Blood inside the body is either bright red or dark red, depending on whether it is arterial blood or venous blood.
Red blood cells carry oxygen from the lungs to the tissues, where the oxygen is exchanged for carbon dioxide.
Each red blood cell is packed with hemoglobin, a protein that contains iron. Hemoglobin combines chemically with oxygen in the lungs, forming a compound called oxyhemoglobin, which is bright red.
When the blood gives up the oxygen to the tissues and enters the veins for the return trip to the lungs, it is a darker red, closer to a purple.
The circulating blood in the veins appears blue from outside the body because of a combination of factors.
First, veins are thin-walled and close to the surface, so the darker red or purplish color may show blue filtered through the layers of the skin.
Another factor in the apparent color is the degree of fairness of the skin, providing contrast.
Compared with veins, arteries are thick-walled and deep in the body.
Arterial blood is seldom seen unless an artery is accidentally or surgically severed.