What is Normal Blood Pressure?

Blood pressure is the force exerted against the walls of arteries as blood is flowing through them. As the heart contracts to delivery more blood into the arteries the force reaches a maximum, known as the systolic blood pressure, which is the top number of the blood pressure reading. When the heart stops contracting to fill with blood again the pressure within the arteries recedes to a minimum, which is the diastolic blood pressure or the lower number of the BP reading. When a blood pressure measurement is being taken in either in the upper arm or the wrist, the measuring device is inflated until the pressure generated around the artery exceeds the pressure within the artery resulting in collapse of the artery. The measuring device is then slowly deflated to the point that the pressure around the artery falls below the pressure within the artery while the heart is contracting but exceeds the pressure in the artery when the heart relaxes, at which point the artery opens when the heart contracts but collapses when the heart ceased to contract. The audible sound detected at this point if blood pressure is being measured manually or the signal that is generated from an automated device at this point is the collapsing of the artery from is expanded state immediately at the end of contraction of the heart, which is the systolic blood pressure. As the deflation continues and the pressure around the artery continues to drop, a point is reached at which the pressure generated around the artery by the device and the pressure within the artery when the heart relaxes are equal resulting in the artery ceasing to collapse and the sound ceasing to be produced.  The beginning of this period of silence in which a sound is no longer heard and a signal is no longer sensed by an automated 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 of less than 80 but greater than 60. Although there are trends characteristic of aging these numbers don't change with respect to defining what is normal. High blood pressure or hypertension is defined as a systolic pressure of 140 or greater and they diastolic blood pressure of 90 or greater. This definition of hypertension was reached by virtue of the fact that studies have shown that the damage to vessels and related organs caused by elevated resting arterial pressure does not begin to occur until pressures reach these levels. What used to be gray areas between systolic blood pressure readings of 120 and 140 and diastolic readings of 80 and 90 are now the ranges that define what is known as pre-hypertension, i.e. readings between 120/80 and 140/90. The reason 120/80 is an abnormal reading is that studies have shown that over time many people with a reading of 120 or higher will eventually develop hypertension. 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 more. More realistically, this explains why two thirds of the population greater than 65 years of age has high blood pressure. 

Low blood pressure generally is not a concern of most people, but if a person has hypotension, i.e. a systolic reading of 90 or less or a diastolic reading of 60 or less symptoms of lightheadedness and/or fainting can occur and may be representative of an underlying disease process in need of evaluation and treatment. 

Both abnormally high and abnormally low blood pressure readings need to be recognized and appropriately addressed because either can be associated with morbidity. Frequent home blood pressure monitoring is probably the most cost-effective way of recognizing high blood pressure and thwarting the untoward outcomes associated with high and low blood pressure.