc, but I think the question was why the feet can survive with 22c blood (from the graphic)
The warm blood going into the feet transfers its heat to the cold blood going back. That means less heat going into the feet where it would otherwise be lost to the environment.
Humans minimize heat loss by simply restricting blood flow to the extremities. It's uncomfortable, but not as uncomfortable (or dangerous) as a cold core.
I'm curious what the evolutionary tradeoff is for having this.
Most of my life I was jealous we weren't cold blooded as it sounded like a plus to be the ambient temp all the time. I eventually learned it was a great evolutionary leap to become warm blooded so our internal systems will generally always be operating at peak conditions despite our surroundings.
It would really change life if a cold day would start shutting down your organ functions or leave you unable to digest food and you die of septic shock because you had a meal before going outside too long.
I'm better than that. My organs can shut down at any temperature!
Congrats,, you beat the system! 😅
Cool.
They must be pros at Oxygen Not Included then.
I hadn't heard of that game. The art style looks so fun!
Uh oh. Birds do not have a diaphragm to allow them to breath via muscle control and need atmospheric pressure to force air through their respiratory system.
Thread on bird breathing
Short animated video on the process
Thermodynamics Simulator
Slightly dodgy thermodynamics simulator, extremely dodgy electricity simulator, and supremely dodgy gas and liquid flow simulator.
I love it though - it's my favourite game.
That's amazing! I misunderstood this as I watched my ducks happily sift through ice cold mud.