Why is blood red?
Blood gets its colour from a red chemical called haemoglobin, which is stored in red blood cells. When blood flows through the lung, the haemoglobin collects oxygen and carries it to other parts of the body. Haemoglobin that is carrying oxygen is bright red, but it has released its oxygen into the body's cells as it flows past them, it becomes much darker. As a result, the blood flowing in your arteries is brighter red than the blood flowing in your veins.