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Understanding Blood Pressure>>http://www.heart.org/HEARTORG/Conditions/HighBloodPressure/AboutHighBloodPressure/Low-Blood-Pressure_UCM_301785_Article.jsp

Blood pressure: How low can you go?

What's considered low blood pressure for you may be normal for someone else. Most doctors consider chronically low blood pressure too low only if it causes noticeable symptoms.

Some experts define low blood pressure as readings lower than 90 systolic or 60 diastolic — you need to have only one number in the low range for your blood pressure to be considered lower than normal. In other words, if your systolic pressure is a perfect 115, but your diastolic pressure is 50, you're considered to have lower than normal pressure.

A sudden fall in blood pressure can also be dangerous. A change of just 20 mm Hg — a drop from 110 systolic to 90 systolic, for example — can cause dizziness and fainting when the brain fails to receive an adequate supply of blood. And big plunges, especially those caused by uncontrolled bleeding, severe infections or allergic reactions, can be life-threatening.

Athletes and people who exercise regularly tend to have lower blood pressure and a slower heart rate than do people who aren't as fit. So, in general, do nonsmokers and people who eat a healthy diet and maintain a normal weight.

But in some rare instances, low blood pressure can be a sign of serious, even life-threatening disorders.

Read more here>>http://www.mayoclinic.org/diseases-conditions/low-blood-pressure/basics/causes/con-20032298