Blood pressure is the force your blood exerts against the walls of your arteries as it moves through your body. It’s recorded as two numbers, like 120/80 mmHg, and those numbers reflect two distinct moments in every heartbeat. Understanding what they mean, what’s normal, and what pushes them too high or too low gives you a practical framework for one of the most important measurements in medicine.
What the Two Numbers Mean
The top number, called systolic pressure, measures the force against your artery walls when your heart contracts and pushes blood out. The bottom number, diastolic pressure, measures that same force while your heart relaxes between beats. Together, they tell you how hard your cardiovascular system is working at any given moment.
Two main factors determine your blood pressure: how much blood your heart pumps with each beat (cardiac output) and how much resistance your blood vessels create (vascular resistance). If your heart pumps more blood per beat, or if your arteries are narrower or stiffer, pressure goes up. If your vessels relax and widen, pressure drops. This is why blood pressure changes constantly throughout the day, rising during exercise or stress and falling during rest and sleep.
Normal, Elevated, and High Ranges
The 2025 guidelines from the American Heart Association and American College of Cardiology divide blood pressure into four categories:
- 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
If your systolic and diastolic numbers fall into two different categories, the higher category applies. So a reading of 135/72 would be classified as stage 1 hypertension based on the systolic number alone, even though the diastolic is normal.
Low Blood Pressure
On the other end of the spectrum, a reading below 90/60 mmHg is generally considered low blood pressure, or hypotension. For some people, naturally low pressure causes no symptoms at all. For others, it leads to dizziness, lightheadedness, blurred vision, fatigue, trouble concentrating, or fainting. The main practical risk is falling and injuring yourself during a dizzy spell.
Severe drops in blood pressure can cause shock, which is a medical emergency. Signs include confusion (especially in older adults), cold and clammy skin, rapid shallow breathing, and a weak pulse.
How High Blood Pressure Damages Your Body
Hypertension rarely causes obvious symptoms in its early years, which is why it’s sometimes called a silent condition. The damage accumulates gradually across multiple organs.
Your kidneys are especially vulnerable. They rely on a dense network of tiny blood vessels to filter waste from your blood, and sustained high pressure damages those vessels over time. Kidney function declines, fluid and waste build up, and in severe cases the result is kidney failure requiring dialysis or a transplant. High blood pressure is one of the most common causes of kidney failure.
Your eyes take a similar hit. The delicate blood vessels supplying the retina, the light-sensing layer at the back of the eye, can bleed or become damaged. This can cause blurred vision and, in the worst cases, complete vision loss. Having diabetes alongside high blood pressure makes retinal damage significantly more likely.
In the brain, chronically elevated pressure raises the risk of stroke, vascular dementia, and mild cognitive impairment. The brain depends on steady, well-regulated blood flow, and years of excess pressure compromise the vessels that deliver it.
Getting an Accurate Reading
Blood pressure is surprisingly easy to measure incorrectly. Small errors in positioning can shift your numbers by several points, which may be enough to change your category. The CDC recommends sitting in a comfortable chair with your back supported for at least five minutes before taking a reading. Both feet should be flat on the ground with legs uncrossed. Rest your arm on a table at chest height, and place the cuff against bare skin, not over a sleeve. The cuff should be snug but not tight.
Even with perfect technique, your reading can vary depending on the setting. Roughly 9 to 23 percent of the general population has what’s known as white coat hypertension, where pressure reads high in a clinic but is normal at home. Another 7 to 20 percent have masked hypertension, the opposite pattern: normal in the office but elevated at home. This is why many doctors recommend home monitoring to confirm a diagnosis, especially if a single office reading seems out of character.
What Affects Your Numbers
Age is one of the strongest predictors. Arteries stiffen over time, which tends to push systolic pressure higher with each decade. A large NIH-funded trial called SPRINT found that lowering systolic pressure to below 120 in adults age 50 and older significantly reduced the risk of cardiovascular disease and death. Still, the right target for any individual depends on other health conditions and overall fitness, so goals for older adults sometimes differ from those for younger people.
Diet plays a measurable role. Reducing salt intake by about 4.4 grams per day for four weeks or more lowers systolic pressure by an average of 5 mmHg in people with hypertension and about 2 mmHg in people with normal pressure. Larger reductions in sodium produce larger drops: cutting roughly 6 grams of salt per day is associated with a systolic decrease of nearly 11 mmHg in hypertensive individuals. Those numbers may sound modest, but at a population level, even a few points of reduction meaningfully lower the risk of heart attack and stroke.
Physical activity, body weight, alcohol intake, stress, and sleep quality all influence blood pressure as well. During exercise, systolic pressure rises naturally because the heart pumps more blood to working muscles, while resistance in those muscle blood vessels drops to accommodate the increased flow. Over time, regular exercise tends to lower resting blood pressure by keeping arteries flexible and reducing baseline vascular resistance.
Blood Pressure in Older Adults
Managing blood pressure becomes more nuanced after 65. Systolic pressure often climbs while diastolic may actually drop, creating a wide gap between the two numbers. This “isolated systolic hypertension” is the most common form in older adults and carries its own set of risks, particularly for stroke.
Treatment decisions weigh the benefits of lower pressure against the risks of side effects like dizziness and falls, which are more consequential in older bodies. The general target remains below 120 systolic based on the SPRINT findings, but individual targets vary. Doctors consider factors like kidney function, medication tolerance, and how many other conditions a patient is managing simultaneously.