The second (bottom) number in a blood pressure reading is called diastolic pressure. It measures the force of blood pushing against your artery walls while your heart rests between beats. In a reading like 120/80, the 80 is your diastolic pressure, recorded in millimeters of mercury (mm Hg).
What Diastolic Pressure Actually Reflects
Your heart works in a two-phase cycle: it contracts to push blood out, then relaxes to refill. The first number (systolic) captures the peak pressure during that contraction. The second number captures the baseline pressure that remains in your arteries during the resting phase, when your heart is refilling with blood.
That resting pressure matters more than it might seem. Your coronary arteries, the vessels that feed the heart muscle itself, receive most of their blood supply during this relaxation phase. A healthy diastolic pressure ensures your heart gets adequate oxygen between beats. When diastolic pressure is consistently too high, it forces the heart and blood vessels to work harder even during what should be a rest period. When it’s too low, the heart may not receive enough blood flow to sustain itself.
How It’s Measured
During a manual blood pressure reading, a clinician inflates a cuff around your arm tight enough to temporarily stop blood flow, then slowly releases the pressure while listening with a stethoscope. As the cuff deflates, they hear thumping sounds created by blood forcing its way through the compressed artery. The point where those sounds first appear marks the systolic (top) number. As the cuff continues to deflate, the sounds change in quality, become muted, and eventually disappear. The moment of silence marks the diastolic (bottom) number, because the cuff pressure has dropped below the resting pressure in the artery and blood is flowing freely again.
Automatic digital monitors use sensors to detect the same thing, though the method is slightly different. Either way, the principle is identical: diastolic pressure is the lowest point of pressure in your arteries between heartbeats.
Healthy Ranges for the Bottom Number
The American Heart Association breaks diastolic pressure into these categories:
- Normal: below 80 mm Hg
- Elevated: 80 to 89 mm Hg
- High blood pressure, stage 1: 90 to 119 mm Hg
- High blood pressure, stage 2: 120 mm Hg or higher
A diastolic reading of 120 or above, especially paired with a systolic reading of 180 or above, qualifies as a hypertensive crisis and requires immediate medical attention.
When Only the Bottom Number Is High
Sometimes the diastolic number climbs into the elevated or high range while the systolic number stays normal. This is called isolated diastolic hypertension, and it’s more common in younger adults. It typically doesn’t cause immediate symptoms or problems, but it does raise the long-term risk of heart attack, congestive heart failure, and death from cardiovascular disease. Those risks are greatest for women and people under 60.
Isolated diastolic hypertension often signals early changes in blood vessel stiffness or increased resistance in smaller arteries. It can serve as a warning that broader cardiovascular changes are underway, even when the top number looks fine.
When the Bottom Number Is Too Low
There’s no single cutoff that defines “too low” for diastolic pressure. Most clinicians consider it a concern only when it causes symptoms: dizziness, lightheadedness, blurred vision, fainting, fatigue, or a weak and rapid pulse. Common causes include dehydration, prolonged bed rest, pregnancy, and certain medications. Older adults with conditions affecting the autonomic nervous system, like Parkinson’s disease, are at higher risk.
A very low diastolic number can be particularly concerning because it may starve the heart muscle of blood during its resting phase. This becomes more relevant in people who already have coronary artery disease.
How the Bottom Number Changes With Age
The two blood pressure numbers don’t age the same way. Systolic pressure tends to climb steadily throughout life as arteries stiffen. Diastolic pressure follows a different pattern: it rises gradually from early adulthood, peaks around age 60, and then begins to decline. This is why older adults often have a wide gap between the two numbers, such as 150/70, with a high top number and a relatively low bottom number.
That widening gap (called pulse pressure) reflects increasing arterial stiffness. It’s one reason clinicians pay more attention to systolic pressure in older adults, while diastolic pressure carries more predictive weight in younger people.
Lifestyle Changes That Lower It
The same habits that lower systolic pressure also lower diastolic pressure, though usually by a smaller amount. Reducing salt intake is one of the most well-studied interventions. A modest, sustained reduction in daily salt lowers diastolic pressure by about 1 to 3 mm Hg on average, with greater effects in people who already have high blood pressure (closer to 3 mm Hg) compared to those with normal readings (closer to 1 mm Hg). The effect also varies by ethnic background.
Regular aerobic exercise, maintaining a healthy weight, limiting alcohol, and eating a diet rich in fruits, vegetables, and whole grains all contribute to lowering the bottom number. These changes tend to have a larger impact on systolic pressure, but diastolic pressure benefits too, and even small reductions in diastolic pressure reduce long-term cardiovascular risk.