A person has high blood pressure when the systolic pressure reading is 140 or above and the diastolic pressure is 90 or above, according to WebMD. Both numbers need not be over the limit, as one or the other indicates high blood pressure.
When the heart beats, it pushes blood through the arteries and exerts force on the arterial walls, according to WebMD. This is the first number on a blood pressure measurement, called systolic pressure. When the heart is at rest, less pressure is put on the arterial walls. This is the second number, called diastolic pressure.
Too much pressure can cause blood vessels to bulge, according to Mayo Clinic. The blood vessel may even burst, which can be a life-threatening condition.