While it seems logical that reducing any fluid in a body would reduce the pressure does the body regulate it's own pressure after donating blood or is it lowered?
Blood pressure is not significantly affected in the short term. The body has all sorts of homeostatic mechanisms to maintain blood pressure, such as increasing heart rate and constricting blood vessels. However, if these mechanisms are not functioning well (or if someone already has low blood pressure), then it might well reduce.
Interestingly, your haemoglobin (the red oxygen-carrying pigment) level does not reduce immediately, as it is a measure of the concentration. The body replaces the lost fluid more quickly than the haemoglobin, so the level does not reduce until the blood is diluted over the following days (a process called haemodilution). This can have an impact on aerobic exercise.
However, blood pressure is not significantly affected.
CONCLUSION: Among individuals who are hypertensive at initial donation, reductions in blood pressure at subsequent donations appear to result from regression to the mean as opposed to a salutary physiologic process.