Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of the story. The models are simplified to show the idea; depths are not to scale.
Tap the buttons and slide the sliders.
Pick a colour of light. Which one goes deeper into the skin?
Short (blue) wavelengths scatter back within a fraction of a millimetre; long (red) ones reach roughly 5–10 mm.
Tap a colour. Which one goes deep?
Drag the vein deeper or closer to the surface. When does it look most blue?
Drag the vein: the patch above it loses red light where blood absorbs it. Very deep veins fade out.
Slide the vein up and down. Watch the colour on top.
Switch skin tones. Does the vein look the same colour on each?
The same reflected tone reads differently against different backgrounds; melanin lowers the contrast.
Tap a skin colour. How does the vein look?
Switch between a vein and an artery. Which is closer to the surface?
Arteries lie deeper and have thicker, muscular walls; superficial veins sit just under the skin.
Tap each one. Which is near the top?
Switch the lamp to red or blue. When are the veins easiest to see?
Under red light veins appear as dark lines; under blue light they almost vanish.
Tap red, then blue. When can you see the veins?
Look at your wrist. You may see thin blue lines. Those are veins. But the blood inside is red! Blood is always red, never blue.
Lamp light and sunlight have every colour mixed in. Some of the light goes into your skin.
Blue light only goes in a tiny bit. Then it bounces back out. Red light goes much deeper, deep enough to reach a vein.
The dark red blood in the vein soaks up the red light. So less red comes back. The blue still comes back. Less red, same blue: the vein looks blue!
Your brain looks at colours side by side. On light skin, veins look very blue. On darker skin, they may look green, or be hard to see. It is the same red blood inside everyone.
More: why blood is red · why the sky is blue · all explainers
This page explains how the body works; it isn't medical advice.
Veins aren't blue, and neither is your blood. The blood inside your veins is dark red.53 Veins only look blue because of the way light travels through skin. Blue light is scattered back out near the surface, while red light travels deeper, where the blood in a vein soaks much of it up. So less red light comes back from the spot above a vein than from the skin around it. Your brain compares that patch with the warmer skin next to it and sees it as bluish.24
Scientists tested this properly in 1996. Alwin Kienle and his colleagues photographed skin with a vein in different colours of light and ran computer simulations of light bouncing through tissue. They found that the colour of a vein depends on four things: how skin scatters and absorbs each colour of light, how much oxygen the blood carries, how deep and wide the vein is, and how our eyes and brain judge colour.1
Human blood is always red. Blood that has just picked up oxygen in your lungs, flowing through your arteries, is bright cherry red. After it has delivered oxygen to your body, the blood heading back to your heart in your veins is a darker red.53 The colour comes from haemoglobin, the iron-containing protein in red blood cells. With oxygen attached it absorbs blue-green light and reflects red-orange light.3 Want the full story of that iron? See why blood is red.
Sunlight and the light from most lamps is "white" light, which is really a mix of all the colours of the rainbow. Each colour is a wave with its own wavelength: red is long (about 700 nanometres), blue is shorter (about 475 nanometres).2 (For more on how light moves, see how light travels. The same stronger scattering of blue light is why the sky is blue.)
Skin is not a clear window. It is packed with cells and fibres that knock light off course, a bit like a pinball bouncing between pins. Shorter, bluer light is scattered much more easily. It only gets a fraction of a millimetre into your skin before most of it is sent straight back out. Longer, redder light is scattered less, so it can wander far deeper, up to about 5 to 10 millimetres below the surface. That's the depth where many veins are.24
| Colour of light | Wavelength | How far into skin | What happens above a vein |
|---|---|---|---|
| Blue | about 475 nm | a fraction of a millimetre | comes back out near the surface, same as anywhere else |
| Red | about 700 nm | up to about 5–10 mm | reaches the vein and much of it is soaked up by the blood |
In the explainer's light toy, the depths are drawn as a simple picture, not to scale.
