What is the color of human blood? Red, Blue, Green?

July 25, 2021

Ever wondered, if our blood is red in color, then why our veins are blue? Is blood sometimes blue, sometimes red? Moreover, we have heard of white blood cells also then why isn’t our blood white in color? Too much confusion!! Isn’t it?

What is blood?

Blood is a fluid that flows throughout our body. It has three basic components:

Red Blood cells (RBCs): They carry oxygen from our lungs to rest of our body and takes carbon dioxide back to the lungs to be exhaled.

White blood cells (WBCs): In simple language these are the weapons of our body for fighting against viruses and bacteria.

Platelets: They come together when we have some injury so that we don’t lose much of our blood. They help our body form clots to stop bleeding.

Plasma is a liquid that holds these three together.

The answer to our first question lies in the amount of these cells in our blood. Since the ratio of WBCs to RBCs is very low, therefore, blood is not white in color. But that partially answers our question. For more detailed answer, lets delve deeper into the structure of RBCs. Do not worry, I won’t bore you with giant horrible structures.

All of us must have heard of hemoglobin sometimes. That’s completely fine if you haven’t! I’ll explain. Our RBCs contain an iron rich protein called hemoglobin, made up of subunits called hemes. The red color of the blood is due to the interaction of oxygen with the heme proteins.

Many of us are being told that since veins carry deoxygenated blood, so it is blue in color, while arteries carry oxygenated blood so is red. It is a very common myth. Infact, deoxygenated blood is dark red not blue. You must have seen this in real life too. Most of us have gone for blood test at least for once. When the blood is drawn from our body, it is taken to a vacuum sealed (completely devoid of air) container so ideally it should be blue in color, since it is deoxygenated, but it is rather dark red in color. Shades of red color may vary with the amount of oxygen your RBCs are carrying.

Deoxygenated blood

Unless you are an octopus, squid or a horseshoe crab your blood is not blue. These animals have blue blood because of the molecule hemocyanin (contains copper), which is being used to carry oxygen in their body rather than hemoglobin we use.

Blue blood of horseshoe crab

Veins appear blue because they are under our skin. When white light hits the surface of our skin, most of the blue light is scattered or deflected while the red light penetrates the skin more and gets absorbed while blue light doesn’t penetrate far into the skin. If you flash red light on your skin, you will see your veins dark red in color that’s how you can spot the veins for drawing out the blood. There are various vein finder devices available in the market which make use of this very concept.

Image showing Illumevein vein finder device holding hand

It may sound bizarre, human blood appears green in the deep ocean (if you get cut while diving). This is because, when the white light passes through the water red component gets absorbed by the water so there is not red component left in water. Without red color in the light, only green is reflected by the blood which we can see in the image below.