Diastolic refers to the phase of your heartbeat when the heart muscle relaxes and fills with blood. In a blood pressure reading like 120/80, the diastolic number is the bottom one, representing the pressure in your arteries during that resting moment between beats. A normal diastolic reading falls below 80 mmHg.
What Happens During Diastole
Your heart works in a two-phase cycle: it squeezes to push blood out (systole), then relaxes to refill (diastole). Diastole begins the instant the valve between the heart and the aorta snaps shut, and it ends when the heart begins its next contraction. During this pause, blood flows from the lungs and body back into the heart’s chambers, stretching them like a balloon filling with water.
The filling process isn’t powered by the heart itself. Instead, the elastic walls of your blood vessels act like a recoil mechanism, using stored energy from the previous heartbeat to push blood forward and into the relaxing chambers. This is why your arteries still maintain pressure even when the heart is at rest. That residual pressure is exactly what the diastolic number measures.
Why Diastolic Pressure Matters for Your Heart
Diastolic pressure plays a unique role that systolic pressure does not: it’s responsible for feeding blood to the heart muscle itself. Your coronary arteries, the small vessels that supply oxygen to the heart, can only fill during diastole. When the heart contracts, it physically squeezes those arteries shut, blocking its own blood supply. So your heart can only nourish itself during the relaxation phase, and adequate diastolic pressure is what drives that flow.
This means a diastolic reading that’s too low can starve the heart of oxygen, while a reading that’s too high forces the heart to work against constant resistance. Both extremes create problems over time.
How Diastolic Differs From Systolic
The top number (systolic) reflects how forcefully blood slams into artery walls when the heart contracts. It’s largely determined by the heart’s pumping strength and the flexibility of your largest arteries. The bottom number (diastolic) is shaped by a different factor: the resistance in your smaller, peripheral blood vessels. When those tiny vessels tighten, blood drains out of the large arteries more slowly between beats, and diastolic pressure rises.
This distinction matters because the two numbers can move independently. Stiff, aging arteries tend to raise systolic pressure while actually lowering diastolic pressure, because rigid vessels can no longer store and slowly release blood between beats. Narrowed small vessels, on the other hand, push diastolic pressure up. That’s why high diastolic readings are more common in younger adults, while high systolic readings dominate in older populations.
Normal, High, and Low Ranges
Current guidelines from the American Heart Association and American College of Cardiology define the categories this way:
- Normal: below 80 mmHg
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
- Low (hypotension): below 60 mmHg
These thresholds apply even when your systolic number looks fine. A reading of 118/92, for example, still qualifies as stage 2 hypertension because of the diastolic value alone.
Isolated Diastolic Hypertension
When only the diastolic number is elevated while systolic stays normal, it’s called isolated diastolic hypertension. This pattern typically affects people under 55 and is more common in that age group because their large arteries are still flexible enough to keep systolic pressure in check. The issue is concentrated in the smaller vessels.
Isolated diastolic hypertension usually doesn’t cause immediate symptoms, but it raises the lifetime risk of heart attack and heart failure. These risks are greatest for women and people under 60. Because it tends to be silent, it’s often caught only during routine checkups.
How Diastolic Pressure Changes With Age
Diastolic pressure follows a predictable arc over a lifetime. CDC data from a large national survey shows it rises gradually from early adulthood, peaks around age 55 to 60, then begins to decline. This drop happens because arteries lose elasticity with age. Stiffer arteries can’t expand as much during a heartbeat, so they release blood into the smaller vessels faster, leaving less residual pressure between beats. The same stiffening pushes systolic pressure higher, which is why older adults commonly have a wide gap between their two numbers.
When Diastolic Pressure Is Too Low
A diastolic reading consistently below 60 mmHg can be a concern, particularly because of the coronary perfusion issue described earlier. If diastolic pressure drops too far, the heart may not receive enough oxygen during its resting phase, especially during exercise or periods of high demand.
Common causes of low diastolic pressure include dehydration, prolonged bed rest, pregnancy, and certain medications. Symptoms often show up as lightheadedness, fatigue, or feeling faint when standing. Some people naturally run low without any symptoms, which is generally not dangerous. The concern increases when low readings accompany dizziness or fainting episodes.
How the Diastolic Number Is Measured
When a healthcare provider takes your blood pressure with a manual cuff and stethoscope, they inflate the cuff to temporarily block blood flow, then slowly release it while listening. As air escapes, they hear a series of thumping sounds caused by blood forcing its way through the compressed artery. The diastolic reading is recorded at the exact moment those sounds disappear completely, meaning blood is now flowing freely and the artery is no longer being compressed at all, even at its lowest pressure point.
Automatic home monitors use sensors instead of sound, but they’re detecting the same transition. For the most accurate reading, sit quietly for five minutes beforehand with your arm supported at heart level. Take two or three readings a minute apart and average them. Single readings can vary by 10 points or more depending on stress, caffeine, or even a full bladder.