Hypertension develops when blood pressure consistently reaches 130/80 mmHg or higher, and in most cases, no single cause is responsible. About 90% to 95% of people with high blood pressure have what’s called primary (or essential) hypertension, meaning it results from a combination of genetic predisposition, dietary habits, aging blood vessels, and hormonal signaling gone slightly off course. The remaining 5% to 10% have secondary hypertension, where a specific medical condition is driving the numbers up.
How Blood Pressure Is Classified
The 2025 guidelines from the American Heart Association and American College of Cardiology define four categories. Normal blood pressure is below 120/80 mmHg. Elevated blood pressure falls between 120 and 129 systolic with diastolic still under 80. Stage 1 hypertension is 130 to 139 systolic or 80 to 89 diastolic. Stage 2 hypertension is 140/90 mmHg or above. These thresholds matter because the causes described below can push your numbers from one category to the next over months or years, often without symptoms.
The Hormonal System That Controls Pressure
Your body has a built-in pressure regulator called the renin-angiotensin-aldosterone system. It works like a chain reaction: when blood flow to the kidneys drops, the kidneys release an enzyme that eventually produces a powerful hormone called angiotensin II. This hormone tightens blood vessels, raises resistance, and signals the adrenal glands to release aldosterone. Aldosterone tells the kidneys to hold onto sodium and water, which expands blood volume and pushes pressure higher. Over 90% of angiotensin II is produced locally in tissues rather than circulating through the bloodstream, which means this system is active throughout your body, not just in your kidneys.
When this system stays chronically activated, angiotensin II does more than constrict vessels. It promotes inflammation in artery walls, stimulates the growth of muscle cells within those walls, and damages the inner lining of blood vessels. Aldosterone compounds the problem by driving inflammation and scarring in both the heart and kidneys. This is why so many blood pressure medications target different points along this hormonal chain.
Salt, Fluid, and Kidney Function
High sodium intake raises blood pressure through a straightforward mechanism: more salt in the bloodstream pulls in more water, which expands circulating blood volume and increases the output of the heart. Your kidneys normally compensate by filtering out the excess sodium, a process that requires temporarily raising the pressure inside the kidney’s filtering units. In healthy kidneys, this self-corrects. But when the kidneys’ ability to excrete sodium is even slightly impaired, whether from genetics, aging, or early kidney damage, the pressure stays elevated to force out the extra salt. Over time, that sustained pressure becomes the new baseline.
Beyond fluid retention, high salt intake also remodels blood vessels and impairs the ability of artery walls to relax. These structural changes mean the vessels can’t accommodate surges in blood flow as easily, contributing to persistently higher readings even between meals.
Genetics and Family History
Twin studies estimate that 50% to 60% of blood pressure variation is inherited. This isn’t a single “hypertension gene” at work. Research using polygenic risk scores has identified over one million common genetic variants that each nudge blood pressure up or down by tiny amounts. Individually, these variants are insignificant. Collectively, they can predispose someone to hypertension years before lifestyle factors come into play. If both of your parents had high blood pressure, your risk is substantially higher than average, and it tends to appear at a younger age.
Genetics also influence how your body handles salt, how reactive your blood vessels are to stress hormones, and how efficiently your kidneys filter sodium. This explains why two people with identical diets and exercise habits can have very different blood pressure readings.
How Aging Stiffens Your Arteries
Arteries are built to stretch with each heartbeat, absorbing the pulse of blood and then gently recoiling. They get this flexibility from elastic fibers woven through their walls. With age, those elastic fibers break down from decades of repetitive stretching, and the body replaces them with collagen, a stiffer structural protein. The ratio of elastic tissue to collagen drops, and the artery walls become rigid.
Several processes accelerate this stiffening. Chronic high blood sugar promotes the formation of chemical cross-links between collagen fibers, essentially gluing them into a rigid mesh. Insulin resistance amplifies collagen production and triggers low-grade inflammation. Even the pattern of blood flow matters: turbulent flow at artery branch points activates signaling pathways that deposit more collagen in those specific areas. The result is that the heart has to pump harder to push blood through less compliant vessels, and systolic pressure (the top number) rises disproportionately. This is why isolated systolic hypertension is so common in people over 60.
