BP stands for blood pressure, the force your blood exerts against the walls of your arteries as your heart pumps it through your body. It’s measured in millimeters of mercury (mmHg) and written as two numbers, like 120/80. Normal blood pressure falls below 120/80 mmHg, and nearly half of U.S. adults (about 120 million people) have blood pressure that’s too high.
What the Two Numbers Mean
The top number is your systolic pressure. It measures the force of blood being pumped out of the heart with each beat. The bottom number is your diastolic pressure, measured between heartbeats when the heart is relaxing and filling with blood. Both numbers matter, though systolic pressure gets more attention as a risk factor, especially as you age.
When you see a reading like 130/85, that means your blood pushes against artery walls with a force of 130 mmHg during each heartbeat and 85 mmHg during the pause between beats.
Blood Pressure Categories
The American Heart Association breaks 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
On the other end, readings below 90/60 mmHg are considered low blood pressure (hypotension). Low blood pressure often causes no symptoms at all, but when it does, dizziness and lightheadedness are the most common. A significant drop can cause fainting, confusion, or fatigue.
How Your Body Controls Blood Pressure
Blood pressure comes down to two things: how much blood your heart pumps per minute and how much resistance your blood vessels create. Your average arterial pressure equals cardiac output multiplied by the resistance in your blood vessels. Anything that increases either one raises your blood pressure.
Your kidneys play a central role in this regulation. When blood pressure drops, the kidneys release an enzyme that triggers a chain reaction: your liver and lungs produce hormones that tighten the walls of small arteries, pushing pressure back up. These same hormones signal your kidneys to hold onto more sodium, which causes your body to retain water. More water in the bloodstream means more blood volume, which means higher pressure. This system works constantly in the background to keep your blood pressure in a livable range.
Why High Blood Pressure Is Dangerous
High blood pressure damages your body quietly, often over years, which is why it’s called a “silent killer.” The damage starts in the arteries themselves. Elevated pressure injures the inner lining of artery walls, making it easier for fats in the bloodstream to collect there. Over time, the artery walls become less elastic and increasingly narrow.
That narrowing creates problems everywhere blood flows. In the heart, it leads to coronary artery disease, chest pain, irregular heart rhythms, and heart attacks. The heart also has to work harder against all that resistance, which causes the lower left chamber to thicken and enlarge. An enlarged heart raises the risk of heart failure and sudden cardiac death.
In the brain, hardened or blocked arteries can cause mini-strokes (brief interruptions in blood supply that serve as warning signs) or full strokes, where part of the brain is cut off from oxygen. The constant pressure on weakened artery walls can also cause them to bulge outward, forming an aneurysm. Aneurysms are most common in the aorta, the body’s largest artery, and can rupture with life-threatening consequences.
High blood pressure also damages the small blood vessels in the kidneys, the eyes (potentially causing vision loss through optic nerve damage), and the reproductive organs. Reduced blood flow can contribute to erectile dysfunction in men and decreased arousal and vaginal dryness in women. Only about 1 in 4 adults with hypertension actually have it under control.
How to Get an Accurate Reading
Blood pressure fluctuates throughout the day based on activity, stress, food, and even body position. A single high reading doesn’t necessarily mean you have hypertension, so accuracy matters. The CDC recommends these steps for a reliable measurement:
- Avoid eating or drinking for 30 minutes beforehand
- Empty your bladder before the reading
- Sit with your back supported for at least 5 minutes before measuring
- Keep both feet flat on the ground, legs uncrossed
- Rest your arm on a table at chest height with the cuff against bare skin
- Don’t talk during the measurement
Skipping any of these steps can inflate your reading by several points, enough to push a borderline number into a higher category.
White Coat and Masked Hypertension
Some people consistently show high readings at the doctor’s office but normal readings at home. This is called white coat hypertension, and it’s one of the most common reasons doctors order 24-hour ambulatory monitoring, where you wear a portable cuff that takes readings throughout your normal day. The diagnosis applies when office readings run above 140/90 but daytime readings outside the clinic stay below 135/85.
The opposite pattern, masked hypertension, is more concerning. Your readings look fine in the office (below 140/90) but run high during everyday life (135/85 or above). Because the elevated pressure goes undetected during routine visits, masked hypertension can cause organ damage without anyone realizing the numbers are off. Home monitoring or ambulatory monitoring is the only way to catch it.