What Is the Difference Between Systolic and Diastolic?

Systolic blood pressure is the peak force your blood exerts against artery walls when your heart contracts and pushes blood out; diastolic blood pressure is the lower, resting force that remains between beats while the heart refills. On a reading of 120/80, the top number (120) is systolic and the bottom (80) is diastolic. That much is straightforward, but the two numbers are not just a “high point” and “low point” of the same thing. They reflect different aspects of your cardiovascular system, respond differently to aging and stress, and carry different clinical weight depending on how old you are.

What Creates Each Number

Every heartbeat has two phases. During systole, the left ventricle contracts and forces blood into the aorta, the body’s largest artery. The pressure spike that occurs at that moment is your systolic reading. During diastole, the ventricle relaxes and refills with blood from the lungs. No new blood is being pushed out, so pressure in the arteries falls. The lowest point it reaches before the next contraction is your diastolic reading.

A key reason diastolic pressure stays as high as it does, rather than dropping to zero, is that the aorta acts like an elastic reservoir. During systole, the aorta stretches to absorb roughly half the blood the heart ejects. Then, during diastole, the elastic walls of the aorta recoil and push that stored blood forward into the rest of the circulation. This creates a nearly continuous flow of blood to your organs even though the heart pumps in bursts.¹ Without that buffering effect, systolic pressure would spike much higher and diastolic pressure would plummet between beats.

How a Blood Pressure Cuff Captures Two Separate Numbers

When a cuff inflates around your arm, it compresses the artery completely, stopping blood flow. As the cuff slowly deflates, there is a moment when pressure in the cuff drops just below your systolic peak. At that instant, a small spurt of blood forces through the compressed artery during each heartbeat, creating a tapping sound. Those tapping sounds, called Korotkoff sounds, mark the systolic reading. As the cuff continues to deflate, the sounds change character because the artery is open for a longer portion of each cycle. When the cuff pressure drops below the diastolic level, the artery stays fully open the entire time and the sounds disappear. That disappearance marks the diastolic reading.²

Automated cuffs used at home work slightly differently, detecting oscillations in cuff pressure rather than listening for sounds, but they are calibrated to produce numbers that correspond to the same systolic and diastolic thresholds.³ One practical wrinkle worth knowing: the systolic pressure measured at your upper arm can run several millimeters of mercury higher than the pressure actually present in the aorta itself, because pressure waves amplify as they travel to smaller arteries. The gap averages about 8 to 9 mm Hg and tends to be larger in younger, more elastic arteries.⁴ This is why researchers sometimes measure “central” blood pressure at the aorta for a more precise picture, especially in studies of arterial stiffness.

Which Number Matters More for Your Health

The short answer is both, but their relative importance shifts with age. Data from the Framingham Heart Study showed a clear pattern: in people under 50, diastolic pressure was the strongest predictor of coronary heart disease risk. Between 50 and 59, the two numbers were roughly equal. From age 60 onward, systolic pressure became the dominant predictor, and diastolic pressure actually showed a negative relationship with risk, meaning very low diastolic readings in older people were not reassuring but instead signaled widened pulse pressure, which is a marker of stiff arteries.⁵

A similar age-dependent pattern shows up for stroke risk. A large European analysis found that both systolic and diastolic readings predicted stroke until about age 62, but systolic pressure became the stronger predictor from the late 40s onward and remained significant longer. The researchers also flagged that very low diastolic pressure in older adults could itself be harmful.⁶

The practical takeaway: if you are younger, do not dismiss a high diastolic number just because your systolic looks fine. And if you are older, focus on systolic but also watch for a diastolic that has dropped unusually low, because the gap between the two numbers carries its own meaning.

Pulse Pressure and What Stiff Arteries Do to the Numbers

Pulse pressure is the difference between your systolic and diastolic readings. A reading of 130/70 gives a pulse pressure of 60. In younger people with healthy, elastic arteries, pulse pressure tends to stay in the 30 to 50 range. As arteries stiffen with age, the pattern described above takes hold: systolic pressure climbs, diastolic pressure drops, and pulse pressure widens.

The mechanism is straightforward. A stiff aorta cannot stretch to absorb the surge of blood during systole, so the systolic peak goes higher. And because the aorta cannot recoil as forcefully during diastole, the pressure floor drops lower. On top of that, the pressure wave generated by each heartbeat travels faster through rigid arteries, bounces back from branch points sooner, and arrives back at the heart while it is still contracting rather than after it has relaxed. That reflected wave adds to the systolic peak and subtracts from the diastolic trough.⁷ It is a self-reinforcing cycle: higher systolic pressure further damages and stiffens the arterial walls.⁸

A wide pulse pressure is itself an independent marker of cardiovascular risk, separate from either systolic or diastolic pressure alone.⁹ This is why doctors do not just look at whether your numbers are above or below a threshold. Two people can both have a systolic of 140, but the one with a diastolic of 60 is in a different vascular situation from the one with a diastolic of 90.

Isolated Systolic Versus Isolated Diastolic Hypertension

Not everyone with high blood pressure has both numbers elevated. Isolated systolic hypertension, where the top number is high but the bottom is normal, is overwhelmingly common in older adults and tracks closely with arterial stiffening. Isolated diastolic hypertension, where only the bottom number is elevated, is a different animal. It tends to show up in younger adults and reflects a different hemodynamic profile.

Research comparing the two subtypes has found something interesting: despite having lower arm-measured systolic readings, people with isolated diastolic hypertension can have central aortic systolic pressures that are comparable to those of people with isolated systolic hypertension. The difference in arm readings comes from how much the pressure wave amplifies between the aorta and the arm, which is greater in the isolated systolic group. The two subtypes also produce distinctly different pressure waveform shapes.¹⁰ This is a good example of why both numbers deserve attention: a “normal” systolic reading at the arm can mask elevated central pressure if the diastolic number is high.