Weight and Physical Activity
Excess body weight raises blood pressure through multiple routes. Fat tissue is metabolically active, producing hormones and inflammatory signals that activate the sympathetic nervous system (your body’s “fight or flight” wiring) and stimulate the kidneys to retain sodium. Visceral fat, the kind packed around abdominal organs, is particularly potent at driving these effects.
A meta-analysis of randomized trials found that each kilogram of weight lost (about 2.2 pounds) reduces systolic blood pressure by roughly 1 mmHg and diastolic by about 0.9 mmHg. That means losing 10 kilograms could lower your systolic reading by around 10 points, which is comparable to what a single blood pressure medication achieves. Physical inactivity compounds the problem by keeping resting heart rate elevated and reducing the flexibility of blood vessel walls.
Sleep Apnea and Nighttime Breathing
Obstructive sleep apnea is one of the most underdiagnosed contributors to high blood pressure. During an apnea episode, the airway collapses, oxygen levels drop, and the body responds with a surge of stress hormones. These repeated cycles of low oxygen and reoxygenation throughout the night trigger inflammatory pathways and essentially retrain the nervous system to maintain a higher baseline level of sympathetic activation.
This heightened nervous system activity doesn’t stop when you wake up. It persists into daytime hours, keeping blood vessels constricted and signaling the kidneys to hold onto sodium and increase resistance to blood flow. People with untreated sleep apnea are particularly likely to have resistant hypertension, the kind that doesn’t respond well to standard medications. Their blood pressure also tends to stay elevated at night rather than dipping during sleep, which is an independent risk factor for heart and kidney damage.
Medical Conditions That Raise Pressure
Secondary hypertension has a specific, identifiable cause. One of the more common culprits is renal artery stenosis, a narrowing of the arteries feeding the kidneys. When one kidney receives less blood flow, it interprets the drop as a signal that the whole body’s pressure is too low and activates the renin-angiotensin-aldosterone system aggressively. Even though the rest of the body has normal or high pressure, the underperfused kidney keeps pumping out signals to raise it further. When both kidney arteries are narrowed, or the person has only one functioning kidney, the problem is compounded because the body also loses its ability to excrete excess sodium and fluid.
Primary aldosteronism is another important cause. The adrenal glands overproduce aldosterone, driving sodium retention and blood volume expansion. High blood pressure is typically the only symptom, which is why the Endocrine Society has called for broader screening. Other endocrine causes include conditions where the adrenal glands overproduce cortisol or adrenaline-like hormones, and thyroid disorders that speed up or slow down the heart’s output. Chronic kidney disease of any cause can also sustain hypertension by impairing the kidneys’ ability to manage sodium and fluid balance.
Medications That Raise Blood Pressure
Several common medications can push blood pressure higher, sometimes enough to tip someone from elevated into hypertensive range. All anti-inflammatory painkillers (NSAIDs) taken at doses strong enough to reduce pain and inflammation can raise blood pressure in both normotensive and hypertensive people. The average increase is about 3/2 mmHg, but it varies widely between individuals. NSAIDs also blunt the effectiveness of most blood pressure medications, with calcium channel blockers being the exception. Notably, low-dose aspirin (75 mg daily) does not share this effect.
Decongestants containing pseudoephedrine or phenylephrine constrict blood vessels and can cause significant spikes. Stimulant medications used for attention disorders raise both heart rate and vascular resistance. Oral contraceptives containing estrogen, certain antidepressants, and long-term corticosteroid use are also well-established contributors. If your blood pressure is newly elevated or harder to control than expected, a medication review is a reasonable starting point.
The Sympathetic Nervous System’s Role
Running through many of these causes is a common thread: overactivation of the sympathetic nervous system. This is the branch of your nervous system that raises heart rate, tightens blood vessels, and tells the kidneys to conserve salt, all useful responses in a genuine emergency, but damaging when they’re sustained. Obesity, sleep apnea, chronic stress, excess aldosterone, and even high salt intake can all ramp up sympathetic tone. Over time, the blood vessels remodel to accommodate the higher pressure, their walls thicken, their inner diameter shrinks, and the elevated pressure becomes self-reinforcing even if the original trigger is removed. This is why hypertension caught and treated early is easier to control than hypertension that has been present for years.