Understanding Blood Color: Why Veins Appear Blue

The colour of blood depends on its oxygen content and the way light interacts with it. When oxygenated, blood appears bright red, while deoxygenated blood is deep, bluish-red.

Inside the body, arteries carry oxygen-rich blood away from the heart to the tissues and organs. This oxygenated blood is bright red. As the oxygen is used by the tissues, the blood becomes deoxygenated and changes colour to bluish-red. This deoxygenated blood is carried back to the heart by the veins.

When light hits blood, some wavelengths are absorbed and others are reflected. The colour of the blood that we see is the result of the wavelengths that are reflected. Oxygenated blood reflects more red light, so it appears bright red. Deoxygenated blood reflects more blue light, so it appears bluish-red or purple.

The bluish-purple colour of blood inside the veins is also due to the fact that veins are typically located deeper in the body than arteries. This means that the light that reaches the blood in the veins has to travel through more tissue and skin. This tissue and skin absorbs some of the red light, so the blood in the veins appears more blue.