Your Two Numbers: What the Top, the Bottom, and the Gap Mean

"When you can measure what you are speaking about, and express it in numbers, you know something about it." — Lord Kelvin (1883)

A woman gets her reading back: 138 over 88. She knows lower is supposed to be better, and that is the entire extent of what the two numbers mean to her. Which one is which, what each is measuring, whether the top or the bottom is the one to worry about, none of it has ever been explained. So she does what most people do with a number she can't read. She worries about it vaguely and understands it not at all.

The reading is not one measurement. It is two, taken at two different moments of a single heartbeat, and the space between them is a third piece of information. Once you can read all three, the rest of this course stops being a foreign language. This is the short literacy lesson that should come before everything else.

The heartbeat has two moments

Blood pressure is the force of blood pushing on the artery walls, and that force is not steady, it pulses, because the heart pumps in beats. The cuff captures the two extremes.

The top number (systolic) is the push. When the heart squeezes, it drives a surge of blood into the arteries, and the pressure peaks. That peak is the systolic number. It reflects how hard the pump is working and how much the pipes resist that surge.

The bottom number (diastolic) is the rest. Between beats, the heart relaxes and refills, and the pressure falls to its lowest point. That trough is the diastolic number. It reflects the baseline tension in the pipes while the pump is idling.

So "138 over 88" means the pressure peaked at 138 during the push and never dropped below 88 during the rest. Two moments, one beat.

Mapping the numbers to the levers

This course runs on three levers, the volume of fluid, the strength of the pump, and the tightness of the pipes (Volume, Pump, Pipes - The Three Levers Underneath Your Number). The two numbers are windows onto them.

• A high top number points toward a hard-driving pump, stiff pipes, or extra volume to push against, especially as the arteries stiffen with age.
• A high bottom number points more specifically at the pipes staying tense even at rest, which is why isolated high diastolic shows up more in younger adults with springy arteries.

Neither number alone tells you which lever is yours. But knowing what each one is reporting is the first step toward asking "high because of what."

The gap between them is a third signal

Here is the part almost nobody is taught. The distance between the top and bottom numbers, the pulse pressure, is itself meaningful. A reading of 120 over 80 has a 40-point gap. A reading of 160 over 70 has a 90-point gap, and that wide gap is a flag.

A widening gap usually means the large arteries have stiffened, losing the springiness that used to cushion each heartbeat. Stiff pipes let the peak run higher and the trough fall lower. That is why, as people age, the top number tends to climb while the bottom number can actually drift down, and the gap stretches. A wide pulse pressure in an older adult is its own quiet signal about arterial aging, separate from either number on its own.

Which number should you watch?

Both, but the emphasis shifts with age.

In adults over about 50, the top number is the stronger predictor of cardiovascular risk, because stiff-artery, high-systolic hypertension is the dominant pattern later in life. In younger adults, the bottom number carries more weight. Either number being high counts as high blood pressure, so neither gets ignored, but if you are older and your top number is up while your bottom number looks fine, that is the common and important pattern, not a reason to relax.

What to actually do with this

Three things to carry out of here.

1. Read both numbers and the gap. Top is the push, bottom is the rest, and a wide gap points to stiff arteries.
2. Don't fixate on one number in isolation. Either being high matters. The pattern (which one, and the gap) is the clue to which lever is yours.
3. Bring the full picture to the visit. Logged readings with both numbers, over two weeks, are what make the pattern visible (Bring Your Own Data - What Your Doctor Cannot See).

With the numbers readable, the next question is what counts as high in the first place, and why the line keeps moving: JNC 8 vs AHA - Why Your Doctor's BP Target Keeps Moving.

References

Sources behind the claims in this piece. Listed at the bottom so the prose reads cleanly. Each entry backs the claim it sits under.

On what systolic and diastolic represent and the blood-pressure categories:

• 2025 AHA/ACC Guideline for the Management of High Blood Pressure in Adults. Jones DW et al., 2025, JACC

On the graded risk per increment and systolic-vs-diastolic prediction by age:

• 2017 ACC/AHA High Blood Pressure Guideline (Executive Summary). Whelton PK et al., 2018, JACC

On pulse pressure, arterial stiffness, and the widening gap with age:

• Large-Artery Stiffness in Health and Disease: JACC State-of-the-Art Review. Chirinos JA et al., 2019, JACC