High blood pressure happens when the force of blood pushing against your artery walls stays consistently too high. This comes down to a simple relationship: blood pressure equals the amount of blood your heart pumps multiplied by the resistance your arteries put up against that flow. Anything that increases either side of that equation, whether it’s more blood volume or narrower arteries, raises your pressure. About 1.4 billion people worldwide lived with hypertension in 2024, and only about one in five had it under control.
The Two Forces Behind Every Reading
Your blood pressure is determined by two things working together: how much blood your heart pushes out with each beat (cardiac output) and how much your smaller arteries resist that flow (vascular resistance). When either one goes up without the other dropping to compensate, your blood pressure rises.
In the earliest stages of high blood pressure, the problem often starts on the output side. Studies in both animals and humans show that borderline hypertension is frequently characterized by increased cardiac output without any change in vascular resistance. Your heart is simply pumping harder or faster, often driven by stress hormones or an overactive nervous system. Over time, though, the arteries themselves start to narrow and stiffen, shifting the problem toward the resistance side. This is why untreated high blood pressure tends to get worse: the initial increased flow eventually damages vessel walls, which then tighten up and create even more resistance.
How Your Kidneys Control the Volume
Your kidneys act as the body’s pressure regulators, and they do it primarily by controlling how much sodium and water stay in your bloodstream. Sodium is the main substance that determines how much fluid sits in your blood vessels. More sodium means more water retained, which means a larger blood volume pressing against artery walls.
This process is governed by a hormonal chain reaction that starts in the kidneys. When blood pressure drops, your kidneys release an enzyme called renin, which triggers a cascade: renin breaks down a liver protein into an inactive hormone, which then gets converted in the lungs and kidneys into an active hormone called angiotensin II. This hormone does two things simultaneously. It causes the muscular walls of small arteries to constrict, immediately raising resistance. And it signals your adrenal glands to release aldosterone, which tells your kidneys to hold onto sodium and water. The result is higher volume and tighter vessels at the same time.
This system exists to protect you during moments of low blood pressure, like after blood loss. But when it stays chronically activated, perhaps because of kidney disease, obesity, or genetic tendencies, it keeps your pressure elevated around the clock.
Why Salt Matters So Much
The connection between salt and blood pressure is rooted in basic physics. Sodium is the dominant particle controlling fluid balance outside your cells. When you take in more sodium than your kidneys can efficiently clear, water follows the sodium into your bloodstream by osmosis. This expands your blood volume, which increases the pressure on your artery walls. For people whose kidneys are slower to excrete sodium (a trait influenced by genetics, age, and kidney health), even moderate salt intake can meaningfully raise blood pressure.
Your Arteries Lose Their Stretch With Age
Healthy arteries are elastic. When your heart contracts and sends a surge of blood outward, large arteries like the aorta stretch to absorb that pulse, then gently recoil to keep blood flowing smoothly between heartbeats. This cushioning effect keeps your systolic pressure (the top number) from spiking too high and your diastolic pressure (the bottom number) from dropping too low.
As you age, the elastic fibers in your artery walls gradually break down. These fibers don’t regenerate well once damaged. At the same time, stiffer collagen fibers accumulate, and in people with diabetes, sugar molecules can cross-link with these collagen fibers, making the walls even more rigid. The result is arteries that no longer absorb the pulse of each heartbeat. Instead, each contraction sends a sharper pressure wave through the system. This is why isolated systolic hypertension, where only the top number is high, becomes so common after age 60.
The Role of Your Inner Artery Lining
The cells lining the inside of your arteries produce a molecule called nitric oxide, which signals the surrounding muscle to relax and widen. This is one of the body’s most important tools for keeping blood pressure in check. Research over the past 40 years has identified the loss of nitric oxide production as one of the earliest events in the development of hypertension.
When this lining gets damaged, whether by high blood sugar, smoking, chronic inflammation, or simply aging, nitric oxide production drops. Without enough of it, arteries stay constricted, resistance goes up, and blood pressure climbs. This damage is both a cause and a consequence of high blood pressure, creating a cycle where elevated pressure further injures the lining, which further reduces nitric oxide, which further raises pressure.
How Excess Weight Raises Pressure
Obesity raises blood pressure through several overlapping pathways, but one of the most important involves insulin. When the body becomes resistant to insulin, as commonly happens with excess weight, the pancreas compensates by producing more of it. These chronically high insulin levels activate the sympathetic nervous system, your body’s “fight or flight” wiring, which constricts blood vessels and raises heart rate.
In people at a healthy weight, insulin actually has a mild blood vessel-relaxing effect that offsets its nervous system activation. But in obesity, that balance tips. The vessel-relaxing action weakens while the sympathetic drive gets exaggerated. Research published in Circulation Research found that insulin-triggered increases in sympathetic activity and stress hormone release are substantially amplified in people with hypertension. This creates a feedback loop: sympathetic activation constricts blood vessels in muscles, which reduces sugar uptake, which worsens insulin resistance, which drives even more insulin production.
Genetics Set the Starting Line
Twin studies estimate that 50% to 60% of hypertension risk is heritable. This doesn’t mean a single gene is responsible. Hundreds of genetic variations, each with a small effect, collectively influence how your kidneys handle sodium, how your arteries respond to stress hormones, and how readily your blood vessels produce nitric oxide. If both your parents developed high blood pressure early in life, your own risk is substantially higher. But genetics load the gun; lifestyle pulls the trigger. The remaining 40% to 50% of risk comes from diet, activity level, weight, and other modifiable factors.
When a Specific Condition Is the Cause
In roughly 5% to 10% of cases, high blood pressure has a single identifiable cause, known as secondary hypertension. These cases are important to recognize because treating the underlying condition can sometimes resolve the blood pressure problem entirely.
Kidney diseases are the most common culprit. Conditions like polycystic kidney disease, diabetic kidney damage, or narrowing of the arteries that supply the kidneys all impair the organ’s ability to regulate sodium and fluid balance. When the kidneys can’t properly filter and excrete, blood volume rises and pressure follows.
Hormonal disorders account for another significant group. In a condition called aldosteronism, the adrenal glands overproduce aldosterone, causing the kidneys to retain far too much sodium and water while dumping potassium. Cushing syndrome, where the body produces excess cortisol, also raises blood pressure. A rare adrenal tumor called a pheochromocytoma floods the body with adrenaline, causing dramatic blood pressure spikes. Some people are born with a narrowed aorta, which forces the heart to pump harder to push blood past the constriction.
What the Numbers Mean
The most recent guidelines from the American Heart Association, updated in 2025, define four categories of blood pressure:
- Normal: below 120/80 mm Hg
- Elevated: systolic 120 to 129 with diastolic still below 80
- Stage 1 hypertension: systolic 130 to 139 or diastolic 80 to 89
- Stage 2 hypertension: systolic 140 or higher, or diastolic 90 or higher
If your systolic and diastolic numbers fall into different categories, the higher category applies. These thresholds are based on the point at which cardiovascular risk begins to climb meaningfully, not an arbitrary cutoff. Someone at 138/86 isn’t fundamentally different from someone at 142/88, but both carry more risk than someone at 118/76.