5 ms·

I knew that breathing through the nose heats up the air before it hits the lungs but the energy recovery aspect is news to me. How does that work? Do we have a

I knew that breathing through the nose heats up the air before it hits the lungs but the energy recovery aspect is news to me. How does that work? Do we have a heat pump in our noses?

nine_k 3y ago
Think of the nose tissues as of a simple heat reservoir. When we inhale cold air, these structures warm it, and thus become colder. When we exhale hot air, it heats these structures, making them warmer again, to allow heating the next breath in. It's of course not 100% efficient, but it's better than nothing. I wonder uf there are more advanced strictures, like circulating liquid, to improve efficiency.
- codethief 3y ago
  > When we inhale cold air, these structures warm it, and thus become colder. When we exhale hot air, it heats these structures, making them warmer again How so? When you breathe in, your nose tissues will cool down to a temperature T_N and the air you inhale will heat up to a temperature T_A. Physics (the second law of thermodynamics) dictates that T_N >= T_A. Moreover, the nose tissues are connected to a large heat reservoir (the body), so we might as well assume they go back up to body temperature very quickly, so T_N = 37°C after a second or two. Either way, when you breathe out you still have T_N >= T_A. In this situation, there is no way the air could transfer back heat to the nose without a violation of the second law or, well, a heat pump. Hence my question.
  - pfdietz 3y ago
    The key point is that the temperature varies across the nose tissues. Close to the outside, it's low; farther in, it's high.
  - codethief 3y ago
    I realized I made a mistake: A quick Google search reveals that air gets heated up to full body temperature in the lungs while the nasal turbinates, after inhalation, remain below body temperature for a bit longer. So upon breathing out T_N < T_A and heat indeed gets transferred back from the air to the nose.
pfdietz 3y ago
It's an example of countercurrent heat exchange, basically exchanging heat between outgoing and incoming air with very high efficiency. When breathing in, the air is gradually heated by the nasal passages. The temperature in the nasal passages increases along the channel, so at all points there's a small temperature drop, minimizing production of entropy. When breathing out, the air is cooled in the same way. Again, there's a small temperature drop (now of the opposite sign) along the channel, again minimizing entropy production. Moisture is also conserved here in a similar way. Countercurrent heat exchange is also used in biology to minimize heat loss in extremities. The arteries of your arm, and the deep vein returning blood there, are wrapped around each other to transfer heat. In some vertebrates elaborate artery/vein structures have evolved to make this more effective. https://en.wikipedia.org/wiki/Rete_mirabile https://en.wikipedia.org/wiki/Rete_mirabile Countercurrent exchange is a fundamental engineering concept I feel more people should know about.
- codethief 3y ago
  Thanks, I hadn't heard of countercurrent heat exchange! I do find it remarkable, though, that the nasal turbinates don't seem to thermalize with the rest of the body in the time between inhale and exhale (which is of the order of several seconds). Or maybe it does thermalize quite a bit (even though not fully): A quick search for "nasal turbinates countercurrent heat exchange" reveals that human nasal turbinates are not particularly great heat exchangers, with air exiting at temperatures of up to 33°C during exhales.
  - pfdietz 3y ago
    Many examples of the principle in action: https://en.wikipedia.org/wiki/Countercurrent_exchange https://en.wikipedia.org/wiki/Countercurrent_exchange

I knew that breathing through the nose heats up the air before it hits the lungs but the energy recovery aspect is news to me. How does that work? Do we have a heat pump in our noses?

nine_k 3y ago

Think of the nose tissues as of a simple heat reservoir. When we inhale cold air, these structures warm it, and thus become colder. When we exhale hot air, it heats these structures, making them warmer again, to allow heating the next breath in. It's of course not 100% efficient, but it's better than nothing. I wonder uf there are more advanced strictures, like circulating liquid, to improve efficiency.

codethief 3y ago
> When we inhale cold air, these structures warm it, and thus become colder. When we exhale hot air, it heats these structures, making them warmer again How so? When you breathe in, your nose tissues will cool down to a temperature T_N and the air you inhale will heat up to a temperature T_A. Physics (the second law of thermodynamics) dictates that T_N >= T_A. Moreover, the nose tissues are connected to a large heat reservoir (the body), so we might as well assume they go back up to body temperature very quickly, so T_N = 37°C after a second or two. Either way, when you breathe out you still have T_N >= T_A. In this situation, there is no way the air could transfer back heat to the nose without a violation of the second law or, well, a heat pump. Hence my question.
- pfdietz 3y ago
  The key point is that the temperature varies across the nose tissues. Close to the outside, it's low; farther in, it's high.
- codethief 3y ago
  I realized I made a mistake: A quick Google search reveals that air gets heated up to full body temperature in the lungs while the nasal turbinates, after inhalation, remain below body temperature for a bit longer. So upon breathing out T_N < T_A and heat indeed gets transferred back from the air to the nose.

pfdietz 3y ago

It's an example of countercurrent heat exchange, basically exchanging heat between outgoing and incoming air with very high efficiency. When breathing in, the air is gradually heated by the nasal passages. The temperature in the nasal passages increases along the channel, so at all points there's a small temperature drop, minimizing production of entropy. When breathing out, the air is cooled in the same way. Again, there's a small temperature drop (now of the opposite sign) along the channel, again minimizing entropy production. Moisture is also conserved here in a similar way. Countercurrent heat exchange is also used in biology to minimize heat loss in extremities. The arteries of your arm, and the deep vein returning blood there, are wrapped around each other to transfer heat. In some vertebrates elaborate artery/vein structures have evolved to make this more effective. https://en.wikipedia.org/wiki/Rete_mirabile https://en.wikipedia.org/wiki/Rete_mirabile Countercurrent exchange is a fundamental engineering concept I feel more people should know about.

codethief 3y ago
Thanks, I hadn't heard of countercurrent heat exchange! I do find it remarkable, though, that the nasal turbinates don't seem to thermalize with the rest of the body in the time between inhale and exhale (which is of the order of several seconds). Or maybe it does thermalize quite a bit (even though not fully): A quick search for "nasal turbinates countercurrent heat exchange" reveals that human nasal turbinates are not particularly great heat exchangers, with air exiting at temperatures of up to 33°C during exhales.
- pfdietz 3y ago
  Many examples of the principle in action: https://en.wikipedia.org/wiki/Countercurrent_exchange https://en.wikipedia.org/wiki/Countercurrent_exchange