Systolic blood pressure is the force your blood exerts on artery walls when your heart contracts and pushes blood out. Diastolic blood pressure is the force that remains when your heart relaxes and refills between beats. Together, these two numbers form your blood pressure reading, written as systolic over diastolic (for example, 120/80 mmHg).
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 inward. Pressure inside the ventricles rises until it exceeds the pressure in the large arteries leaving the heart, at which point the exit valves pop open and blood surges into the aorta and pulmonary artery. This burst of blood is what creates the higher number in your reading.
During diastole, the ventricles relax. The exit valves snap shut so blood doesn’t flow backward, and the intake valves between the upper and lower chambers open. Blood that has been pooling in the atria rushes down to fill the ventricles. Most of the filling happens quickly in the first moment after the valves open, with a slower trickle continuing until the next contraction. Throughout this entire resting phase, there is still pressure in your arteries from the blood that was just pumped out. That residual pressure is your diastolic number.
How the Two Numbers Are Measured
When a clinician wraps a cuff around your arm and inflates it, the cuff temporarily squeezes your artery shut. As air is slowly released, the first moment blood manages to push through the compressed artery produces a tapping sound heard through the stethoscope. The pressure reading at that instant is your systolic number, because only the peak force of a heartbeat is strong enough to break through first. As the cuff continues to deflate, the tapping eventually disappears altogether. The reading at that moment of silence is your diastolic number, representing the lowest pressure between beats. Automated cuffs use sensors to detect the same transition points.
What the Numbers Mean for Your Health
Current guidelines classify blood pressure 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 that either number being too high is enough to qualify as hypertension. You don’t need both to be elevated.
Why Systolic Tends to Rise With Age
Over time, artery walls stiffen as plaque gradually builds up inside them. Stiffer arteries can’t expand and absorb the force of each heartbeat the way flexible ones do, so the peak pressure during contraction climbs. This is why many older adults develop what’s called isolated systolic hypertension, where the top number is high but the bottom number stays normal or even drops. The condition raises the risk of heart attack, stroke, and long-term heart damage.
Why Diastolic Matters Too
High diastolic pressure on its own is less common and tends to show up in people under 55. The most significant risk factors include carrying excess weight, sleep apnea, and smoking. As people age, their diastolic number naturally trends downward while the systolic number rises, which is why isolated diastolic hypertension is primarily a younger person’s concern.
That doesn’t make it harmless. A persistently elevated diastolic number increases the lifetime risk of heart attack, heart failure, and death from cardiovascular disease. These risks are especially pronounced for women and people under 60.
Pulse Pressure: The Gap Between the Two
The difference between your systolic and diastolic numbers is called pulse pressure. If your reading is 120/80, your pulse pressure is 40 mmHg, which is considered a healthy baseline. A pulse pressure that’s significantly wider than 40 often signals stiff arteries. A very narrow pulse pressure can indicate that the heart isn’t pumping forcefully enough. Both extremes give your doctor additional information beyond what either number reveals alone.
Practical Differences at a Glance
- Systolic (top number): measures peak pressure during a heartbeat, tends to increase with age, is the primary driver of cardiovascular risk in older adults
- Diastolic (bottom number): measures resting pressure between beats, tends to decrease after middle age, is more relevant to cardiovascular risk in younger adults
Both numbers matter. A reading where one is normal and the other is elevated still counts as high blood pressure and still carries health consequences. Tracking both over time gives you the clearest picture of how your cardiovascular system is holding up.