Blood pressure is the force exerted against the walls of the arteries when blood flows through them. As the heart contracts to send more blood to the arteries, the force reaches a maximum, known as systolic blood pressure, which is the top number in the reading. When the heart stops contracting to fill with blood again, the pressure inside the arteries falls back to a minimum, which is the diastolic blood pressure, or the bottom number of the reading. When a pressure measurement is taken in the upper arm or wrist, the measurement device inflates until the pressure generated around the artery exceeds the pressure inside the artery, causing the artery to collapse. The measuring device is then slowly deflated to the point where the pressure around the artery falls below the pressure inside the artery while the heart contracts, but exceeds the pressure in the artery when the heart relaxes, at which point in which the artery opens when the heart contracts. but it collapses when the heart stops contracting. The audible sound detected at this point whether the reading is being taken manually or the signal that is generated from an automatic device at this point is the collapse of the artery from its expanded state immediately at the end of the contraction of the heart, which is the pressure. systolic arterial As deflation continues and the pressure around the artery continues to drop, a point is reached where the pressure generated around the artery by the device and the pressure inside the artery when the heart relaxes are equal, which which causes the artery to stop collapsing and the sound stops being produced. The beginning of this silent period in which a sound is no longer heard and a signal is no longer detected by an automatic device is the diastolic blood pressure.

Normal resting blood pressure is a systolic reading less than 120 but greater than 90 and a diastolic reading less than 80 but greater than 60. Although there are trends of aging, these numbers do not change in defining what is normal. Hypertension is defined as a systolic blood pressure of 140 or higher and a diastolic blood pressure of 90 or higher. This definition of hypertension was arrived at because studies have shown that damage to vessels and related organs caused by elevated resting blood pressure does not begin to occur until pressures reach these levels. What were previously considered gray areas between systolic readings of 120 and 140 and diastolic readings of 80 and 90 are now the ranges that define what is known as prehypertension, meaning readings between 120/80 and 140/90. The reason 120/80 is an abnormal reading is that studies have shown that many people with a reading of 120 or higher will eventually develop hypertension over time. In fact, one study showed that virtually 100% of people with blood pressure readings of 120/80 will develop hypertension if they live to be 90 years of age or older. More realistically, this explains why two-thirds of the population over the age of 65 have high blood pressure. Low blood pressure is usually not a concern for most people, but if a person is hypotensive, meaning a systolic reading of 90 or less or a diastolic reading of 60 or less, symptoms of dizziness and/or fainting and may be representative of an underlying disease process that needs evaluation and treatment. Both abnormally high and abnormally low blood pressure readings must be recognized and appropriately addressed because either can be associated with morbidity.