The lower number in a blood pressure reading is called diastolic pressure, and it measures the force of blood pushing against your artery walls when your heart is resting between beats. If your reading is 120/80, the 80 is your diastolic pressure. While most attention goes to the top number (systolic), the bottom number tells you something distinct about the health of your blood vessels and how well your heart muscle gets its own blood supply.
What Happens During the Lower Number
Your heart works in a pump-and-rest cycle. When it squeezes, blood surges out and creates the top number. When it relaxes and refills, the pressure in your arteries drops to its lowest point. That lowest pressure is your diastolic reading. It reflects how much resistance your blood vessels are creating even when the heart isn’t actively pumping.
The main factor controlling that bottom number is the tightness of your smaller arteries and blood vessels throughout your body. When those vessels are relaxed and flexible, blood flows through easily and diastolic pressure stays low. When they’re stiff or constricted, the pressure stays elevated even during the heart’s resting phase. This is why diastolic pressure is sometimes described as a window into your vascular resistance, the baseline tension in your circulatory system.
What the Numbers Mean
Under the 2025 American Heart Association and American College of Cardiology guidelines, the diastolic categories are:
- Normal: below 80 mm Hg
- Stage 1 hypertension: 80 to 89 mm Hg
- Stage 2 hypertension: 90 mm Hg or higher
European guidelines set a slightly higher threshold, defining hypertension as 90 mm Hg or above. Either way, the consistent message is that a diastolic reading in the low-to-mid 70s paired with a systolic under 120 represents healthy arterial pressure.
Why the Bottom Number Matters for Your Heart
Your heart muscle has a unique blood supply. Unlike most organs, the heart receives the majority of its own blood flow during diastole, the resting phase between beats. That means diastolic pressure directly determines how much oxygen-rich blood reaches the heart muscle itself through the coronary arteries.
This creates what researchers call a J-curve relationship. Diastolic pressure that’s too high damages arteries over time, but pressure that drops too low can starve the heart of blood. Data from the Multi-Ethnic Study of Atherosclerosis found this risk is most pronounced in people who already have calcium buildup in their coronary arteries. If someone has narrowed heart vessels and their diastolic pressure drops very low, the reduced flow can cause ischemia, a shortage of oxygen to the heart tissue. Notably, this J-curve pattern shows up specifically for heart events, not for strokes, which supports the idea that it’s genuinely about coronary blood flow rather than a statistical quirk.
High Diastolic Pressure
Some people have an elevated bottom number while their top number stays normal, a pattern called isolated diastolic hypertension. This tends to be more common in younger adults and is often driven by high vascular resistance rather than the arterial stiffness that typically raises systolic pressure in older people.
The health risk of isolated diastolic hypertension is a subject of genuine debate. A large study of over 5,100 U.S. participants found that having a diastolic reading between 80 and 89 (with normal systolic pressure) was not associated with coronary artery calcium buildup, and had only a weak link to coronary plaque on imaging. However, when diastolic pressure climbs above 100 mm Hg, the relationship with cardiovascular disease becomes clear and linear. So a mildly elevated bottom number may not carry the same urgency as a high top number, but significantly elevated diastolic pressure is a real concern.
Low Diastolic Pressure
A diastolic reading below 60 mm Hg generally qualifies as low. Common symptoms include dizziness, weakness, and fainting, all of which stem from reduced blood flow to the brain. Falls and fall-related injuries are a practical risk, especially for older adults. Severely low pressure can reduce oxygen delivery enough to damage the heart and brain.
Low diastolic readings are especially important to monitor if you’re already being treated for high blood pressure. Medications that lower both numbers can sometimes push diastolic pressure into a range where the heart muscle isn’t getting adequate perfusion. If you notice lightheadedness after starting or adjusting blood pressure medication, that’s worth flagging at your next appointment.
Getting an Accurate Reading
Diastolic pressure is actually harder to measure accurately than systolic pressure. Automated home monitors can underestimate the bottom number by 4 to 6 mm Hg compared to manual readings, according to validation studies. That gap is large enough to shift someone from one category to another.
Arm circumference plays a role too. People with smaller arms tend to get less accurate diastolic readings, with the error being most noticeable when the true diastolic is below 70 mm Hg. Using the correct cuff size for your arm makes a meaningful difference. A cuff that’s too large for your arm can artificially lower the reading, while one that’s too small can inflate it. Some people also have what’s called an auscultatory gap, a silent zone caused by stiff arteries that can make the diastolic reading appear higher than it actually is.
For the most reliable results, sit quietly for five minutes before measuring, keep your arm supported at heart level, and take two or three readings a minute apart. The average of those readings is more trustworthy than any single one.
Lowering a High Diastolic Reading
Lifestyle changes can meaningfully reduce your diastolic pressure. In a clinical trial, participants who followed the DASH eating pattern (heavy on fruits, vegetables, and low-fat dairy, light on saturated fat and sodium) lowered their diastolic pressure by about 7.5 mm Hg. When that same diet was combined with weight loss through calorie reduction and aerobic exercise, the drop reached nearly 10 mm Hg. The usual care group, by comparison, saw only a 3.8 mm Hg reduction.
Weight loss on its own produces roughly a 1 mm Hg drop in blood pressure for every kilogram (about 2.2 pounds) lost. That’s a dose-dependent relationship: the more weight you lose, the more your pressure falls. The participants who combined diet changes with exercise lost an average of 8.7 kg (about 19 pounds) and improved their exercise capacity by 19%, with the largest reductions in both systolic and diastolic pressure as well as cholesterol.
Physical activity alone also lowers diastolic pressure, particularly when paired with dietary changes. The effect is amplified rather than simply additive, meaning people who do both see bigger improvements than you’d expect from adding the benefits of each one separately.