Biddy,

Thanks for your question. 

Though there are parameters for high blood pressure, there are no set parameters for low blood pressure.  People tolerate lower pressure differently.  As long as a person has no symptoms with low pressure, doctors do not consider it a problem.  Usual symptoms of low pressure include lightheadedness, dizziness, vision changes, nausea, vomiting, and sometimes loss of consciousness.  These symptoms are accentuated with changes in body position, such as sitting up from a lying position, or standing up from a sitting or lying position.  With low pressure, blood tends to pool in the legs and abdomen leaving less blood to travel to the brain, which causes these symptoms.

If these symptoms occur, it usually indicates another problem such as dehydration, blood loss, anemia, or side effect of blood pressure medication. 

I hope this has been helpful.

Martin Cane, M.D.