blue blood
Don Leonard: Anyone know the origin of the term "blue blood" to describe the upper-class folks?
Michael kuter: If you look at the eyelids of a sleeping child, especially when the temperature is low, you will see that the eyelids appear blue. This is a demonstration of the phenomenon which led to the tag 'blue blood' being applied to the aristocracy-the privileged and ruling classes of medieval society-in central and northern Europe.
Blood contains about five million million red blood cells per litre; each red blood cell contain many molecules of a pigment called haemoglobin which loosely binds oxygen as the blood is pumped through the lungs. Fully oxygenated blood is bright cherry red. On reaching the tissues of the body the oxygen leaves the haemoglobin and diffuses to regions of lower oxygen concentration. Blood which has given up its oxygen is a darker bluish-red. In cool weather an automatic control system reduces blood flow through the skin, to conserve body heat.
The skin, or at least the lower layers of it, is a living tissue which uses oxygen in its metabolism. Under conditions of reduced blood flow the skin cells use up the oxygen from the blood, turning it dark blue-red. In cold weather the de-oxygenated blood is not flushed through the skin capillaries very rapidly, so the bluish tinge persists and is seen on exposed cold regions of the body like the hands and face, especially the eyelids and ears.
Very white skin has almost no melanin, the pigment in the skin which increases and darkens the skin on exposure to the sun. Unpigmented skin which has been drained of blood appears blue-white, the colour of the connective tissue it contains. What then is the connection between the term 'blue blood' and the members of the privileged or 'upper' classes to whom it is applied?
Medieval society was divided into four classes: royal, noble, gentle and simple, the last-mentioned, the lowest social class, including peasants, villeins, serfs and slaves. (Medicinal preparations and herbal remedies used by the peasants were called 'simples'.)
Those fortunate people in positions of power used a number of means to demonstrate and reinforce their superiority - dress, regalia, language, legal and social discrimination, conspicuous consumption and a reluctance to indulge in the heavy, repetitive physical labour of the outdoor worker-peasant.
The agricultural labourer and their families, on the other hand, inevitably gained a better sun-tan, a darker skin pigmentation, than did the indoor-dwelling nobles and gentry. These horny-handed sons of toil also gained a thickened skin from exposure to the weather, and several daily coatings of dust and dirt engrimed with sweat. It was uncommon in the period for the lower classes to bathe in the way that we do today; toilet soap was an expensive luxury.
The aristocracy generally ate far smaller quantities of 'earthy' foods, such as fruits and vegetables, than did their social inferiors, the tenant farmers and labourers, and the ruling classes suffered for it with anaemia. In anaemia the blood cells are low in iron; an atom of iron in each haemoglobin molecule is essential for binding oxygen, so anaemic blood does not contain as much oxygen as normal, and is therefore less bright red to start with.
The elements which created in the popular mind the association between 'blue blood' and aristocracy were: a thin, white (relatively clean or less grimy) skin which allowed the colour of the blood to show through, and a low rate of blood flow through the skin in cold climates. This effect would be even more pronounced in people suffering from anaemia.

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