High blood pressure doesn’t have a single root cause in most people. Between 85% and 95% of cases are classified as “primary” or “essential” hypertension, meaning no one identifiable trigger explains it. Instead, blood pressure creeps up over years as several biological systems drift out of balance simultaneously: your kidneys retain too much sodium, your blood vessels stiffen or lose their ability to relax, your nervous system runs hotter than it should, and your genes set the stage for all of it. The remaining 5% to 15% of cases are “secondary” hypertension, caused by a specific medical condition that can often be treated directly.
How Your Body Regulates Blood Pressure
Understanding what goes wrong starts with understanding what’s supposed to happen. Your kidneys act as the master control for blood pressure through a hormone chain reaction. When pressure drops, your kidneys release an enzyme called renin into the bloodstream. Renin breaks apart a protein made by your liver, eventually producing a hormone called angiotensin II. This hormone does two things at once: it tightens the muscular walls of small arteries, raising pressure immediately, and it signals your adrenal glands to release aldosterone. Aldosterone tells your kidneys to hold onto sodium instead of flushing it out. That extra sodium pulls water into your bloodstream, increasing blood volume and pushing pressure up further.
In a healthy system, this cycle turns on when pressure is too low and shuts off when it normalizes. In primary hypertension, parts of this system become overactive or lose their off switch. Your kidneys may retain sodium even when blood volume is already adequate, or your arteries may stay constricted when they should relax. The genes most strongly linked to inherited hypertension are the ones that control this exact hormone chain, particularly the genes for angiotensinogen (AGT) and its receptor (AGTR1).
Blood Vessel Damage and Stiffness
The inner lining of every blood vessel, called the endothelium, produces nitric oxide, a molecule that keeps arteries relaxed and open. When endothelial cells stop producing enough nitric oxide, blood vessels narrow instead of staying dilated. This forces your heart to pump harder to move the same volume of blood, raising pressure. Over time, the higher pressure damages the vessel lining further, which reduces nitric oxide production even more, creating a self-reinforcing cycle.
Aging accelerates this process. Large arteries like the aorta gradually lose their elasticity. Before age 50, arterial stiffness increases by less than 1% per year. After 60, the rate of stiffening roughly doubles. Stiffer arteries can’t absorb the pulse of blood that comes with each heartbeat, so the peak pressure (systolic reading) climbs while the resting pressure between beats (diastolic reading) may actually drop. This is why “isolated systolic hypertension,” where only the top number is high, becomes so common in older adults. Genetic variations affecting the blood vessel lining also contribute. Several genes linked to essential hypertension are involved in endothelial function, and changes in these genes can leave vessels abnormally constricted from a relatively young age.
Sodium, Potassium, and Fluid Balance
The relationship between salt and blood pressure is more nuanced than “eat less salt.” What matters is the balance between sodium and potassium. Your kidneys use a molecular switch in their filtering tubes that responds to potassium levels. When potassium intake is low, the switch activates a transporter that aggressively holds onto sodium and chloride. A potassium-deficient diet can flip this switch in as little as 12 hours, causing your body to retain salt and water it doesn’t need.
When potassium intake is adequate, that same transporter quiets down, and your kidneys let sodium pass through more freely. This is why increasing potassium (through foods like bananas, potatoes, beans, and leafy greens) can lower blood pressure even without cutting sodium. The ideal scenario is higher potassium and lower sodium together, which keeps the kidney’s filtering system in a relaxed, balanced state.
Insulin Resistance and Metabolic Shifts
People with insulin resistance, where cells stop responding normally to insulin, often develop high blood pressure well before they’re diagnosed with diabetes. When cells resist insulin, the body compensates by producing more of it. Chronically elevated insulin affects blood pressure through at least three pathways: it makes your kidneys retain more sodium, it ramps up your sympathetic nervous system (the “fight or flight” wiring that constricts blood vessels), and it activates the same kidney hormone system described above. This is one reason hypertension, prediabetes, and abdominal obesity so frequently travel together.
Stress and the Nervous System
Stress causes sharp, temporary blood pressure spikes. Your body releases hormones that speed up your heart rate and narrow blood vessels, preparing you for action. Once the stressor passes, pressure returns to baseline. There is no proof that stress alone causes long-term high blood pressure. However, repeated short spikes can damage blood vessels, the heart, and the kidneys over time in ways that resemble the damage from chronic hypertension. Stress also tends to drive behaviors that do raise pressure persistently: poor sleep, excess alcohol, overeating, and physical inactivity.
Secondary Causes Worth Knowing
In the 5% to 15% of cases where a specific cause exists, identifying and treating it can sometimes resolve the hypertension entirely. The most common secondary causes, ranked roughly by frequency:
- Obstructive sleep apnea: Repeated drops in oxygen overnight trigger surges in stress hormones and keep the nervous system in a heightened state.
- Narrowed kidney arteries: Reduced blood flow to a kidney tricks it into thinking pressure is low, activating the renin-aldosterone cycle inappropriately.
- Excess aldosterone production: The adrenal glands overproduce aldosterone on their own (Conn’s syndrome), causing the kidneys to hold sodium and water constantly.
- Medications and substances: Certain pain relievers, decongestants, birth control pills, and heavy alcohol use can raise blood pressure.
- Kidney disease: Damaged kidney tissue loses its ability to filter sodium and regulate fluid volume properly.
- Thyroid disorders: Both underactive and overactive thyroid conditions alter heart rate, blood vessel tone, and fluid balance.
- Adrenal tumors: Rare tumors called pheochromocytomas release bursts of adrenaline, causing dramatic pressure spikes.
Secondary hypertension is more likely when blood pressure rises suddenly, is unusually resistant to medication, or appears before age 30.
Current Blood Pressure Categories
The 2025 guidelines from the American Heart Association 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 is 130 to 139 systolic or 80 to 89 diastolic. Stage 2 hypertension is 140/90 or higher. If your top and bottom numbers fall into different categories, the higher category applies.
These thresholds matter because the biological mechanisms behind hypertension operate on a continuum. Arterial stiffening, sodium retention, and endothelial damage don’t suddenly begin at 130/80. They accumulate gradually, and the categories represent the points at which cardiovascular risk increases enough to warrant action.