Blood pressure reflects the force exerted by circulating blood against the walls of the body’s arteries. This measurement is a primary indicator of cardiovascular health, showing how efficiently the heart and blood vessels are operating. A blood pressure reading is always presented as two distinct numbers, which correspond to the two phases of the heart’s continuous cycle. These values reveal the maximum and minimum pressures within the arterial system during a single heartbeat.
Defining the Two Pressures
The heart operates in a rhythmic cycle alternating between a working phase and a resting phase, each corresponding directly to one of the two blood pressure numbers. The first, higher number is the systolic pressure, which is the maximum force exerted on the arterial walls. This peak pressure occurs during the phase known as systole, when the heart muscle actively contracts, pushing blood out of the left ventricle and into the aorta.
The systolic value reflects the integrity of the large arteries and the pressure generated by the heart’s pump action, indicating its ability to effectively distribute blood. This pressure also shows how well the arteries maintain appropriate elasticity to absorb the sudden surge of blood flow.
The second, lower number is the diastolic pressure, which represents the minimum pressure in the arteries. This occurs during diastole, the heart’s resting phase, when the chambers relax and fill with blood returning from the body. During this period between beats, the pressure in the arteries drops to its lowest point.
Diastolic pressure indicates the resistance within the blood vessels and the pressure maintained when the heart is not actively pumping. This sustained pressure ensures that organs and tissues receive continuous blood flow and oxygenation during the brief resting period.
Understanding the Measurement and Notation
When a blood pressure measurement is taken, the two values are recorded in a specific format, typically written as a fraction, such as 120/80. The systolic pressure is always noted as the first, or top, number, representing the maximum pressure. The diastolic pressure is always the second, or bottom, number, representing the minimum pressure.
The unit of measurement used for both pressures is millimeters of mercury, abbreviated as mmHg. This historical unit refers to the height to which the pressure can push a column of mercury. Although modern measurement devices rarely use mercury, the unit remains the international standard for reporting blood pressure readings.
Both numbers are essential indicators of cardiovascular function. The difference between the systolic and diastolic numbers is known as the pulse pressure. This value provides additional insight into the stiffness and elasticity of the arteries, as arterial walls tend to become less compliant with age.
Blood Pressure Categories and Health Implications
Medical professionals classify blood pressure readings into several categories to determine potential health risks, using the most recent guidelines.
- Normal: Systolic pressure is less than 120 mmHg and diastolic pressure is less than 80 mmHg.
- Elevated: Systolic reading is between 120 and 129 mmHg and diastolic pressure is less than 80 mmHg.
- Stage 1 Hypertension: Systolic pressure is between 130 and 139 mmHg, or diastolic pressure is between 80 and 89 mmHg.
- Stage 2 Hypertension: Systolic pressure is 140 mmHg or higher, or diastolic pressure is 90 mmHg or higher.
Sustained high pressure, medically termed hypertension, forces the heart to work harder. This staging is important because chronic high pressure can cause the heart muscle to thicken, leading to a diminished ability to pump blood effectively over time.
Elevated pressure also damages the inner lining of the arteries, making them less flexible and promoting the buildup of plaque. This damage increases the long-term risk of severe health events, including heart attack, stroke, and kidney disease.
In some cases, a person may experience Isolated Systolic Hypertension (ISH), a condition where the systolic number is elevated (130 mmHg or higher) but the diastolic number remains normal (less than 80 mmHg). This is the most common form of high blood pressure in adults over the age of 50. ISH is often linked to the natural stiffening of large arteries that occurs with age.
Isolated Systolic Hypertension is a risk factor for cardiovascular disease and stroke. Conversely, a reading surpassing 180 mmHg systolic and/or 120 mmHg diastolic is categorized as a hypertensive crisis, which demands immediate medical attention.