Blood pressure is the physical force your blood exerts against the walls of your arteries as it moves through your body. Every time your heart beats, it pumps blood into a network of arteries, and the push of that blood against the vessel walls creates a measurable pressure. The two numbers in a blood pressure reading capture two distinct moments: the peak force when your heart contracts and the residual force when it rests between beats.
What the Two Numbers Mean
A blood pressure reading like 120/80 describes two phases of every heartbeat. The top number, called systolic pressure, measures the force inside your arteries at the exact moment your heart squeezes and pushes blood out. The bottom number, diastolic pressure, measures the pressure that remains in your arteries while your heart relaxes and refills with blood. Both numbers matter, but they tell you different things about your cardiovascular system.
Think of it like a garden hose. Systolic pressure is the surge when you first turn the spigot on. Diastolic pressure is the baseline pressure that stays in the hose even between surges. A healthy heart and flexible arteries produce a systolic push that’s noticeably higher than the diastolic baseline, but not dramatically so.
Why It’s Measured in Millimeters of Mercury
Blood pressure is expressed in millimeters of mercury (mmHg), a unit that dates back to the earliest measurement devices. The original instruments used a column of liquid mercury that rose and fell with each heartbeat, and the height of that column in millimeters became the standard unit. The first blood pressure measurement on record happened in 1733, when Reverend Stephen Hales inserted a tube into a horse’s artery and watched a column of blood rise over eight feet. Mercury, being far denser than blood, allowed the same pressures to be captured on a much shorter, more practical scale. Even though most modern devices are digital and contain no mercury, the unit stuck.
What Determines Your Blood Pressure
Blood pressure comes down to two variables: how much blood your heart pumps per minute and how much resistance your blood vessels create. Multiply those together and you get your overall blood pressure. Doctors sometimes write this as a simple equation: blood pressure equals cardiac output times the resistance in your blood vessels.
Cardiac output increases when your heart beats faster or pumps more forcefully, both of which raise pressure. Vascular resistance increases when the small arteries throughout your body narrow, which also raises pressure. Anything that changes either variable, whether it’s exercise, stress, dehydration, or medication, will shift your blood pressure reading.
How Your Body Adjusts Pressure in Real Time
Your body has a rapid-response system for keeping blood pressure stable from moment to moment. Specialized nerve endings called baroreceptors sit in the walls of major arteries near your heart and in your neck. These sensors detect how much the artery wall is stretching. When the stretch is normal, they signal your brain that pressure is adequate. When stretching drops, as it does when you stand up suddenly, your brain responds within seconds by telling your heart to beat faster and your blood vessels to tighten.
This reflex is why you can go from lying flat to standing upright without passing out. It’s also why a sudden drop of just 20 mmHg in systolic pressure, say from 110 to 90, can make you dizzy or faint. That’s what happens when the reflex can’t compensate quickly enough, a condition called orthostatic hypotension.
How Your Body Controls Pressure Over Hours and Days
For longer-term regulation, your kidneys run a hormonal system that adjusts blood volume and vessel tightness over hours to days. When blood pressure falls, your kidneys release an enzyme that triggers a chain reaction: it activates a hormone that narrows small arteries (raising resistance) and signals your adrenal glands to release another hormone that causes your kidneys to retain sodium. Where sodium goes, water follows. More water in your bloodstream means more blood volume, which raises pressure back up.
This system works in reverse too. When pressure is adequate, the chain reaction quiets down, your kidneys release more sodium and water, and blood volume decreases. Many common blood pressure medications work by interrupting specific steps in this chain, either blocking the hormone that narrows arteries or helping the kidneys release more sodium and water.
The Clinical Categories
The 2025 guidelines from the American Heart Association define four categories of blood pressure in adults, based on readings taken in a healthcare setting:
- Normal: below 120/80 mmHg
- Elevated: systolic 120 to 129 and diastolic below 80
- Stage 1 hypertension: systolic 130 to 139 or diastolic 80 to 89
- Stage 2 hypertension: systolic 140 or higher, or diastolic 90 or higher
If your systolic and diastolic numbers fall into different categories, the higher category applies. So a reading of 135/75 counts as stage 1 hypertension because the systolic number is in that range, even though the diastolic number is normal. On the low end, blood pressure below 90/60 is generally considered hypotension, though some people run naturally low without symptoms.
Why Blood Pressure Changes With Age
Arteries aren’t rigid pipes. In a young, healthy person, the large arteries, especially the aorta, are elastic. They stretch when the heart pumps and then recoil between beats, which smooths out the pressure wave and keeps systolic and diastolic numbers relatively close together. With age, calcium deposits accumulate in artery walls, elastic fibers fray, and collagen fibers stiffen and cross-link. Research from the American Heart Association shows that large artery compliance (a measure of flexibility) declines steadily with age.
A stiffened aorta can’t absorb the heart’s pumping force as well, so more of that force transmits directly into the blood. The result is a pattern common in older adults: systolic pressure climbs while diastolic pressure stays the same or even drops. This widening gap between the two numbers reflects arteries that have lost their ability to buffer each heartbeat. It’s one reason isolated high systolic pressure becomes more common after age 60.
Getting an Accurate Reading
Blood pressure is surprisingly sensitive to how and when you measure it. The CDC recommends a specific routine for reliable results: avoid eating, drinking, or using caffeine for 30 minutes beforehand. Empty your bladder. Sit with your back supported and both feet flat on the floor for at least five minutes before the measurement. Rest the arm with the cuff on a surface at chest height, and keep your legs uncrossed.
The cuff should sit against bare skin, not over a sleeve, and should be snug without squeezing. Don’t talk during the reading. Even a casual conversation can raise your systolic pressure by several points. Taking two or three readings a minute apart and averaging them gives a more reliable number than any single measurement, which is why your doctor may check more than once during a visit.