Blood pressure is the force your blood exerts against the walls of your arteries as your heart pumps it through your body. It’s determined by two factors: how much blood your heart pushes out with each beat (cardiac output) and how much resistance your blood vessels create against that flow. The simple relationship is pressure equals flow times resistance, which means anything that changes either your heart’s output or the tightness of your blood vessels will change your blood pressure.
What the Two Numbers Mean
A blood pressure reading gives you two numbers, like 120/80. The top number, systolic pressure, is the maximum pressure inside your large arteries when your heart muscle contracts and pushes blood outward. The bottom number, diastolic pressure, is the lowest pressure in those same arteries when your heart relaxes between beats. Both numbers are measured in millimeters of mercury (mmHg), a unit that dates back to the original mercury-column pressure gauges doctors used.
Systolic pressure tells you how forcefully blood hits your artery walls during each heartbeat. Diastolic pressure reflects the baseline tension your arteries are under even when the heart is resting. Both matter for your health, but they can shift independently. As people age, systolic pressure tends to climb while diastolic pressure may actually drop, which is why doctors pay close attention to both numbers rather than just one.
Current Blood Pressure Categories
The 2025 guidelines from the American College of Cardiology classify blood pressure into four ranges:
- Normal: below 120/80 mmHg
- Elevated: 120 to 129 systolic with diastolic still below 80
- Stage 1 hypertension: 130 to 139 systolic or 80 to 89 diastolic
- Stage 2 hypertension: 140 or higher systolic, or 90 or higher diastolic
The World Health Organization uses a slightly different threshold, defining hypertension as 140/90 or above on two separate days. This means someone diagnosed with Stage 1 hypertension in the U.S. might not meet the WHO’s definition. If you’re comparing your numbers to a standard, it helps to know which guideline your doctor is using.
How Your Body Regulates Blood Pressure
Your body doesn’t leave blood pressure to chance. It runs two overlapping control systems: one for moment-to-moment adjustments and another for longer-term regulation.
The fast system relies on pressure sensors called baroreceptors, embedded in the walls of major arteries near your heart and neck. These sensors detect how much the artery wall stretches with each heartbeat. When your blood pressure drops suddenly, say when you stand up quickly, the baroreceptors fire fewer nerve signals. Your brain responds within seconds by tightening blood vessels and increasing heart rate to push pressure back up. The reverse happens when pressure spikes: baroreceptors fire more rapidly, your brain dials back the heart rate, and blood vessels relax. This feedback loop works beat to beat, keeping pressure stable through position changes, exercise, and stress.
The slower system involves your kidneys and a hormone chain called the renin-angiotensin-aldosterone system. When blood pressure or blood volume drops, your kidneys release an enzyme that triggers a cascade of hormonal signals. The end result is a molecule that tightens blood vessels, prompts your adrenal glands to release a hormone that makes your kidneys hold onto sodium and water, and even stimulates thirst so you drink more fluid. All of this expands blood volume and raises pressure over hours to days. This is the system that many common blood pressure medications target.
What Happens When Blood Pressure Stays High
Persistently high blood pressure damages the body in ways you can’t feel until the harm is significant. The constant excess force batters the inner lining of arteries, making them stiffer and more prone to plaque buildup. This damage doesn’t stay in one place. It spreads across four major organ systems: blood vessels, heart, brain, and kidneys.
The heart has to work harder against elevated pressure, which causes the muscle wall to thicken over time, much like any muscle that’s chronically overworked. That thickening eventually makes the heart less efficient at pumping. Hypertension more than doubles the risk of coronary heart disease and triples the risk of both heart failure and stroke. In the kidneys, high pressure damages the tiny filtering units, gradually reducing their ability to clean your blood. In the brain, it contributes to both stroke and a form of cognitive decline called vascular dementia. The eyes are vulnerable too, with high pressure damaging the small blood vessels of the retina.
Getting an Accurate Reading
Blood pressure is surprisingly easy to measure incorrectly. A standard reading uses an inflatable cuff on your upper arm, a pressure gauge, and either a stethoscope or an automatic sensor. But where your arm is positioned during the measurement makes a real difference. In one study, standing with the arm hanging at the side produced readings about 8 mmHg higher for systolic and 9 mmHg higher for diastolic compared to holding the arm at heart level. That gap is enough to push a normal reading into the elevated range.
For the most accurate result, sit quietly for at least five minutes before the reading. Your arm should be supported at heart level, not dangling or raised. The cuff needs to fit properly: too small and it will read artificially high, too large and it may read low. Because blood pressure fluctuates throughout the day, a single reading is just a snapshot. Doctors typically want measurements from multiple visits, or a series of home readings over several days, before making any diagnosis.