Blue Blood?

[natalie_1982]

Why do veins look blue if blood is red?

[tartanwiz]

The blood turns blue when it runs out of oxygen, as it gets carried to the lungs ,it collects oxygen again (turning red), carriying it to the heart. When it leaves the heart it turns blue again, and so round and round the circulatory system. Of course, if you cut a blue vein, the blood would be immediately exposed to the oxygen in the air and would turn red instantly.

[Sam172]

Of course, that would be different if you happened to cut yourself in a vaccum....though you would msot probably be very dead by the time you were in a vaccum anyway. So in a sense blood is blue, just when the haemoglobin in the blood absorbs oxygen the blood cells itself turns red

[zeus]

its not actually the haemoglobin that is red in the blood, its the red iron bearing pigment. When the o2 attaches to the iron oxyhaemoglobin is formed. (red) the dissociation of the O2 thus leaves deoxygenated blood. (blue)

[Gef]

Just to clear up zeus' answer. Blood contains a complex molecule with an iron atom at the centre. When oxygen binds with this molecule we get oxyhaemoglobin which is red. If there is no oxygen present then the de-oxyhaemoglobin is blue. By the way, carbon monoxide is binds more strongly to haemoglobin than oxygen. This is why it is so poisonous. Carboxyhaemoglobin is pink and so people who have sufferred carbon monoxide poisoning look quite rosy!

[crispy]

Venous blood is not blue. Veins are blue because of the type of endothelial cell which forms the walls. It is a common misconception that venous blood is "empty" of Oxygen and thus blue. The oxygen content of arterial blood (CaO2) is around 20.4 ml/100 ml, were as the oxygen content of mixed venous blood is around 15.2 ml/100 ml. Thus the difference is only a 25% drop. Venous blood is still 75% saturated with oxygen and has a deep dark red colour c.f. arterial blood which has a more intense and brighter red colour.

[natalie_1982]

That's great, I think I get the jist at least so thank you!

[sylviak]

blood is blue inside you. it becomes red when it comes out in the open

[dblondfemme]

SDSTAFF Hawk replies:


Blood is a bright red in its oxygenated form (i.e., leaving the lungs), when hemoglobin is bound to oxygen to form oxyhemoglobin. It's a dark red in its deoxygenated form (i.e., returning to the lungs), when hemoglobin is bound to carbon dioxide to form carboxyhemoglobin.


Veins appear blue because light, penetrating the skin, is absorbed and reflected back to the eye. Since only the higher energy wavelengths can do this (lower energy wavelengths just don't have the *oomph*), only higher energy wavelengths are seen. And higher energy wavelengths are what we call "blue."


In an experiment, glass tubes were filled with blood and immersed in milk, milk having a similar ratio of fat, proteins, and water in emulsion as skin. At a certain depth, the tubes appeared blue.


As a phlebotomist (a person who draws blood) at the local university hospital, I drew hundreds of samples of venous blood. Almost all samples were dark red. Those that weren't were usually because people were on enriched oxygen systems.


At the same time, respiratory therapists drew blood from an artery, not a vein. These samples were characteristically bright red, unless the patient was having difficulty breathing (asthma, people with one lung, pneumonia, emphysema, whatever).


So, your veins may look blue, but even a blueblood will bleed red.


--SDSTAFF Hawk

Straight Dope Science Advisory Board

Blood is not blue.....ever.... dark red maybe....never blue