Diastolic pressure is the lowest pressure in your arteries, measured during the brief pause between heartbeats when your heart muscle relaxes and refills with blood. It’s the bottom number in a blood pressure reading. A normal diastolic pressure is less than 80 mm Hg, and it reflects how much resistance your blood vessels are putting up even when the heart isn’t actively pumping.
What Happens During Diastole
Your heart cycles between two phases: systole (contraction) and diastole (relaxation). During systole, the heart squeezes blood into your arteries, creating the peak pressure you see as the top number. Then the heart relaxes, the chambers refill, and the pressure in your arteries drops. The lowest point it reaches, just before the next contraction begins, is your diastolic pressure.
Even though the heart isn’t pumping during this phase, pressure doesn’t drop to zero. Your artery walls are elastic, and they recoil inward after being stretched by each heartbeat, pushing blood forward. This stored energy in the vessel walls is what maintains diastolic pressure. Meanwhile, the smallest arteries (arterioles) act as gatekeepers, resisting blood flow and keeping pressure from falling too quickly. The stiffer your arteries and the tighter those small vessels, the higher your diastolic number will be.
Diastole also plays a unique role for the heart itself. The coronary arteries, which supply oxygen to your heart muscle, receive most of their blood flow during this relaxation phase. That means your diastolic pressure directly affects how well your heart feeds itself. If diastolic pressure is too low, the heart may not get enough blood. If it’s too high, the extra force damages vessel walls over time.
Normal, High, and Low Ranges
Under the widely used 2017 ACC/AHA guidelines in the United States, diastolic blood pressure is categorized alongside systolic pressure:
- Normal: less than 80 mm Hg (with systolic less than 120)
- Elevated: less than 80 mm Hg, but systolic is 120 to 129
- Stage 1 hypertension: 80 to 89 mm Hg
- Stage 2 hypertension: 90 mm Hg or higher
European guidelines from 2024 set the hypertension threshold slightly differently, at 90 mm Hg diastolic (paired with 140 systolic). On the low end, diastolic pressure at or below 60 mm Hg is generally considered low. Not everyone with a reading in that range has symptoms, but when they do, dizziness, lightheadedness, and fainting are the most common signs.
What Determines Your Diastolic Number
Two main factors set your diastolic pressure: how much resistance your small arteries create and how elastic your larger arteries are. The arterioles throughout your body are the single biggest source of vascular resistance. When they tighten (from stress hormones, high sodium intake, or chronic inflammation), diastolic pressure rises. When they relax (during sleep, with certain medications, or after exercise), it falls.
Arterial stiffness matters just as much. Young, healthy arteries stretch with each heartbeat and then gently push blood forward during diastole, cushioning pressure swings. As arteries stiffen with age or disease (a process called arteriosclerosis), they lose that cushioning ability. Stiff arteries raise systolic pressure because they can’t absorb the surge, but they also change the diastolic picture in more complex ways, which is why the two numbers don’t always move in the same direction.
How Diastolic Pressure Changes With Age
Diastolic pressure follows a distinct arc over a lifetime. CDC data from a large national survey shows that average diastolic pressure rises from early adulthood until about age 60, then begins to decline. The overall average for U.S. adults is around 71 mm Hg, with normotensive adults averaging 69 mm Hg and untreated hypertensive adults averaging 84 mm Hg.
The decline after 60 is driven largely by arterial stiffening. As vessels lose elasticity, they stop maintaining pressure between beats as effectively, so the diastolic number drops even while systolic pressure continues climbing. This is why older adults often have a wide gap between their two numbers, a pattern called isolated systolic hypertension. It also means that in older adults, a very low diastolic reading (below 60) deserves attention, since it may signal that coronary blood flow is compromised during the relaxation phase when the heart muscle depends on it most.
Why High Diastolic Pressure Matters
For years, systolic pressure got most of the clinical attention, and diastolic hypertension on its own was sometimes dismissed. That picture has shifted, especially for younger adults. Isolated diastolic hypertension, where the bottom number is elevated but the top number is normal, accounts for up to 20% of all hypertension cases.
A large Korean study of over 6 million adults aged 20 to 39 found that stage 1 isolated diastolic hypertension raised cardiovascular disease risk by 32%, and stage 2 raised it by 82%, compared to people with normal readings. An international database of ambulatory blood pressure monitoring found an even sharper signal: among people under 50, isolated diastolic hypertension nearly tripled the risk of cardiovascular events. For people over 50, the association weakened considerably, possibly because arterial stiffening makes diastolic readings less reliable as an independent risk marker in that age group.
The takeaway is straightforward. If you’re under 50 and your bottom number consistently runs at 80 or above while your top number looks fine, that’s not something to ignore. It’s linked to heart failure, atrial fibrillation, and chronic kidney disease in addition to general cardiovascular risk.
When Diastolic Pressure Is Too Low
A diastolic reading at or below 60 mm Hg raises a different set of concerns. For some people, particularly younger and physically fit individuals, a naturally low diastolic pressure causes no symptoms and isn’t harmful. But when low diastolic pressure produces dizziness, lightheadedness, or fainting, the body is signaling that organs aren’t getting enough blood flow during the heart’s relaxation phase.
The concern is most relevant for the heart itself. Because the coronary arteries fill during diastole, a persistently low diastolic pressure can reduce the oxygen supply to heart muscle. In people who already have narrowed coronary arteries, this reduction can tip the balance toward ischemia, a state where the heart isn’t getting enough blood to meet its needs. Research from the American Heart Association shows that during physical stress, when the heart beats faster and diastole shortens, people with coronary artery narrowing lose their ability to compensate. Blood flow to the inner layers of the heart wall drops, and the risk of damage increases.
Dehydration, prolonged bed rest, blood loss, and certain medications can all push diastolic pressure below that 60 mm Hg threshold. If you notice persistent lightheadedness or have a known heart condition, a consistently low bottom number is worth tracking and discussing with your provider.