Why is blood red?

Blood is red because it is made up of a clear liquid (lymph) carrying large numbers of red blood cells, which carry oxygen to the body. Red blood cells are red due to hemoglobin, an iron-based protein. In the hemoglobin are four atoms of iron which can be in an oxidized state, i.e with oxygen attached (making it red), or in a non-oxidized state (when it is dark red-blue or maroon color).

Blood is pumped through the lungs, where it is oxygenated. Then it travels around the body to where the oxygen is required. When the oxygen is used up, the hemoglobin is routed back via the veins, and then back through the lungs again.

When arterial blood leaves the lungs to circulate through the body, it has just a little bit more oxygen in it than does venous blood. The reason it appears bright red is that the combination of iron, oxygen, and hemoglobin absorbs higher energy wavelength light (blue and green) which leaves the red wavelengths available for our eyes to sense.

The venous blood is never blue; it is a darker color of red than arterial blood, not blue. The color is a burgundy red or maroon color. Blood is bright red in the arteries and dark red in the veins. The reason venous blood is a darker red can be partially attributed to the slightly less oxygen in the blood in the veins. But its color change is more due to the "waste" it carries away from the body tissues and back to the kidneys for filtering and elimination. This "waste" darkens the red color of the blood (think of it as a little like dirty dish water).

Although a popular belief, being in contact with air does not cause venous blood to instantly oxygenate and turn red. It is red outside the body because it is red inside the body as well. When you look at unopened veins inside the body, in endoscopy, for example, they are a dark red color.

The blue appearance of the veins that you see when looking at them through the skin is not caused by blue venous blood. As explained, that is always red. The blue appearance is caused by a reflective factor of the skin itself. It is an optical property of the reflection of light off light colored skin and the difference in that reflection from the veins under the skin (but near the surface). That reflective process is complex, but the blue-looking veins are mostly all about the skin and reflection.

If arteries were not too deep to be visible through the skin, then they, too, would have a blueish appearance, the same as the veins for the same reasons.