blood

[Smartboy]

Why does the blood vessels look blue on the outside when it is red in colour?

[alancohen]

The blood vessels that you are referring to are veins. Blood flowing in veins is bluey purple because the blood is flowing away from body tissues back to the heart. The body tissues have removed about 25 to 35% of the oxygen from the blood. Such blood which is only partly oxygenated is blue. When you cut yourself the blood is directly exposed to oxygen in the air. The cells in the blood take up this oxygen so that the blue blood quickly becomes red. Does that answer your question? If you were to look at blood flowing from the heart and lungs it would also be red because it would have more oxygen in it. However, you can't normally see blood vessels carrying blood from the heart and lungs because these are carried in arteries which are firstly less superficial than veins (to protect them better, as damaging an artery would be more significant because the blood in them is at a high pressure) and also because the walls of arteries are thicker than veins and not easy to see through.