Let�s get the facts right here once and for all. As I said in the first answer, blood is red. If it�s venous blood (poor in oxygen) it is dark red and if it�s arterial blood (rich in oxygen) it�s bright red.
If you cut a vein, the blood which comes out will not become oxygenated and turn bright red on contact with the air; that can only happen in the lungs or in an artificial lung such as the oxygenator in the extracorporeal blood circuit of a heart-lung machine. The reason for this is that a very large surface area of the red cells of the blood must come into contact with a membrane (the walls of the alveoli of the lungs or the membrane of an oxygenator) in order to allow oxygen to diffuse through that membrane. This diffusion takes place because of the difference in the partial pressures of oxygen in the air we breathe (or the gas supplied to an oxygenator) and the blood.
Red blood cells (which transport oxygen) are about 7 microns in diameter. The capillaries of the alveolar walls in the lungs have a similar diameter: consequently, there is a very high surface contact. This cannot be achieved in an oxygenator, but is compensated for by lowering the patient�s temperature and/or increasing the percentage of oxygen in the gas supplied.