Temporarily anyway?
Sure, any decrease of volume within your arteriovascular system will decrease the pressure. You’ll see this on people who are extremely dehydrated due to low plasma volume. We often give people IV fluids to bolster low BP.
Of course it’s not a long term answer. 
Demo
It’s not clear to me that you actually will have any benefit from donating blood – your body isn’t a glass bottle, after all – it’s an organic collection of sacs, with feedback and control mechanisms built in. Whatever was keeping your pressure high would, I suspect, tend to keep it high even after you lost a quantity (as long as it wasn’t too much).
Side note: They won’t let you donate blood if your pressure is too high. I speak from experience here.
Specifically, 180 systolic and 100 diastolic.
Here is some suggestion of a link between donating blood and reduced risk of heart disease, but it includes the idea that donors may live more healthy lives overall.
Yet the overall volume is still decreased, in turn decreasing the BP:
"Immediately after donating a unit of blood, the circulating blood volume is reduced by 9%. This will reduce the blood pressure and make the person more susceptible to fainting, one of the reasons you are escorted and watched closely immediately after donating blood, and why you are fed and forced to drink fluids. The fluid lost by donating blood is replaced in a few hours, however, divers are also prone to dehydration/fainting. "
From http://divermag.com/archives/april2000/divedoctor_apr00.html
I don’t know anything about that source, but the quote above concurs with what I’ve been taught.
I’ll take your word for it that it goes low immediately, as I have no training in this area. I’ll just repeat that decreasing the volume doesn’t automatically lower the pressure in a feedback system or a system in which things can mechanically adjust.-- imagine a flexible bag or, better yet, a cylinder with a piston atop it and a heavy weight. If you draw off a bit of fluid you have reduced the fluid volume, but the weight is still on top keeping the pressure up (you might lose a bit of pressure because of the loss of fluid weight, but I’m assuming the weight atop the piston is much greater). So taking out fluid doesn’t really change the pressure.
Yes, you’re right. The cardio vascular system does have feedback mechanisms to keep the BP at a certain level, such as increasing the heart rate or using vasoconstriction (analogous to your piston example).
I think the key word in the OP is “temporarily”.
I was wondering about this. The following is anectodal evidence from my experience. Two weeks ago I went and had a physical. My diastolic was a little high so they asked to come back in a week and recheck it. During the week I donated blood at Red Cross, 3 days before the appointment. I went back in and my diastolic was down by over 10. Could just be I was more relaxed but donating blood could have helped.
I’ve seen blood pressures rise after blood donations. This is due to the body’s sensing that volume is depleted, and thus tightening vascular tone, heart rate, and cardiac output to compensate, but here overcompensating.
Not if needles give you the heebie-jeebies. :D:D
What actually causes high blood pressure, anyway? Never really understood it.
Great question. The truth is, high blood pressure is not really understood that well. Most of the time the cause is unknown. There are a few cases where things like kidney disfunction or narrowing of aortic arteries can cause it, but that is rare.
Generally it’s handled with medications that either control the heart rate, dilate vessels or use diuretics to decrease water and sodium in the body.
The most common type of hypertension is “essential hypertension”, which means you got it, we don’t know why, there doesn’t seem to be any other disease causing it. Lose weight, eat less salt, exercise more and you’ll do fine. What? You don’t want to do that? OK, here, take these drugs for the rest of your life.
Otherwise, less common causes include renal and thyroid disease, along with connective tissue abnormalities, to list 3 out of a thousand causes of “secondary hypertension”.
Well, I know someone who is 5’2", weighs 93lb and has extremely high BP for which there is no known cause. She walks a lot even if she doesn’t get much other exercise and is on all the known drugs.
True, Hari. I shouldn’t generalize. There are quite a few fit people with essential hypertension. However, they will generally benefit from salt restriction.
Other research in 1999 offered an explanation of the mechanism behind the reduction in health risks.
Not the actual title of the SD column, but anyway.
A quantity of perfumed blessings to your enviorns, Qadgop. I’ve been waiting, indefinitely stalled, to hear such a clear statement of the current recommended practice about this, and hearing little but “Be Good”. Nice to hear that other people can’t figure it out, either. Ta.
Can someone here please give me the one-line idiot’s guide to the difference between systolic and diastolic?