OT Biological counter-current exchange a.k.a rete mirabile

Josh Hayes wrote:
maryann kolb <mkolb@xxxxxxxxxx> wrote in news:be89p35p2tp1eance968410129pq50lapo@xxxxxxx:

Having twice had Chicadees land on my finger, I was surprised at how
hot their little feet felt even in the fleeting moments that they
stood there. They trap air under their feathers and their boody heat
makes it even cozier that an L.L. Bean parka.

You could find out more about how this works by looking for web pages devoted to "counter-current exchange".

We do it too, but not as efficiently (well, of course: consider how superior the bird lung is as well!). Heck, even tuna have counter current exchange systems in their skulls to keep their brains cool. (tuna are incredible fish -- don't get me going on that!)

With efficient heat exchange, the chickadee's feet would seem cool, not warm. A seal's flipper and the blood in it is nearly at sea temperature, but the exchange of heat between blood entering and leaving brings the return (venous) blood nearly to body temperature. Such exchanges are not always of heat nor do they always involve blood. A birds lung and windpipe (So /that's/ why geese have such long necks!) is heat and air columns, the network in a dog's snout uses air to cool blood going to the brain. Humans and canids are cursive hunters by biological nature. A fit human can go farther "in the long run" than any animal but a canine. Dogs and humans have different mechanisms, both involving exchange. With canids, it is in the snout, as I wrote above. Humans cool by sweating, and counter-current exchange in the kidneys allows them to process the prodigious amounts of water sometimes required. (Our kidneys are inefficient at conserving water, a price paid for enabling rapid refill when supplies are low.)

This also explains how, for instance, gulls can stand around on ice which must be well below zero C without having their feet fall off.

That's the point: those feet must feel cold to the touch. Water birds' feet need heat exchangers not only to stand on ice, but to remain submerged for long times.

Obviously it's been WAY too long since I taught vertebrate physiology; I have lectures bubbling up out of me.

I eagerly await whatever insights you can provide.

Jerry
--
Engineering is the art of making what you want from things you can get.
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