Lupinus gredensis Gand.
Blushing is the most peculiar and the most human of all expressions. Monkeys redden from passion, but it would require an overwhelming amount of evidence to make us believe that any animal could blush. The reddening of a face from a blush is due to the relaxation of the muscular coats of the small arteries, by which the capillaries become filled with blood; and this depends on the proper vaso-motor centre being affected. No doubt if there be at the same time much mental agitation, the general circulation will be affected; but it is not due to the action of the heart that the network of minute vessels covering the face becomes under a sense of shame gorged with blood. We can cause laughing by tickling the skin, weeping or frowning by a blow, trembling from the fear of pain, and so forth; but we cannot cause a blush (...) by any physical means - that is, by any action on the body. It is the mind which must be affected. Blushing is not only involuntary; the wish to restrain it, by leading to self-attention, actually increases the tendency.
We have now to consider, why should the thought that others are thinking about us affect our capillary circulation?
Charles Darwin, The Expression of the Emotions in Man and Animals (The Folio Society, 2008)