Systolic blood pressure is the force your blood exerts against artery walls when your heart contracts and pumps, while diastolic blood pressure is the pressure that remains when your heart relaxes between beats. When you see a reading like 120/80, the top number is systolic and the bottom number is diastolic, both measured in millimeters of mercury (mmHg). Together, they give a picture of how hard your cardiovascular system is working.
What Happens During Each Phase
Your heart cycles between two states roughly once per second. During systole, the muscular walls of the lower chambers (ventricles) squeeze together. Pressure builds rapidly until it exceeds the pressure already in your arteries, at which point the valves leading to your aorta and pulmonary artery snap open and blood surges out. This burst of pressure is your systolic reading.
During diastole, the ventricles relax. The outflow valves close as blood tries to flow backward, and the valves between the upper and lower chambers open, letting blood pour back into the ventricles from above. The pressure in your arteries drops to its lowest point during this filling phase. That resting pressure is your diastolic reading.
How the Two Numbers Behave Differently
Systolic pressure is the more reactive number. Anxiety, caffeine, and physical exertion all cause immediate, temporary spikes. During intense cardiovascular exercise, systolic pressure can climb to 200 mmHg or higher. Diastolic pressure, by contrast, stays relatively stable throughout the day. It doesn’t fluctuate much in response to outside influences, and during cardio exercise it may actually drop slightly.
Age affects the two numbers in opposite directions. As you get older, arteries naturally lose flexibility and become stiffer. After about age 50, systolic pressure tends to keep rising while diastolic pressure gradually drifts downward. People with diabetes or chronic kidney disease tend to experience this arterial stiffening earlier. In younger adults, high diastolic pressure is more closely linked to increased resistance in smaller blood vessels, while high systolic pressure reflects stiffness in the aorta itself.
What the Numbers Should Be
The 2025 blood pressure guidelines classify readings into four categories:
- Normal: below 120 systolic and below 80 diastolic
- Elevated: 120 to 129 systolic with diastolic still below 80
- Stage 1 hypertension: 130 to 139 systolic or 80 to 89 diastolic
- Stage 2 hypertension: 140 or higher systolic or 90 or higher diastolic
Notice the word “or” in the hypertension stages. Only one number needs to be elevated for the diagnosis to apply. A reading of 145/78, for instance, qualifies as stage 2 hypertension even though the diastolic number looks fine.
Readings above 180 systolic or above 120 diastolic are considered markedly elevated and can signal a hypertensive emergency if there are signs of organ damage, such as chest pain, vision changes, or severe headache.
Which Number Matters More
Both matter, but they carry somewhat different weight depending on your age. For people over 50, systolic pressure is generally the stronger predictor of heart attack and stroke, likely because of the greater force placed on artery walls each time the heart pumps. For younger adults, the picture is more nuanced. A large study published in Circulation found that among young adults, having both numbers elevated (stage 1, with systolic 130 to 139 and diastolic 80 to 89) carried a 67% higher risk of cardiovascular events compared to normal blood pressure. Having only systolic or only diastolic elevated carried a lower but still meaningful increase of about 32 to 36%.
The same study found that women faced higher relative cardiovascular risk than men at nearly every stage and subtype of hypertension, with women-to-men risk ratios ranging from 1.14 to 1.46.
The Gap Between the Numbers
The difference between your systolic and diastolic readings is called pulse pressure. If your blood pressure is 130/80, your pulse pressure is 50 mmHg. A normal pulse pressure sits around 40 mmHg.
A widening gap can be a sign of arterial stiffness, a leaky or narrowed aortic valve, or atherosclerosis. Every 10 mmHg increase in pulse pressure raises the risk of coronary artery disease by about 23%. Anything above 100 mmHg is considered truly widened. Because systolic pressure rises and diastolic pressure falls with age, pulse pressure naturally increases over time, which is one reason cardiovascular risk climbs in older adults.
How Blood Pressure Is Measured
When a cuff inflates around your arm, it temporarily cuts off blood flow. As the cuff slowly deflates, the first tapping sound heard through a stethoscope marks the moment blood begins pushing past the cuff. That’s your systolic pressure. As the cuff continues to release, the sounds eventually disappear entirely. The point where silence returns is your diastolic pressure. Automated home monitors use sensors instead of sound, but they’re detecting the same two transition points.
Long-term factors that push both numbers up include smoking, excess body weight, high salt intake, certain medications, hardening of the arteries, and chronic stress. Unlike short-term spikes from exercise or caffeine, these influences create sustained elevation that gradually damages blood vessels over months and years.