Yesterday I donated blood for the American Red Cross for the first time. It was pretty neat; it didn’t hurt at all and I didn’t suffer any ill effects. It got me wondering though – they took a little more than a pint of blood from me, I think, a pint into the bag that they actually use, then a couple of vials to test for AIDS and whatnot. How long does it take my body to completely replenish this blood and be back to normal?
I’ll just answer my own question: “you will have given about a pint of blood when finished. Your body will replace the plasma (liquid part) in hours and the cells in a few weeks” (Red Cross).
Ok then, slight change of question: is there any effect of having fewer cells for that time period?
I give blood three times a year, and have never experienced any negative effects.
I’ve heard that some people experience dizziness and lethargy for a brief time.
I guess it varies with the individual.
Back in high school, I once gave blood in the morning and then ran in a cross-country meet in the afternoon. It was stupid and not too fun.
Nowadays it’s fun to give blood and then go drinking (also stupid, but at least it’s fun), as your BAC will be greatly affected by each drink. So it takes less to get a little tipsy.
Neighbor of a good friend of mine had given blood on a regular basis his whole life. Maybe every 8 weeks, I think, but I could be wrong about that.
Anyway, he never, ever, ever missed his blood-giving appointment, in something like 40 years of giving blood.
Well, when my friend knew him, he was already an old man, in this 70’s. And the blood people or his doctor told him that he should stop giving blood because he was too old, it wasn’t good for him, whatever. So he did. Or rather, he tried.
A week or two past the time he would usually give blood, he became extremely ill. Unfortunately, I heard about this years ago so I don’t remember exactly the nature of his symptoms. But he felt like shit and ended up at the doctor.
Well, they determined that what was wrong with him: ** too much blood!** Yep, no lie! His body had become so adapted to producing an extra pint on a regular, fixed schedule, that it went ahead and made the extra pint even though he hadn’t lost it! Isn’t that amazing??
Epilogue: he kept giving blood. 
stoid
Too much blood is a condition called polycythemia vera or hemochromatosis (too much iron). One of the recommended therapies is blood donation. Women are less prone to hemochromatosis (which is fairly common) because of menstrual bleeding.
The half life of a red blood cell is 120 days. That is the speed at which they are normally replenished as well, although in cases of trauma or disease, this will of course go up. Your kidney releases a chemical called epoeitin that stimulates your bone marrow to start making more blood. Epoeitin (or EPO) is what some cyclists and long distance runners use to “dope” their blood. The legal way to “dope” your blood is to train at high altitude, which will increase the number of RBCs you have because there is less oxygen around. Another way it is done is to bank your blood a few weeks before a race and then give yourself a transfusion before the race. EPO is also used in patients who have some forms of anemia where they don’t make or respond to their endogenous epoeitin normally. This often happens in kidney disease or during cancer chemotherapy.