Yesterday was my regular every-56-days appointment to donate plasma across the street at Canadian Blood Services.

I’ve come to appreciate donation day, for several reasons.

The first has nothing to do with blood at all: I find the regular marking of each 56 day period a useful milestone — more useful than the passing of the months. It’s like a regular planetary event.

Second, I admire and appreciate the system the whole plasma collecting system, from paperwork through to the equipment used to actually extract the plasma, that Canadian Blood Services has in place: it works well, is designed to have several redundant checks to make sure the right plasma gets labeled as coming from the right person, and gets me in and out in about an hour and a half.

Third, it offers me some comfort to have my weight, blood pressure, and blood chemistry screened every couple of months. No, it’s not a thorough checkup; but if they’re willing to take my plasma, it means I must be at least somewhat healthy.

Finally, I just plain like the staff over there: they are friendly and helpful and dedicated to a fault, always remember my name, and somehow manage to make even the litany of “have you traded money or drugs for sex”-type questions flow with a minimum of embarrassment.

On the wall of the room where I go to have my blood pressure and temperature taken, and to answer the aforementioned set of “high risk behaviours” questions, there’s a photocopied piece of paper that’s a simplified version of this chart of blood types and compatibilities.

I know from my donor sheet that I’ve got O+ — “O positive” — type blood, a type I share with 39% of Canadians, and the most common blood type in the country. As such, I can donate blood to people with type A+, B+, AB+ and O+ blood, and I can receive type O+ and O- blood.

Most interesting — and something that explains a lot of the yelling on the television show er — is that people with type O- blood are “universal donors” in that their blood can be given to people with any blood type. That’s why you hear things like “four litres of O-neg, stat!” on medical shows — in an emergency room, where time is of the essence, and you don’t want to have to wait around to type a patient’s blood, giving them O- blood is a safe bet.

This Wikipedia page tells the whole story, with all the details you could ever want.