So. I gave blood yesterday – my first time because my iron has always been too low before. I weighed 2.5 pounds less this morning than I did yesterday morning. My cow-orker remembers hearing that the body uses up a ton of calories rebuilding blood volume.
Here are my questions:
1 - How long does it take to replace lost blood volume.
2 - Does it really burn calories to do it? If so, how much?
Thanks all. My Google-fu just ain’t that strong.
Anyone?
Hello? hellohellohellohello
IANA medical professional, but I have donated blood a lot. Blood volume is easy to recover - drink, and the fluid will make up for the lost volume. On the other hand, it takes a while for your body to make up for the lost blood cells. Googling gives a bunch of sites that say that it takes 3-4 weeks, but I don’t have anything particularly authoritative on that front.
The Perfect Master says that donating a unit of blood is the equivalent of losing about 600 calories, or the amount that it takes to make about 1/6 of a pound. I don’t think that the whole pint, even including all the water that makes up most of your plasma, weights 2.5 pounds. Did you sweat a lot yesterday?
IANAD, just a lab rat. 
The “pint” they take out of you is roughly ≈ 500 mLs. Converted to weight that’s roughly ≈ 500 grams. And once again roughly that’s about 1.1 lbs.
GilaB was correct that the “volume” can be more or less replaced in a couple of hours with adequate hydration. Sure your serum proteins and electrolytes will be slightly wacky (can you tell I love speaking in strictly scientific terms?) but your general equilibrium should return soon enough.
The reason we ask you wait eight weeks to before donating again is to allow your blood cell counts to rebound. After donating your lowered arterial blood pressure triggers your kidneys into releasing a enzyme known as renin, which stimulates your bone marrow to increase blood cell production (it’s more complicated than that, but it’s the basic shtick). Most healthy donors are fine after 2-4 weeks, but to be safe we generally ask you to wait longer.
Thanks for donating!
I thought it was Erythropoietin that was the big factor in signaling RBC prouction?
Either way, great explanation.
Cool. That means I actually lost that weight (I’ve been working on it).
Thanks for the info everyone.
Unless your kidneys were stolen while you were sleeping.
If it’s true that giving blood makes you lose weight I’d be down the blood bank tomorrow! Unfortunately even if it is they don’t take dirty tainted homosexual blood so I can’t (and no I’m not HIV positive).
Aw crap, I’ve been out of school too long and just stuck in the practical lab medicine field. You’re right, renin I believe is the enzyme I sort of skimmed over regarding electrolyte balance in the kidney function. I’m digging out my old school notes and will be right back with corrections.
Thank you for catching that.
ETA:
Don’t get me started on this rant. Please understand that this is an FDA law, and has nothing to do with the Red Cross, or any of your other donation centers. And it pisses off a lot of people in the medical field as well.
It’s the same in the UK. Well it at least lets me feel absolved of any responsibility whenever I read the pleadings of the blood service for people to donate. Personally I don’t understand the rule at all - what is it about being gay that makes your blood so undesirable? Obviously there’s the STD aspect but it’s a very simple matter to get a test to show negative status, why not simply accept that?
Odd.
So donating blood reduces your overall blood volume as well as causing an imbalance normal blood oxygen levels by reducing the number of circulating red blood cells.
As far as replacing the volume that was removed, proper post-donation rehydration is important (drink that free apple juice). And the the enzyme I mentioned earlier, renin is an important part of regulating your electrolytes. The decrease in arterial blood pressure, from the loss of volume causes your kidneys to release this enzyme. This activates angiotensin which leads to vasoconstriction and retention of sodium and water. There, that’s the volume replacement.
The kidneys are involved in the replacement of all those red blood cells you just donated as well. So forgive me for confusing the two enzymes, I was at work and didn’t have my dusty text books available ( :smack: ). A reduction of your normal blood oxygen levels (a slight hypoxia from a decreased RBC count) causes your kidneys to release more erythropoietin, which stimulates your red cell bone marrow into creating more RBC’s. This increases your RBC count, increasing the O2 carrying ability in your blood, correcting your blood oxygen levels.
This is all fantastic since this was never the actual question asked. So scroll back up and read Cecil’s article dammit!