How Stress and Sleep Move Each Number

Acute stress raises both systolic and diastolic pressure, but systolic tends to jump more. An ecological momentary assessment study that tracked people in real time found that moments of acute stress were associated with an average systolic increase of about 1.5 mm Hg and a diastolic increase of about 0.8 mm Hg, with larger bumps when the stressor felt more severe than usual.¹¹ These are average effects spread across many readings throughout the day; in-the-moment spikes from intense stress can be much larger. How quickly your diastolic pressure rises at the start of a stressor also appears to influence whether your nervous system dials back its stress response via a reflex loop involving pressure sensors in the arteries.¹²

During sleep, both numbers normally dip. Healthy sleepers see about a 10 to 20 percent drop in systolic pressure overnight. People who sleep poorly show a blunted dip; in one study, those with low sleep efficiency had a nighttime systolic dip of around 10 percent compared to 14 percent in good sleepers.¹³ Interestingly, diastolic pressure can be selectively affected by psychological habits. People who score high on trait rumination, the tendency to replay stressful thoughts, show blunted nighttime diastolic dipping specifically, without a corresponding effect on systolic dipping.¹⁴ Inadequate nighttime dipping in either number is associated with cardiovascular trouble down the road, including early signs of coronary artery disease.¹⁵

What Happens When You Stand Up

Standing shifts blood downward by gravity, and your body responds quickly to compensate. A study of postural blood pressure changes found that, on average, systolic pressure stayed about the same on standing but varied widely between individuals, with a standard deviation of about 8 mm Hg. Diastolic pressure, by contrast, reliably increased by about 6 mm Hg upon standing.¹⁶ That makes sense: your body constricts smaller arteries to keep blood flowing to your brain, and peripheral constriction shows up more in the diastolic number. The same study found that whether your systolic pressure goes up or down on standing is partly heritable, with genetics accounting for about a quarter of the variation, whereas the diastolic rise appeared to be driven mainly by the reflex response itself rather than genetic differences.

Exercise and the Two Numbers

During physical activity, systolic pressure climbs substantially, sometimes exceeding 200 mm Hg during vigorous effort, because the heart is pumping harder and faster. Diastolic pressure, by contrast, usually stays the same or even falls slightly during aerobic exercise, because blood vessels in working muscles dilate and lower the overall resistance the heart pumps against. If your diastolic pressure rises significantly during a stress test, it can be a sign of stiffer or poorly functioning blood vessels.

The Framingham Heart Study looked at what predicts an exaggerated systolic response during exercise and found it was associated with arterial stiffness, higher resting blood pressure, smoking, higher body mass, and impaired blood-vessel dilation. The diastolic response during exercise was related to different vascular factors, including the amplitude of the forward pressure wave and overall mean arterial pressure.¹⁷ So the two numbers during exercise tell you somewhat different things about vascular health.

Systolic and Diastolic Heart Failure

The terms “systolic” and “diastolic” show up in heart failure as well, and the distinction mirrors the two phases of the heartbeat. About half of all heart failure cases involve reduced pumping ability, formally called heart failure with reduced ejection fraction. In these cases, the heart muscle has weakened and cannot contract forcefully enough to push blood out.¹⁸ The other half involve a heart that contracts normally but does not relax and fill properly, known as heart failure with preserved ejection fraction.¹⁹ In these patients, the filling (diastolic) function is consistently impaired.²⁰

The two types produce similar symptoms, including shortness of breath and fatigue, but they differ in the underlying structural changes and respond to different treatments. The preserved-ejection-fraction type has proven particularly stubborn to treat, partly because the problem is not a weak contraction but a stiff, poorly relaxing ventricle. Recognizing whether the systolic or diastolic phase is the root of the problem is one of the most consequential distinctions in cardiology.

Blood Pressure Shifts During Pregnancy

Pregnancy rearranges the usual relationship between the two numbers. Early in pregnancy, both systolic and diastolic pressure typically drop as blood vessels dilate to accommodate increased blood volume. As pregnancy progresses into the second and third trimesters, both numbers gradually climb back up. By late pregnancy, systolic pressure tends to exceed its pre-pregnancy level, while diastolic pressure returns close to but stays slightly below where it started.²¹

In pregnancies complicated by gestational hypertension or preeclampsia, the circadian rhythm of blood pressure also changes. Healthy pregnant women maintain a normal daily rhythm with overnight dipping, but women who develop complications show altered patterns as early as the first trimester. By the third trimester, differences in the circadian mean for both systolic and diastolic pressure between healthy and complicated pregnancies are large and statistically clear.²²

How a Giraffe Handles the Problem

One of the more vivid ways to appreciate what systolic and diastolic pressure do is to look at an animal that has pushed them to extremes. A giraffe needs to pump blood roughly two meters uphill from its heart to its brain. The result is a systemic blood pressure about double that of similar-sized mammals, driven by a massively thickened heart and reinforced arteriole walls that increase vascular resistance.²³ When a giraffe bends down to drink water, the head suddenly drops below the heart, and distal carotid systolic and diastolic pressures both rise sharply. When the head comes back up, there is a brief drop in pressure lasting five to ten seconds before the system stabilizes.²⁴ The giraffe manages this without fainting or blowing out cerebral blood vessels, thanks to specialized valves and thick-walled vessels in the neck. It is an extreme case of the same systolic-diastolic balancing act your own body performs every time you stand up from a chair.