The bottom number of a blood pressure reading is called diastolic pressure, and it measures the force of blood against your artery walls during the brief pause between heartbeats. A normal diastolic reading is below 80 mm Hg. While most attention goes to the top number (systolic pressure), the bottom number tells you something important about how well your arteries handle pressure even when your heart is at rest.
What Happens Between Heartbeats
Your heart doesn’t squeeze continuously. It pumps in a rhythmic cycle of contraction and relaxation. The top number captures the peak pressure during contraction. The bottom number captures the lowest point, when your heart muscle relaxes and fills with blood for the next beat.
This resting phase matters more than you might expect. Your heart muscle actually receives most of its own blood supply during this pause, not during the powerful squeeze. When the heart contracts, it compresses its own blood vessels so tightly that flow is restricted. As the muscle relaxes, it creates a suction effect that pulls oxygen-rich blood into the coronary arteries. So the diastolic phase is when your heart feeds itself. If the pressure during this phase is too high or too low, that feeding process can be disrupted.
How the Bottom Number Is Measured
When a healthcare provider takes your blood pressure manually, they inflate a cuff around your arm and listen through a stethoscope for specific sounds. As the cuff slowly deflates, the first thumping sound they hear marks the systolic (top) number. As the cuff continues to deflate, those sounds change in quality, becoming muted and then disappearing altogether. The point where silence returns is your diastolic pressure. It’s the moment when blood is flowing freely through the artery again, with no turbulence from the cuff. Automated monitors use sensors to detect the same transition, though you won’t hear the sounds yourself.
Diastolic Blood Pressure Ranges
The American Heart Association classifies diastolic pressure into clear categories:
- Normal: below 80 mm Hg
- Stage 1 hypertension: 80 to 89 mm Hg
- Stage 2 hypertension: 90 mm Hg or higher
- Severe hypertension: above 120 mm Hg
The “elevated” blood pressure category applies only to the top number (120 to 129 systolic), while the bottom number remains below 80. This is worth knowing because it means you can fall into the elevated category based on your top number alone, even if your bottom number looks fine.
When Only the Bottom Number Is High
Some people have a normal top number but a diastolic reading of 80 or above. This is called isolated diastolic hypertension. It tends to be more common in younger adults and is often linked to factors like excess weight, high sodium intake, or lack of physical activity.
Isolated diastolic hypertension usually isn’t an immediate emergency. But over time, it raises the risk of heart attack, congestive heart failure, and death from cardiovascular disease. These risks are greatest for women and people under 60. If your bottom number consistently reads in the 80s or above while your top number stays normal, it’s worth addressing rather than ignoring.
When the Bottom Number Is Too Low
A bottom number that drops too low can also cause problems. Blood pressure below 90/60 mm Hg is generally considered low. If diastolic pressure falls significantly, your organs may not receive enough oxygen-rich blood between heartbeats. Remember, this is the phase when your heart muscle gets its own blood supply, so very low diastolic pressure can compromise the heart itself.
Symptoms of low diastolic pressure include dizziness, weakness, and fainting. Falls are a real concern, especially for adults over 65, who are more prone to blood pressure drops when standing up or after eating. Severely low blood pressure can reduce oxygen delivery enough to damage the heart and brain.
How the Bottom Number Changes With Age
Diastolic pressure doesn’t follow the same lifelong trajectory as systolic pressure. In most people, the bottom number rises gradually through middle age, typically peaking around age 50 to 60. After that, it often begins to decline, even as the top number continues to climb. This creates a widening gap between the two numbers, known as pulse pressure, which becomes its own risk factor for heart disease in older adults.
This pattern is why doctors sometimes focus more on the top number in older patients and more on the bottom number in younger patients. A 35-year-old with a diastolic reading of 88 is facing a different situation than a 70-year-old with the same number.
Lowering a High Diastolic Reading
Lifestyle changes can meaningfully reduce both blood pressure numbers. A diet rich in whole grains, fruits, vegetables, and low-fat dairy while low in saturated fat can lower blood pressure by up to 11 mm Hg. Regular aerobic exercise, like brisk walking, cycling, or swimming, can bring readings down by 5 to 8 mm Hg.
Sodium reduction has a notable effect. Keeping intake below 1,500 mg per day can lower blood pressure by about 5 to 6 mm Hg. Increasing potassium through foods like bananas, potatoes, and beans (aiming for 3,500 to 5,000 mg daily) may lower it by another 4 to 5 mm Hg. Weight loss is one of the most effective strategies: blood pressure drops by roughly 1 mm Hg for every kilogram (about 2.2 pounds) lost.
These effects are additive. Combining several of these changes can bring a borderline diastolic reading back into the normal range without medication, particularly for people whose numbers are only mildly elevated.