Science Puzzle
Is the Blood in Your Veins Blue?
Look at the inside of your wrist and the veins look blue. Diagrams of the body usually colour veins blue too, and plenty of people have been told that blood is blue until it meets the oxygen in the air.
Is the blood in your veins really blue?
The Answer
Human blood is always red. In the veins it's a darker red, but it's never blue.
Blood gets its colour from haemoglobin, the protein in red blood cells that carries oxygen. Haemoglobin loaded with oxygen, in blood flowing out from the lungs, is bright cherry red. Haemoglobin that has given up some of its oxygen, in blood returning through the veins, is a deep maroon. When a nurse takes a blood sample from your arm, the tube fills straight from a vein with no air getting in, and the blood is dark red.
The blue is an illusion made by your skin. Light of different colours travels different distances into skin before bouncing back out. Red light goes deeper, deep enough to reach a vein, where the blood soaks much of it up. Blue light is mostly scattered back from shallower layers before it reaches the vein. So the skin over a vein reflects relatively less red than the skin around it, and your brain, comparing the two, reads the vein as bluish.
The blue in diagrams is just a convention to tell veins from arteries. Some animals really do have blue blood, though: octopuses, squid and horseshoe crabs carry oxygen with a copper-based molecule, haemocyanin, instead of iron-based haemoglobin.
The principle: Colour depends on the light as well as on the object. Skin scatters blue light back and lets red light reach deeper veins, where it's absorbed, so red blood under the skin looks blue.