Diastole is the phase of the heartbeat when your heart muscle relaxes and the chambers fill with blood. It alternates with systole, the contraction phase that pumps blood out. Together, these two phases make up one complete cardiac cycle. The bottom number on a blood pressure reading represents the pressure in your arteries during diastole.
What Happens During Diastole
Each heartbeat has a built-in rhythm: squeeze, then relax. Diastole is the relaxation half. It begins the moment the valve between the ventricle (the heart’s main pumping chamber) and the outgoing artery snaps shut after a contraction. The heart muscle then softens, pressure inside the chamber drops rapidly, and blood from the upper chamber (the atrium) rushes in to refill it.
This refilling happens in two stages. First, blood flows passively from the atrium into the ventricle, driven purely by the pressure difference between the two chambers. This passive flow accounts for roughly 70 to 80 percent of all the blood that enters the ventricle. Then the atrium contracts, sometimes called the “atrial kick,” pushing the remaining 20 to 30 percent in. Once the ventricle is full, the next contraction begins, and the cycle repeats.
On an ECG tracing, diastole starts at the end of the T wave and lasts until the next R wave peak, which marks the onset of the following contraction.
Why Diastole Matters for Heart Health
Diastole isn’t just a rest period. It’s when your heart feeds itself. Blood flow through the coronary arteries, the vessels that supply oxygen to the heart muscle, peaks during diastole rather than during contraction. That’s the opposite of how blood flow works in the rest of the body. The reason: when the heart contracts, the muscle squeezes its own blood vessels so tightly that flow can actually reverse, especially in the thick-walled left ventricle. Only when the muscle relaxes do those coronary vessels open back up and receive a normal blood supply.
This means that anything shortening diastole, like a very fast heart rate, reduces the time your heart has to nourish itself. At a resting heart rate of around 60 to 80 beats per minute, diastole takes up about two-thirds of each cardiac cycle. As your heart rate climbs during exercise or stress, diastole shortens more than systole does. A healthy heart handles this fine, but a diseased heart can struggle when that filling and feeding window shrinks.
Diastolic Blood Pressure
When you get a blood pressure reading like 120/80, the bottom number (80 in this case) is your diastolic blood pressure. It measures the pressure in your arteries during the moment your heart is relaxed between beats. Under the 2025 guidelines from the American Heart Association and American College of Cardiology, the categories break down like this:
- Normal: below 80 mm Hg
- Stage 1 hypertension: 80 to 89 mm Hg
- Stage 2 hypertension: 90 mm Hg or higher
Diastolic pressure behaves differently from systolic pressure as you age. Both numbers tend to rise through middle age, but after about age 50 to 60, diastolic pressure typically starts to fall while systolic pressure keeps climbing. This happens because arteries stiffen with age, which drives up the peak pressure during contraction but lowers the baseline pressure during relaxation. That widening gap between the two numbers is called a wide pulse pressure, and it’s common in older adults.
Diastolic Heart Failure
When the heart muscle becomes stiff or slow to relax, it can’t fill properly during diastole. This leads to a condition called heart failure with preserved ejection fraction, or HFpEF, often referred to as diastolic heart failure. The pumping strength looks normal on imaging (ejection fraction above 50 percent), but the heart can’t accept enough blood during the filling phase, so fluid backs up into the lungs and body.
Several things can make the ventricle stiffer. Increased muscle mass from years of high blood pressure is a major contributor. Changes in the collagen scaffolding within the heart wall, including both more collagen and stiffer cross-links between collagen fibers, play a role too. At the cellular level, the elastic proteins inside heart muscle cells can shift toward a stiffer form. Conditions like obesity, kidney disease, and anemia can add extra fluid volume that worsens the problem.
People with diastolic heart failure often feel fine at rest but become short of breath and fatigued with exertion. Research has shown that their heart muscle has a reduced energy reserve even at rest, with lower levels of the molecules cells use for quick fuel. This energy deficit, potentially linked to insulin resistance and age-related changes in how cells generate power, helps explain why exercise becomes so difficult. The heart simply can’t relax fast enough between beats to keep up with the body’s demand for more blood flow.
How Diastolic Function Is Assessed
If your doctor suspects a problem with how your heart fills, the primary tool is an echocardiogram, an ultrasound of the heart. The test uses Doppler imaging to measure the speed of blood flowing through the mitral valve (the valve between the left atrium and left ventricle) during the passive filling phase and during the atrial kick. The ratio between those two flow speeds, called the E/A ratio, gives a picture of how well the ventricle relaxes.
Another key measurement compares the speed of blood flow through the valve with how fast the heart muscle itself moves during relaxation. When this ratio (called E/e’) climbs above 14, it’s a strong indicator that filling pressures inside the heart are abnormally high. Values below 8 generally indicate normal pressures. Newer techniques also look at how much the left atrium stretches and contracts, since a struggling ventricle forces the atrium to work harder and eventually enlarge.
These measurements help distinguish between a heart that pumps weakly (systolic heart failure) and one that pumps fine but fills poorly (diastolic heart failure), because the symptoms of both can look identical from the outside.