Effects of repeated, long-term blood donation

[banespawn]

I'm writing a fantasy novel in which a character has a curse that makes his blood the source of all magic. This curse only afflicts one person at a time. When that person dies, someone else inherits the curse. This has been going on for a thousand years and the current cursed individual has been held prisoner for 15 years, during which time his

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[lolmac]

Former regular blood donor in the US here.

The regulations here are aimed at making it possible for people to continue donating for years without any ill effects. Your blood pressure has to be acceptable (neither too high nor too low). You're given food and fluids immediately afterwards (cookies and juice), and told to take it easy. I never had any worse aftereffects than a little fatigue.

General requirements for health include a weight of 110 lbs.

Here's the donation frequency, from the Red Cross website:
Blood (whole blood) - Every 56 days
Platelets - Every 7 days, up to 24 times / year
Plasma - Every 28 days, up to 13 times / year
Double Red Cells - Every 112 days, up to 3 times / year

That's how long it takes for the body to fully replace and recover, plus a big fat safety margin. There are people who follow this schedule for years and continue donating into their 70s. Here's a guy who hit 40 gallons (320 pints) in 2009: http://www.oprah.com/spirit/Albert-Fischers-Record-Breaking-Blood-Donations.

One of the other commenters pointed out that 15 years of captivity is going to mess up your MC badly -- major point of truth there, especially since he's being milked for blood. Being imprisoned won't help his physical health, which is the basis for continuous recovery from repeated frequent donation. I'd assume that they'll take as much blood as they can as often as they can without compromising him so badly that it endangers their ongoing supply.

On the needle-tracks etc.: your best source might be looking at people who have to get regular transfusions or who shoot up, which would be much more frequent. When I donated constantly, I did have faint needle-tracks; I can still see them if I peer closely enough.