Systolic blood pressure is the pressure inside your arteries when your heart beats and pushes blood out. Diastolic blood pressure is the pressure that remains in your arteries between beats, while your heart rests and refills. When you see a blood pressure reading like 120/80, the top number is systolic and the bottom number is diastolic, both measured in millimeters of mercury (mmHg).
What Happens During Each Phase
Your heart works in a two-phase cycle that repeats roughly once per second. During systole, the left ventricle contracts and forces blood into the aorta, your body’s largest artery. That surge of blood pushes against the artery walls, creating peak pressure. In a healthy person, this peaks around 120 mmHg.
During diastole, the heart muscle relaxes and the ventricle refills with blood. Pressure inside the ventricle drops to nearly zero, but the arteries don’t go slack. The elastic walls of your arteries recoil and maintain a baseline pressure of about 80 mmHg in the circulatory system. This residual pressure keeps blood flowing forward even between heartbeats.
Diastolic pressure plays a specific role for the heart itself. Your coronary arteries, the vessels that feed oxygen to your heart muscle, receive most of their blood flow during this resting phase. That makes a healthy diastolic pressure essential for keeping the heart well-supplied with oxygen.
What the Numbers Mean for Your Health
The American Heart Association defines five blood pressure categories based on both numbers:
- 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 “or” in the hypertension categories. Only one number needs to be elevated for the reading to qualify. You could have a systolic of 145 with a diastolic of 75 and still be classified as stage 2 hypertension.
Why One Number Can Be High While the Other Is Normal
It’s common to have only the systolic or only the diastolic number elevated. These patterns have names: isolated systolic hypertension (high top number, normal bottom number) and isolated diastolic hypertension (normal top number, high bottom number). A large study of over 6.4 million young adults published in Circulation found that both patterns carry increased cardiovascular risk compared to normal blood pressure. When both numbers are elevated together, the risk is higher still.
The same study found that women faced a relatively greater increase in cardiovascular risk from elevated blood pressure than men, with women-to-men risk ratios ranging from 1.14 to 1.46 across hypertension categories.
How Age Changes Each Number
Systolic and diastolic pressure don’t age the same way. Diastolic pressure typically rises through middle age, peaking around 50, then gradually declines. Systolic pressure, on the other hand, climbs steadily from adolescence into old age. This divergence is largely driven by arterial stiffness: as arteries lose elasticity over the decades, they can’t absorb the force of each heartbeat as effectively, so peak pressure rises. At the same time, stiffer arteries recoil less between beats, which is why diastolic pressure eventually falls.
This is why older adults often develop isolated systolic hypertension. Their arteries have stiffened enough to push the top number up while the bottom number stays the same or drops. Smoking, diabetes, high cholesterol, and higher body weight can accelerate this stiffening process, potentially shifting the pattern to an earlier age.
What Pulse Pressure Tells You
The gap between your two numbers is called pulse pressure. You calculate it by subtracting the diastolic from the systolic. For a reading of 130/70, the pulse pressure is 60.
A pulse pressure above 40 mmHg is generally considered a warning sign, and above 60 mmHg it becomes an independent risk factor for heart disease, particularly in older adults. A wide gap suggests the blood vessels have become stiffer and potentially more damaged. If your systolic number keeps climbing while your diastolic stays flat or drops, a widening pulse pressure is the result, and it’s worth paying attention to.
How to Read Your Own Results
When you get a blood pressure reading, look at both numbers together rather than focusing on just one. A reading of 118/76 sits comfortably in the normal range. A reading of 125/78 is elevated because the top number has crossed 120, even though the bottom number is still fine. A reading of 115/92 qualifies as stage 2 hypertension solely because of the diastolic number, despite a perfectly normal systolic.
Blood pressure fluctuates throughout the day based on activity, stress, caffeine intake, and even body position. A single high reading doesn’t necessarily mean you have hypertension. Patterns over multiple readings matter more than any one measurement. If you’re tracking at home, take readings at the same time each day, sitting quietly for five minutes beforehand, with your arm supported at heart level.