Measurements of blood pressure have two components: one to capture the peak pressure during a heart beat, and the other to capture the resting pressure during the space between heart beats. Both of these measurements are necessary to characterize the blood pressure. Systolic pressure reflects the pressure on the vessel walls when the heart pumps. Diastolic pressure is the pressure on the vessel walls when the heart relaxes.
Blood pressure is measured in a unit called “millimeters of mercury” and abbreviated as mmHg. This unit is based on the original measurements of pressure that resulted from mercury rising in a cylinder. The systolic blood pressure measurement is the higher of the two numbers and is written on top of the diastolic. For example, if one has a systolic of 140mmHg and a diastolic of 80mmHg, it is written as 140/80, and read as "One-forty over eighty".
Normal and Abnormal Blood Pressure
Blood pressure measurements can be classified into four categories depending on how they vary from what is most healthy. The categories in use are: hypotensive, optimal, prehypertensive, and hypertensive. The hypertension category is further subdivided into two subtypes based on severity. Low blood pressure measurements are often variable, and as a category hypotension is mostly only of concern when patients are hospitalized or feeling faint upon standing up or passing out for unknown reasons. For most purposes, therefore, blood pressure readings can be considered optimal, prehypertensive, or varying degrees of hypertensive.
| Pressure Category | Systolic (mmHg) |
Diastolic (mmHg) |
| Optimal | Below 120 | Below 80 |
| Prehypertension | Between 120 and 139 | Between 80 and 89 |
| Hypertension | Above 140 | Above 90 |
| Mild Hypertension | Between 140 and 159 | Between 90 and 99 |
| Moderate to Severe Hypertension | Above 160 | Above 100 |
