High blood pressure develops when the force of blood pushing against your artery walls stays elevated over time. In roughly 85% to 90% of adults with hypertension, no single identifiable cause exists. This is called primary (or essential) hypertension, and it results from a mix of genetics, lifestyle habits, and gradual changes in how your body manages fluid and blood vessel tension. The remaining 10% to 15% of cases stem from a specific medical condition or medication.
How Your Body Regulates Blood Pressure
Your body has a built-in pressure control system that involves your kidneys, blood vessels, and hormones working together. When blood pressure drops, your kidneys release a signal that triggers a chain reaction: your liver and blood vessels produce a hormone that tightens artery walls, raising resistance to blood flow. That same chain reaction tells your adrenal glands to release a second hormone that makes your kidneys hold onto sodium and water, increasing blood volume. Both effects push blood pressure up.
In a healthy system, opposing signals kick in to relax blood vessels and bring pressure back down. When this balance tips, and the constricting signals dominate or never fully shut off, blood pressure stays elevated. Over 90% of the key constricting hormone found in your tissues is produced locally rather than circulating from elsewhere in the body, which helps explain why high blood pressure can be so persistent once it takes hold.
Sodium, Potassium, and Fluid Balance
Excess sodium is one of the most well-established dietary drivers of high blood pressure. When you eat more sodium than your kidneys can efficiently clear, your body retains water to dilute it. That extra fluid increases blood volume and puts more pressure on artery walls.
Potassium plays the opposite role, and not getting enough of it makes things worse. Low potassium levels cause blood vessels to constrict through a direct effect on muscle cells in the vessel walls: potassium depletion changes the electrical charge across those cells, allowing more calcium to flow in, which tightens them. At the same time, low potassium makes it harder for your body to handle sodium, leading to further fluid retention. In studies on healthy people, dietary potassium depletion raised blood pressure, and supplementing potassium blunted the blood pressure increase caused by a high-sodium diet.
Genetics and Family History
Hypertension runs in families, and your risk rises significantly if both parents have it. Researchers have identified more than 100 genetic variations linked to essential hypertension, affecting genes involved in blood vessel tone, kidney sodium handling, and hormone signaling. No single gene causes high blood pressure on its own. Instead, many small genetic contributions add up, and they interact with your diet, weight, and activity level to determine your actual risk. The inheritance pattern is complex enough that scientists still can’t predict it from genetics alone.
How Aging Stiffens Your Arteries
Arteries closer to your heart, especially the aorta, are naturally rich in elastic fibers that stretch with each heartbeat and then snap back to keep blood flowing smoothly between beats. Over decades, the repetitive stretching causes fatigue in the wall material. The elastic fibers gradually break down and get replaced by stiffer collagen, and the smooth muscle cells in the artery walls become less responsive.
This stiffening means arteries can no longer absorb the surge of blood pumped out with each heartbeat. The top number of your blood pressure (systolic) climbs, while the bottom number (diastolic) may stay the same or even drop. This pattern, called isolated systolic hypertension, is the most common form of high blood pressure in people over 60. Heart rate itself increases stiffness through a physical effect related to the composition of the artery wall, which is one reason a chronically elevated resting heart rate is considered a risk factor.
Obesity and the Sympathetic Nervous System
Carrying excess body fat raises blood pressure through several pathways at once. One of the most important involves leptin, a hormone produced by fat cells. Leptin’s normal job is to signal your brain to increase energy expenditure. But it does this partly by activating the sympathetic nervous system, the same system responsible for your “fight or flight” response. In obesity, leptin levels are chronically elevated, which keeps the sympathetic nervous system running at a higher baseline. The result is sustained constriction of blood vessels and a faster heart rate, both of which raise blood pressure.
Obesity also promotes insulin resistance, which further disrupts sodium handling in the kidneys and contributes to fluid retention. These overlapping mechanisms explain why losing even a modest amount of weight often produces a noticeable drop in blood pressure.
Sleep Apnea and Nighttime Blood Pressure
Obstructive sleep apnea is one of the most underdiagnosed causes of resistant high blood pressure. During apnea episodes, your airway collapses repeatedly, causing oxygen levels to drop and carbon dioxide to spike. Each episode triggers a burst of stress hormones and sympathetic nervous system activation. Over time, these repeated surges raise baseline levels of stress hormones that persist even during the daytime, driving blood pressure up around the clock. If your blood pressure is difficult to control despite medication, or if you snore heavily and feel tired during the day, sleep apnea is worth investigating.
Kidney Disease
Your kidneys are central to blood pressure regulation because they control how much sodium and water your body retains. When kidney function declines, the kidneys lose their ability to efficiently excrete sodium during the day. One hypothesis is that the body compensates by keeping blood pressure elevated at night to force more sodium out, a pattern called “nondipping” that shows up on 24-hour blood pressure monitors. This creates a vicious cycle: high blood pressure damages the kidneys further, and worsening kidney function makes blood pressure harder to control.
Medications That Raise Blood Pressure
Several common medications can push blood pressure up, sometimes significantly enough to cause problems even in people who were previously in a normal range.
- Anti-inflammatory painkillers (NSAIDs) cause the body to retain water, which increases blood volume and stresses the kidneys.
- Decongestants narrow blood vessels throughout the body, not just in the nose, making it harder for blood to flow and raising pressure.
- Some antidepressants alter the brain chemicals that influence blood vessel tone.
- Hormonal birth control raises blood pressure in some people.
- Stimulant medications used for ADHD increase heart rate and can raise blood pressure.
- Immunosuppressants can affect kidney function in ways that raise pressure.
Caffeine can cause short-term blood pressure spikes, and certain herbal supplements may either raise blood pressure directly or interfere with blood pressure medications. If you take any of these regularly and your readings are elevated, it’s worth discussing alternatives.
Current Blood Pressure Categories
The 2025 guidelines from the American Heart Association and American College of Cardiology define four categories. Normal blood pressure is below 120/80. Elevated blood pressure is 120 to 129 systolic with a diastolic still under 80. Stage 1 hypertension starts at 130/80, and Stage 2 hypertension begins at 140/90 or higher. If your systolic and diastolic numbers fall into different categories, the higher category applies.
These thresholds matter because cardiovascular risk doesn’t suddenly appear at 140/90. It rises gradually starting well below that point, which is why the guidelines now flag readings in the 130s as a stage of hypertension rather than “prehypertension.”