Systolic blood pressure is the force your blood exerts against artery walls when your heart beats, and diastolic blood pressure is that force when your heart rests between beats. Together, they make up the two numbers in a blood pressure reading, written as systolic over diastolic (like 120/80). Both numbers matter for your health, but they reflect different phases of your heart’s pumping cycle and carry different risks depending on your age.
What Happens During Each Phase
Your heart works in a two-step rhythm, squeezing and relaxing roughly 100,000 times a day. During the squeeze (systole), your heart’s lower chambers contract and push blood out into your arteries. This is the moment of peak pressure, and it produces the top number in your reading. A healthy heart ejects blood forcefully enough to reach every organ, but not so forcefully that it damages the artery walls over time.
During the rest phase (diastole), the heart muscle relaxes and the lower chambers refill with blood from the upper chambers. Pressure in the arteries drops but doesn’t fall to zero because your blood vessels maintain a baseline level of tension. This resting pressure, the bottom number, matters more than many people realize: it’s the phase when your coronary arteries receive most of their own blood supply. If diastolic pressure drops too low, the heart muscle itself can become starved for oxygen, especially if there’s already some narrowing in the coronary arteries.
What the Numbers Mean
The 2025 guidelines from the American Heart Association and American College of Cardiology define four categories based on readings taken in a healthcare setting:
- Normal: below 120 systolic and below 80 diastolic
- Elevated: 120 to 129 systolic and below 80 diastolic
- 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 two different categories, the higher category is the one that applies. So a reading of 135/75 counts as Stage 1 hypertension because of the systolic number, even though the diastolic number is normal. Readings at or above 180/120 are considered a hypertensive crisis and can cause symptoms like chest pain, blurred vision, and confusion.
Which Number Matters More
The answer depends on your age. A large European study tracking apparently healthy adults found that both numbers contribute to stroke risk up to about age 62. Starting around age 47, though, systolic pressure begins to outweigh diastolic pressure as a predictor of stroke. After 62, systolic pressure is clearly the dominant risk factor.
This shift happens because arteries stiffen with age. Young, flexible arteries expand when the heart pumps and spring back between beats, which keeps systolic pressure from climbing too high and maintains healthy diastolic pressure. As arteries lose elasticity, they can’t absorb that force as well. Systolic pressure rises, diastolic pressure often falls, and the gap between the two numbers widens.
That gap has a name: pulse pressure. You can calculate it by subtracting the bottom number from the top number. A reading of 140/70, for example, gives a pulse pressure of 70. A wider pulse pressure generally signals stiffer, more damaged blood vessels and is associated with greater cardiovascular risk in older adults.
When Only One Number Is High
It’s common for just one of the two numbers to be elevated while the other stays normal, and these patterns tend to show up in different age groups.
Isolated systolic hypertension, where the top number is high but the bottom number is normal, is especially common in people over 50. It’s driven by the age-related arterial stiffness described above. Plaque buildup in artery walls makes them rigid, so they can’t flex to absorb the surge of blood with each heartbeat. This isn’t a benign quirk of aging. Without treatment, it raises the risk of heart attack, stroke, chronic kidney disease, and death from cardiovascular disease.
Isolated diastolic hypertension is the opposite pattern: a high bottom number with a normal top number. It affects about 6 in 100 adults in the U.S. and tends to show up in people younger than 55. As you age, diastolic pressure naturally trends downward while systolic pressure trends upward, so this pattern is more of a younger person’s condition. Obesity, sleep apnea, and smoking are three of the biggest risk factors. While it’s generally less immediately dangerous than isolated systolic hypertension, it still raises long-term risk of heart attack, heart failure, and cardiovascular death. Those risks are greatest for women and people under 60.
Getting an Accurate Reading
Blood pressure fluctuates throughout the day, so technique matters. Small errors in positioning can shift your reading by 10 points or more, enough to push a normal reading into the elevated range or mask a problem. The CDC recommends the following steps:
- Sit with your back supported for at least five minutes before the reading.
- Keep both feet flat on the ground with your legs uncrossed.
- Rest the arm wearing the cuff on a table at chest height.
- Place the cuff on bare skin, not over clothing, and make sure it’s snug without being too tight.
- Don’t talk while the reading is being taken.
If you’re monitoring at home, take readings at the same time each day, ideally in the morning before medication and in the evening. Two or three readings a minute apart, then averaging them, gives a more reliable picture than a single measurement. Your healthcare provider will typically base any diagnosis on the average of multiple readings taken on separate occasions, not a single visit.
Why Both Numbers Deserve Attention
It’s tempting to focus on whichever number looks worse, but the two readings tell you different things about your cardiovascular system. Systolic pressure reflects how hard your heart is working and how flexible your large arteries are. Diastolic pressure reflects the resistance in your smaller blood vessels and determines how well your heart muscle gets its own blood supply between beats. A problem with either one, or with the relationship between them, can signal trouble. Tracking both numbers over time gives you and your provider a much clearer picture of where your cardiovascular health is heading than any single reading ever could.