When a medical professional measures blood pressure, he or she is looking for signs that the patient's circulatory system is behaving in a way that could indicate the presence of disease, or point to the likelihood of future illness. Traditionally, two measurements were the most important results from this test, the pressure of the circulatory system when the heart contracts, and the pressure when the heart relaxed. Another measurement that may also be useful in medicine, which is a relatively new concept, is the difference in pressure between the two, called the pulse pressure.
Typically, blood pressure is measured in units called millimeters of mercury. This is because a common blood pressure test uses a piece of equipment called a mercury sphygmomanometer, which is commonly called a "blood pressure cuff." This cuff wraps around a patient's arm, exerts pressure on the mercury meter attached to the cuff, and the doctor reads the pressure of the meter as the pressure of the blood inside the arm.
The circulatory system inside a human body works because the heart is a central pump providing pressure to the blood, moving it through the arteries and veins. All the blood in the body passes through the heart at some point, so the heart can push it onward, through the blood vessels. When the heart squeezes itself, it pushes the blood inside the heart onward. This step produces a blood pressure called systolic, which is the highest of the two blood pressure readings.
After each squeeze, the heart has to relax. During the relaxation, incoming blood can enter the heart, ready for the next squeeze. A doctor measures the lessened pressure through the blood vessels in the arm during the relaxation phase as diastolic blood pressure. A healthy adult typically has a diastolic pressure of below 80 mm of mercury (Hg) and a systolic pressure of under 120 mm Hg.
Values of blood pressure that are too high or too low can indicate the presence of a medical problem. Doctors may recommend that someone with a high blood pressure, such as a diastolic pressure of over 90 mm Hg and a systolic pressure of more than 140 mm Hg, make lifestyle changes and even take medication to improve cardiovascular function, in case of heart disease. Typically, blood pressure rises with age, due to thickened arteries and a decreased ability for blood to move around the body.
Pulse pressure is a number, also expressed in mm Hg, that may be of use to doctors in diagnosis of circulatory problems. It refers to the difference in pressure between the diastolic and systolic measurements. As of 2011, medical professionals in general cannot state exactly which measurements are most useful in diagnosis, although pulse pressure is potentially useful.
A healthy pulse pressure, as of 2011, ranges from 30 to 40 mm Hg. This falls neatly into the normal limits for systolic and diastolic pressures, which are less than 120 mm Hg, and 80 mm Hg, respectively. A pulse pressure of more than 40 mm Hg is an indication of damaged blood vessels, or malfunctioning heart valves. A low pulse pressure, such as under 30 mm Hg, may be a sign of a heart that cannot pump blood properly.