Dear Matthew Alice: Help me! I want to be a blood donor, but the past two times I've tried, they could get only half a bagful out of me. When I was in the service, ol’ Uncle Sam insisted on hitting me up for a vial of blood practically anytime I had to go to the base clinic. Because of that, the veins in the crook of my elbows don’t stand up to prodding real well. My query is this: why can’t blood be taken from some other area of the body for the purpose of donating? How much tougher could it be to take it, say, from behind the knee or from the neck? (Hey, if it works for vampires...)— Liz K. Richards, Normal Heights

Ol’ M.A. applauds your generosity, especially during the summer donor doldrums, when blood supplies are low. If the San Diego Blood Bank only cared about extracting some juice, they could tap you like a beer keg or a maple tree from any number of vessels. But medical needs and public accommodation dictate the site.

To collect useable, undamaged cells, to keep blood flowing smoothly, and to prevent clotting, the procedure requires a fairly large-bore needle. (Oh, stop saying eeeuuuwww, you babies.) The site should be as close to the heart as possible, because blood pressure and pulse aid gravity in the collecting process. Donor comfort (quick, painless access) requires the target to be a large vessel close to the surface, where skin is relatively thin. The inside of the elbow best fits these criteria. Any other site might even require a physician to insert the needle. It’s hard enough to get us wimps to donate under the present comfortable circumstances. Imagine how we’d take to the prospect of being lanced in the jugular. Besides, we’ve all had blood samples drawn from our arms and lived to tell about it, so there’s a certain familiarity factor at work, too.