It doesn't "turn" red. it IS red. The blue you see is the color of your vein, not the blood inside it.
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."
When the blood comes in contact with the air it turns red.
well what do you mean by ''It turns red when we bleed'' ! Its already 'red' in colour ..coz it contains RBCs (red blood cells)
Because OXYGEN gets in it which makes it RED...
Without oxygen in the blood it turns Blue...
when blood is oxygenated it is red. when it comes in contact with the oxygen in the air it will turn red.
this is the reason why blood in your arteries is red and blood in your veins is blue.
Exposure to air, particularly oxygen, reacts with the iron. When iron and air mix and oxidize the resulting color is red, just like rust and it's the same chemical reaction.
When blood has oxygen in it, it's red. So, when coming in contact with the oxygen in the air, it will turn red.
You know how when people choke their face turns blue? It's because the blood flowing through it isn't getting oxygen. Because they're not breathing.
So, blood is red because of oxygen. :)
[Thought, if aliens didn't need oxygen, would their blood be blue, or would it be some other weird color? o.O]