OCT 31, 2008 – 12:00AM

My list of reasons to ride my bike is considerably longer than my list of reasons to drive, but the latter grew by one the other day: altruism.

Let me explain.

I gave blood Wednesday, and I let the vampires talk me into a double-red-blood-cell donation, or, as it’s innocuously described on the facility’s big board illustrating just how dire the blood straits are in this town, a “2RBC” donation.

Normally I give whole blood, and the side effects are minimal. I tend to be a little light of head for a day or two, but that’s about it.

This time, however, I relented and agreed to the 2RBC.

The difference between the two, I was assured, was small. In the 2RBC, twice as many red blood cells are harvested, but – bonus! – the rest of the blood, plus a little saline for good measure, is returned to the donor.

In your arm, I might add, not a doggy bag.

Quick biology lesson: Red blood cells transport oxygen to the body tissues.

The donor center’s literature pooh-poohed any serious effects, suggesting only high-end aerobic exercise should be avoided, and on the day of donation only.

”If you go for a run,” I was told, “you might feel a little out of breath for a couple of days, but that’s it.”

So I gave in, let ’em stick me and hook me up to a whirling, clicking contraption.

Less than half an hour later, I was out the door and on my bike, feeling pretty much the same as I had going in.

The feeling lasted about two blocks.

The road was flat, and I had a tailwind, yet two blocks away, I was sucking wind. And it didn’t get any better.

So I went home and took a nap.

Afterward, I jumped up and headed to the kitchen for a snack, and … let’s just say it’s a good thing the stove has a handle, because I was headed to the floor faster than a seventh-year senior at a graduation-day kegger.

That night, after a few more near-fainting experiences, I turned in early – about six hours early – and woke up feeling great.

Until I headed to the basement for a half hour on the bike trainer.

The legs felt great, but the lungs felt like they belonged to an asthmatic pack-a-day smoker in Beijing.

So rather than ride as usual to my racquetball game, I drove, and I’m sorta glad I did. I sucked wind through two-plus hours of racquetball before heading home for another nap.

I did ride to work twice Thursday, and I’ve held up OK.

I’ve only nodded off a couple of times at my desk.

And I expect tomorrow will be better still as I regenerate those lovely red blood cells.

Do I regret giving blood? Of course not. It’s the gift of life. And the life I save might be my own.

And all that.

But after my 112-day waiting period, I might go back to twice-as-frequent whole-blood donations instead of the big double-red enchilada.

Now I just need another nap.