Hank Green explains us again things about our bodies that we did not know. Our blood is always red, inside or outside of us. The red color is given by the presence of hemoglobin in the red cells. We do, however, have different shades or red blood in our bodies, depending on where you get it from.
Hemoglobin is a protein and it is red because it contains iron and in contact with oxygen it gives out the reddish color that we are used with.
If you could cut your finger, then you could see a bright red blood. That is the oxygenated blood also called capillary blood which is full with oxygen ready to be sent to the cells. On the other hand, or forearm if you wish, the blood that the paramedics take from our forearm is venous blood, which is full of carbon dioxide, comes back from the cells and goes to the heart.
Arteries bring oxygen to the cells from the heart, veins take the carbon dioxide from cells and then guide blood back to the heart. See details about veins here. More details on circulatory system on the Wiki page.
Some animals have blue or green blood, because they do not have hemoglobin in their blood. Instead they might have hemocyanin (copper containing protein, snails, blue blood) or hemorithyn ( marine worms, pink-violet color) or biliverdin (byproduct of a breakdown of hemoblobin, skinks, green blood, New Guinea ).
You can see biliverdin in your own body in the three days old bruises which paint your skin in blue-green-yellow colors. Now, sci-fi movies with aliens that have green blood do not seem so interesting anymore, isn’t it?