Blood pressure is expressed in millimeters of mercury, or mm Hg. High blood pressure is defined as a systolic blood pressure reading greater than 140 mm Hg or a diastolic blood pressure reading greater than 90 mm Hg. The systolic blood pressure is the top number of a blood pressure reading. This shows the maximum pressure in the blood vessels. Pressure is highest as the heart contracts and circulates blood throughout the body. The diastolic blood pressure is the bottom number of a blood pressure reading. It shows the lowest pressure in the blood vessels. Pressure is lowest between heartbeats, when the heart is at rest.

The National Institutes of Health has further defined high blood pressure. These categories are for people 18 years and older who do not take medication for high blood pressure and do not have a short-term serious illness.

• Normal blood pressure is a systolic blood pressure of less than 130, and a diastolic pressure of less than 85.
• High normal blood pressure is a systolic blood pressure of 130 to 139, and a diastolic pressure of 85 to 89.
• Stage 1 high blood pressure is a systolic blood pressure of 140 to 159, and a diastolic pressure of 90 to 99.
• Stage 2 high blood pressure is a systolic blood pressure of 160 to 179, and a diastolic pressure of 100 to 109.
• Stage 3 high blood pressure is a systolic blood pressure of 180 or higher, or a diastolic blood pressure of 110 or higher.

For a more in-depth understanding of high blood pressure as well as the causes and risks of the condition go to the Diseases & Conditions Encyclopedia.