Not exactly my field, but IIRC blood pressure is not a passive result of how much blood is present, but rather is regulated by the body through a series of cont

Not exactly my field, but IIRC blood pressure is not a passive result of how much blood is present, but rather is regulated by the body through a series of control networks. Removing blood does not reduce blood pressure (the body recognizes the dip and will adjust, plus more blood is made again quickly) because the problem is not that there is too much blood, but rather that the pressure setpoint is wrong. Things like atherosclerosis reduce the space available for blood and increase the pressure without the body realizing the setpoint needs to change, thus why pressure increases.