Dr. Craig Heller

We are mammals.

Mammals have fur.

We've lost the fur, but we still have that hairy skin phenotype all over our body, except for those skin surfaces where our mammal relatives didn't have fur.

So the pads of the feet.

And for the primates, upper part of the face.

For rabbits, no portions of the ears, the inner surface of the ears.

I never thought about that.

Our mammalian relatives can't lose heat over their overall body surface.

So probably very early on in mammalian evolution...

They evolved these special blood vessels in the limited surface areas that don't have fur.

And as I said, what these blood vessels are, are shunts between the arteries and the veins.

Arteries and veins are both low resistance vessels.

So you can have high flow rate.

Capillaries, which normally are between arteries and veins, are high resistance because they're very tiny.

If you are warm and you look at the palms of your hands, they are fairly red.

The backs of your hands aren't.

You don't have these vessels in the backs of your hands.

Now, if you take a glass, like a water tumbler, right, and you grab it, you can see if you squeeze a little bit, the hand goes white.

that's because you've shut off that blood flow.

So if you're bicycling on a hot day, you don't want to be grabbing your handlebars all the time.