Knives & Such

Theo Walcott is flagged offside, 16th minute:

You Can See Your Own White Blood Cells Flowing Through Your Eye

If you look up into the blue sky long enough, and with enough attention, you should be able to see tiny blue-white dots flashing around the sky. The phenomenon was so well-known that it acquired the nickname “blue-sky sprites”, but they’re actually white blood cells moving through your eye.

White blood cells make up a very small percentage of blood — as little as only 1%. Red blood cells floating in plasma are the bulk of blood. In the tiny blood vessels in the eye, we don’t see blood in bulk. Cells file through the vessel one at a time. The red blood cells block out light, particularly if the light is blue. White blood cells don’t absorb light well, so they let the light shine through.

When these blood vessels are strung over the receptor cells in the retina, the red blood cells block out the sun. That’s no problem. The eye adjusts so we don’t see dark nets across our vision. Because the eye adjusts, the 1% of the time that a white blood cell moves along the blood vessel, it lets the light through. The retina sends a signal of increased brightness to the brain, an, to the viewer, it looks like there is a tiny spot of blue-white light moving through the sky.

The phenomenon itself is interesting, but what’s really intriguing is how scientists figured it out. They couldn’t use human eyes, as people whose eyeballs are being prodded tend not to notice blue spots in the sky.