QUESTION: Blood is red, so why do veins in your arm look blue? (Asked by a curious elementary school student.)

REPLY: This is an old one. Yes, blood is red, but how red depends on where you find it. Let’s look at the red blood cells that account for this. These cells, which make up about 40 percent of the blood, have an oxygen-carrying pigment called hemoglobin. When red blood cells pass through the vessels in the lungs, the hemoglobin combines with oxygen and forms oxyhemoglobin, which gives the blood a bright red color.

The oxygenated blood is sent out through the arteries and goes to the tissues and into tiny blood vessels called capillaries. A healthy pink skin (if not covered up by a tan) is evidence of the red blood in the capillaries of the skin. And, if you are embarrassed and the capillaries in your face dilate so more blood flows, you really turn red – that’s called blushing.

When the oxygen is distributed to the cells, the hemoglobin then has a dark color with a purplish or bluish hue. Out of the capillaries the blood goes, and into veins for the trip back to the heart and lungs. (If you’ve ever given blood, which is taken from a vein, you know how dark venous blood is.)

The biggest veins within the body tissue cannot be seen, but there are some fairly large veins just under the skin, for example, in the forearm. These have a bluish appearance because of the deoxygenated blood passing through... no, you’re not a blue-blood.

C.P.S. (Curious Postscript): “Science fiction is no more written for scientists than ghost stories are written for ghosts.” -- Brian Aldiss

Curious about something? Send your questions to Dr. Jerry D. Wilson, College of Science and Mathematics, Lander University, Greenwood, SC 29649, or e-mail jerry@curiosity-corner.net. Selected questions will appear in the Curiosity Corner. For Curiosity Corner background, go to www.curiosity-corner.net.