When red light gets deep enough to reach a vein, the haemoglobin in the blood soaks much of it up.2 That means that from the spot right above the vein, less red light makes it back to your eyes. The blue light never went that deep, so the vein makes almost no difference to it. Less red plus the same amount of blue: that patch of skin looks bluer than the skin beside it.2
You can see this with coloured lights. If you shine red light on your arm, the veins show up as dark lines, because the red is being absorbed there. Under blue light, the skin just looks blue and veins are hard to find. Medical staff use the same idea: devices that shine red or infrared light help them find veins.2
Depth matters too. A vein that is very deep gets little light at all and fades from view. Very narrow vessels right near the surface, like the tiny capillaries, don't look blue either.21 In the vein toy, dragging the vein changes a colour swatch "seen from above". That swatch is a simple picture of the idea, not a measurement.
There's a second part to the trick, and it happens in your head. Your brain never judges a colour on its own; it compares it with the colours around it. The same greyish patch looks different on a light background than on a dark one. Next to warm, lighter skin, the slightly less-red patch above a vein looks clearly blue.4 Kienle's study lists this "visual perception" step as one of the four ingredients of vein colour.1
Not everyone sees blue. Under some skin colours veins look bluish-green, and on darker skin they can be faint or hard to see at all.4 Skin colour comes from melanin, a dark pigment made by cells in the epidermis, the outer layer of the skin; people with more melanin have darker skin.711 Melanin absorbs light, and the brain's comparison changes with the colour of the surrounding skin, so veins show the most contrast on very pale skin.211 Whatever colour a vein looks from the outside, the blood inside is the same dark red. In the skin tone toy, the vein colours are an illustration of the idea, not measurements.
Arteries carry blood away from your heart and veins carry it back. Around the body, arteries carry bright red, oxygen-rich blood and veins bring darker red, oxygen-poor blood back (the vessels between the heart and the lungs are the exception).5 (Want to know how the heart pumps blood through them? See what causes a heart attack.) So why aren't there red lines on your arms?
Yes! Octopuses, squid, snails, slugs, spiders and horseshoe crabs carry oxygen with a protein called hemocyanin. Instead of iron, it holds copper.89 Hemocyanin is almost colourless without oxygen and turns blue when it picks oxygen up.9 Humans and other mammals, birds, reptiles and most fish use red, iron-based haemoglobin.8 (Octopuses have other colour tricks too: see how octopuses change colour.)
Horseshoe crab blood is even useful to people: because it reacts quickly to bacterial toxins, it is used to check that vaccines, medicines and medical devices are free of them.10 The phrase "blue blood" for royalty is different again: it may come from pale-skinned nobles whose veins showed clearly through untanned skin.2
Because of how light travels in skin. Blue light only goes a fraction of a millimetre into skin before it is scattered back out. Red light goes much deeper, up to about 5 to 10 mm, where the blood in a vein soaks much of it up. So from the spot above a vein, less red light comes back than from the skin around it, while the blue comes back as usual. Your brain compares that patch with the warmer skin around it and sees it as bluish.
No. Human blood is always red. Blood that is full of oxygen, in your arteries, is bright cherry red. Blood that has handed oxygen out, in most of your veins, is a darker red. It does not turn from blue to red when it touches air; that is a myth.
Mostly because veins look blue through skin, and because diagrams and textbooks often colour veins blue and arteries red to tell them apart. Oxygen-poor blood is actually dark red.
Vein colour depends on your skin, how deep and wide the vein is, and how your brain compares it with the skin around it. Under some skin colours veins look greenish, and on darker skin they may be harder to see at all. It is the same red blood underneath.
Veins stand out more when they are close to the surface and when the skin is thinner or lighter, because there is more contrast between the vein and the skin around it. If your veins suddenly change, bulge, hurt or swell, talk to a doctor.
Most arteries lie deeper in the body than the veins just under your skin, and they have thick, muscular walls because blood is pushed through them at higher pressure. So the vessels we notice on our hands and arms are veins.
Yes. Octopuses, squid, snails, spiders and horseshoe crabs carry oxygen with hemocyanin, a protein that holds copper instead of iron. It is blue when it carries oxygen and almost colourless without it.