Systolic pressure is the force your blood exerts against artery walls when your heart beats, and diastolic pressure is the force that remains between beats when your heart is resting. Together, they make up the two numbers in a blood pressure reading. A normal reading is below 120/80 mmHg, where 120 is the systolic (top) number and 80 is the diastolic (bottom) number.
What Happens During Each Phase
Your heart cycles through two phases roughly once per second. During systole, the ventricles (your heart’s lower chambers) contract and push blood out into the aorta and the arteries that supply the rest of your body. That surge of blood stretches the artery walls and creates peak pressure, which is the systolic number. In a healthy adult, this peaks around 120 mmHg.
During diastole, the ventricles relax and refill with blood. No new blood is being pushed into the arteries, so pressure drops. But it never falls to zero. Your arteries are elastic, and they recoil between beats to keep blood moving forward. That residual pressure is your diastolic number, typically around 80 mmHg in a healthy adult.
What the Numbers Mean Together
The current guidelines from the American Heart Association and American College of Cardiology 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
If your systolic and diastolic numbers fall into different categories, the higher category is the one that applies. So a reading of 138/78 counts as stage 1 hypertension even though the diastolic number looks fine.
On the low end, a reading below 90/60 mmHg is generally considered low blood pressure. Most providers only treat it if you’re having symptoms, though. A sudden drop of just 20 mmHg in the systolic number, say from 110 to 90, can cause dizziness or fainting.
Why One Number Can Be High While the Other Isn’t
It’s common for only one of the two numbers to be elevated. Which one tends to depend on your age.
In people over 50, the arteries gradually stiffen and lose their natural elasticity. Stiffer arteries can’t absorb the surge of blood during each heartbeat as well, so systolic pressure climbs. At the same time, that lost elasticity means the arteries don’t recoil as effectively between beats, so diastolic pressure actually drifts downward. This pattern, where systolic is 130 or higher but diastolic stays below 80, is called isolated systolic hypertension. It’s the most common form of high blood pressure in older adults and raises the risk of serious cardiovascular problems, shortness of breath during light activity, lightheadedness when standing, and falls.
In younger adults, particularly those under 55, the opposite pattern sometimes occurs: diastolic pressure is elevated while systolic remains normal. This isolated diastolic hypertension may not cause immediate symptoms, but it increases the lifetime risk of heart attack, heart failure, and death from cardiovascular disease. Those risks are greatest for women and people under 60.
Pulse Pressure: The Gap Between the Two Numbers
Subtracting diastolic from systolic gives you your pulse pressure. For a reading of 120/80, that’s 40 mmHg. This number reflects how much stretch your arteries experience with every heartbeat. As arteries stiffen with age (or with conditions like diabetes and chronic kidney disease), systolic rises and diastolic falls, widening the gap. A pulse pressure above 100 mmHg is considered truly widened and signals significant arterial stiffness. Even moderately widened pulse pressure can be an early indicator that arteries are losing flexibility.
What Makes Your Numbers Fluctuate
Blood pressure isn’t fixed. It shifts throughout the day in response to what you’re doing and what’s happening around you. Stressful events, intense exercise, and even worrying about finances can trigger short-term spikes. Certain medications, including common pain relievers like ibuprofen, steroids, and oral contraceptives, can also push readings higher. Over the long term, excess body weight, high sodium intake, and heavy alcohol use are the most consistent drivers of sustained hypertension.
Because of these fluctuations, a single high reading doesn’t necessarily mean you have high blood pressure. Patterns over time matter far more than any individual measurement.
Getting an Accurate Reading
Small details during measurement can swing your numbers by 10 to 15 points, enough to shift you into a different category entirely. The CDC recommends following these steps every time:
- Avoid food and drinks for 30 minutes beforehand
- Empty your bladder before sitting down
- Sit with back supported for at least 5 minutes before the reading
- Keep both feet flat on the floor, legs uncrossed
- Support your arm on a table at chest height
- Place the cuff on bare skin, not over a sleeve, and make sure it’s snug without being tight
- Stay silent while the measurement is being taken
Crossing your legs, talking, or letting your arm hang at your side can all inflate the reading. A full bladder alone can add several points to both numbers. If you’re monitoring at home, taking two readings a minute apart and averaging them gives a more reliable picture than relying on a single check.