Blood Pressure Basics: Systolic, Diastolic & Pulse

Three numbers, three different questions

Every blood-pressure reading hands you a little story in three parts. Most people glance at the top number, decide it looks “fine,” and move on. But each figure answers a genuinely different question about how your circulation is working, and when you are recovering from POTS, long COVID or dysautonomia, the differences between them are exactly where the useful signal lives.

A cuff reading looks like 118/76, sometimes with a small heart on the display showing 64. Read left to right, that is:

• Systolic (118): the peak pressure in your arteries during the split second your heart contracts and ejects blood.
• Diastolic (76): the resting pressure between beats, while the heart relaxes and refills.
• Pulse (64): how many times per minute that whole cycle repeats.

The first two are pressures, measured in millimeters of mercury (mmHg). The third is a rate, the same thing as your resting heart rate. Together they describe both how hard your circulation is pushing and how often.

Systolic: the peak of the push

Each time your heart’s main pumping chamber squeezes, it forces a bolus of blood into the aorta and the pressure in your arteries spikes. Systolic pressure is the height of that spike. It reflects the force of each ejection and how stiff or springy your large arteries are as they absorb it.

A systolic number that runs high means your heart is working against more resistance or your vessels have stiffened. A systolic number that runs low (common in slim, well-conditioned people, but also in dehydration and in the blood-volume problems that shadow POTS) means each beat is delivering a gentler push. Neither extreme is automatically good or bad; context is everything.

Diastolic: the pressure that never lets up

Between beats, your heart relaxes and refills, but your arteries do not empty. They stay under pressure, and diastolic is that baseline load: the pressure your vessels carry every second of every day, even at rest. Because diastole (the resting phase) lasts roughly twice as long as systole, this lower number is doing a lot of quiet work keeping your organs perfused.

A low diastolic can mean your arteries are relaxed and elastic, which is often healthy, but a very low diastolic can leave the heart’s own blood supply short, since the heart muscle is fed mostly during diastole. A high diastolic points to vessels that stay tense between beats.

Pulse: how often the cycle repeats

The third number is simply your heart rate at the moment of measurement. It is not a pressure at all, but it belongs on the same reading because pressure and rate are constantly traded off against each other. When blood volume drops or you stand up and blood pools in your legs, the body often defends its pressure by speeding the heart, which is the exact mechanism behind the tachycardia in POTS. That is why the orthostatic stand test watches heart rate and blood pressure together rather than either one alone.

The gap between systolic and diastolic (the pulse pressure) and the average across the whole cycle (the mean arterial pressure) each carry extra meaning of their own, enough that they get their own article.

The clinical chart, for context

The most widely used reference is the 2017 ACC/AHA blood pressure guideline, which sorts readings into categories built around the risk of high pressure over time:

Why recovery tracking uses a tighter, two-sided band

Autonomic grades blood pressure against a recovery optimum that is narrower than “normal” and, crucially, treats both directions as meaningful. The bands are U-shaped: there is a sweet spot in the middle, and drifting too low is graded down just as drifting too high is. That reflects the reality of orthostatic intolerance, where a chronically low baseline, a big drop on standing, or blood pooling in the legs can leave your brain under-perfused even when a clinician would call the number “fine.”

Systolic recovery grade bands:

Diastolic recovery grade bands:

Your overall blood-pressure grade in the app is simply the worse of the two, if your systolic lands in Good but your diastolic slips to Moderate, the reading grades as Moderate. That keeps a weak side from hiding behind a strong one. If you want the wider picture of how single-metric grades roll up into one score, see the autonomic score and grade bands.

Enter a reading to see both the recovery grade and the AHA category side by side:

Low blood pressure and the POTS connection

For most of medicine, “watch your blood pressure” means watch it stay down. In the world of POTS and orthostatic intolerance, the opposite worry is usually more relevant. Many people with post-viral dysautonomia run low blood volume; when they stand, gravity pulls blood into the legs and abdomen faster than the autonomic system can clamp the vessels down, so pressure to the brain sags and the heart races to compensate. The result is the familiar cluster: lightheadedness, a pounding heart, tunnel vision, fatigue.

A single low reading on the couch is rarely a crisis. What matters is the pattern: is your resting pressure trending downward over weeks, does it fall sharply when you stand, and does that line up with how you feel? Those are questions a trend answers and a snapshot cannot. As Dysautonomia International outlines, orthostatic intolerance is defined by what happens to your circulation on standing, which is why posture-matched, repeated readings beat any one number.

The bottom line

Systolic is the peak of each heartbeat’s push, diastolic is the steady pressure your arteries hold between beats, and pulse is how often the cycle repeats. The AHA chart is built to catch pressure that climbs too high, but recovery from POTS and dysautonomia usually turns on the opposite worry, pressure that runs too low or sags on standing, so the app grades a tighter, two-sided band and treats the worse of your two pressures as the reading. Track your own three numbers the same way over weeks, watch them alongside your heart rate and stand test, and the pattern will tell you far more than any single “normal” ever could.