Diastolic pressure is the lowest pressure in your arteries, measured in the brief moment when your heart relaxes between beats. It’s the bottom number in a blood pressure reading. If your reading is 120/80, the 80 is your diastolic pressure, recorded in millimeters of mercury (mmHg). While most attention goes to the top number (systolic pressure), the bottom number tells its own important story about your cardiovascular health.
What Happens in Your Body During Diastole
Your heart works in a two-phase cycle: contraction and relaxation. During contraction (systole), the heart pumps blood out into the arteries, creating peak pressure. During relaxation (diastole), the heart’s chambers refill with blood and no new blood is being pushed out. Diastolic pressure is the reading taken at this lowest point, just before the next contraction begins.
Even though the heart isn’t actively pumping during diastole, your blood pressure doesn’t drop to zero. Your arteries are elastic, and they recoil inward after being stretched by each heartbeat, maintaining enough pressure to keep blood moving forward. The diastolic number reflects how much baseline pressure remains in your arteries during that rest period. It’s shaped largely by the resistance in your smaller blood vessels, the ones that branch out into your organs and tissues. When those vessels are stiff or constricted, diastolic pressure rises. When they’re relaxed, it falls.
Normal Ranges and What the Numbers Mean
The 2025 guidelines from the American Heart Association and American College of Cardiology categorize diastolic pressure as follows:
- Normal: below 80 mmHg
- Hypertension Stage 1: 80 to 89 mmHg
- Hypertension Stage 2: 90 mmHg or higher
For the “elevated” blood pressure category, the systolic number is what’s slightly high (120 to 129), while the diastolic number remains below 80. In other words, your diastolic reading alone can push you into a hypertension diagnosis even if your systolic number looks fine.
How Diastolic Pressure Changes With Age
In younger and middle-aged adults, diastolic and systolic pressure tend to rise together. Up to roughly age 50 to 55, both numbers climb in tandem, primarily driven by increasing resistance in the smaller blood vessels throughout the body. After about age 60, however, the two numbers diverge. Systolic pressure continues to rise as the large arteries near the heart stiffen and lose elasticity, but diastolic pressure often starts to decline. This is why older adults frequently have a wide gap between their top and bottom numbers, a pattern called wide pulse pressure.
This age-related decline doesn’t mean diastolic pressure stops mattering in older adults. A very high diastolic reading later in life, especially paired with a high systolic reading, can signal a serious increase in vascular resistance that may require urgent evaluation.
When Only Diastolic Pressure Is High
Some people have a normal systolic reading (under 130 mmHg) but a diastolic number at 80 or above. This is called isolated diastolic hypertension, and it affects about 6 in 100 people in the U.S. Among all adults diagnosed with high blood pressure, roughly 1 in 5 have this pattern. A diagnosis typically requires elevated diastolic readings at two or more separate office visits.
Isolated diastolic hypertension is more common in younger adults and is generally linked to higher resistance in the peripheral blood vessels. It’s worth paying attention to because it still places extra strain on your heart and arteries over time, even if the top number appears reassuring.
When Diastolic Pressure Is Too Low
Low blood pressure is generally defined as a reading below 90/60 mmHg, so a diastolic number under 60 paired with a low systolic reading falls in that range. Not everyone with low readings has symptoms, but when they do, common signs include dizziness, lightheadedness, blurred vision, fatigue, and difficulty concentrating. Severe drops can cause fainting or, in extreme cases, a dangerous condition called shock, marked by cold and clammy skin, rapid shallow breathing, confusion, and a weak pulse.
A low diastolic reading matters partly because diastolic pressure is when the heart muscle itself receives most of its own blood supply. The coronary arteries fill during the relaxation phase, so a consistently low diastolic number could mean the heart isn’t getting the oxygen-rich blood it needs, particularly in people who already have narrowed coronary arteries.
How Diastolic Pressure Is Measured
When a clinician takes your blood pressure manually, they inflate a cuff around your upper arm until it temporarily stops blood flow, then slowly release the pressure while listening through a stethoscope. As the cuff deflates, they hear a series of sounds called Korotkoff sounds. The first sound marks your systolic pressure. As the cuff continues to deflate, those sounds change character: they go from sharp tapping to thumping, then become muffled, and finally disappear. The point where the sounds go silent is your diastolic pressure, the moment cuff pressure drops below the resting pressure in your artery and blood flows freely again.
Automatic home monitors use sensors to detect these same pressure changes, though the exact method differs. Either way, the principle is the same: diastolic pressure is the point at which blood flow through the compressed artery becomes completely unobstructed.
What Influences Your Diastolic Number
Several factors can push diastolic pressure up or down. The most significant is peripheral vascular resistance, the degree of tension in the walls of your smaller arteries. Things that increase this resistance, like chronic stress, excess sodium intake, lack of physical activity, and carrying extra weight, tend to raise diastolic pressure. Conditions that affect blood vessel flexibility, such as atherosclerosis (plaque buildup in the arteries), also play a role.
Short-term fluctuations are normal. Caffeine, a full bladder, anxiety, or even talking during a measurement can temporarily bump your numbers up. That’s one reason clinicians look for consistent readings across multiple visits rather than acting on a single measurement. If you’re monitoring at home, sitting quietly for five minutes before checking, keeping your feet flat on the floor, and supporting your arm at heart level will give you the most accurate diastolic reading.