How Your Body Controls Blood Pressure
Blood pressure is not one thing your body sets — it is the running product of two: how much blood the heart pumps and how tight the small arteries are. Watch the whole control system work at once: pressure sensors in your neck firing to the brainstem in seconds, and the kidney’s slower, more powerful hormone loop — renin → angiotensin I → angiotensin II → aldosterone — squeezing arteries and holding on to salt and water over hours to days.
Try this: switch to High salt and watch volume and pressure climb while renin is switched off. Then switch to High K⁺ / DASH and watch sodium leave in the urine and pressure fall. Then turn on Chronic hypertension and add an ACE inhibitor to see the lung’s conversion step get blocked.
Live blood-pressure readout
What's happening
The Science in Plain Language
Blood pressure is a product, not a setting
Your body never stores a number called “blood pressure.” Pressure is simply what happens when a pump pushes blood into a closed loop of pipes. Two things decide it:
- Cardiac output — how much blood the heart pumps per minute. That is heart rate × stroke volume: about 72 beats/min × 70 mL = roughly 5 litres a minute, which is your whole blood volume, every minute.
- Peripheral resistance — how tight the small arteries (the arterioles) are. These are the taps of the circulation.
Multiply them and you get mean arterial pressure (MAP), normally about 93 mmHg. The familiar two numbers sit either side of that average: the systolic peak as the heart ejects (~120 mmHg) and the diastolic trough as it refills (~80 mmHg). The gap between them is the pulse pressure, normally about 40 mmHg, and it depends on stroke volume and how stretchy your arteries are.
The arterioles have outsized power because of a piece of physics: flow through a tube depends on the fourth power of its radius. Narrow every small artery by just 6% and resistance rises by about 28%. A change too small to see under a microscope is enough to move you from 120/80 to hypertension — which is exactly why almost every blood-pressure hormone and drug targets these vessels.
The fast loop: baroreceptors, in seconds
Stretch sensors called baroreceptors sit in the wall of the carotid sinus (in your neck) and the aortic arch (just above the heart). The harder the pressure stretches them, the faster they fire — up the glossopharyngeal and vagus nerves to the brainstem. Watch the cyan impulses in the animation: when pressure is high they stream; when pressure falls they thin out.
The brainstem reads that firing rate and answers within one or two heartbeats:
- Pressure too high? More vagal (parasympathetic) traffic → the heart slows, vessels relax.
- Pressure too low? A burst of sympathetic traffic (the magenta particles) → heart rate and force go up, arterioles clamp down, and the kidney is told to release renin.
This is the reflex that stops you fainting when you stand up. It is fast, but it is only a buffer — it cannot make new blood. For that you need the kidney.
The slow loop: renin → angiotensin → aldosterone (RAAS)
The kidney is the body’s real pressure authority, because it controls volume. Specialised juxtaglomerular (JG) cells wrapped around the incoming arteriole act as their own pressure gauge. They release the enzyme renin when any of three things happen: the vessel is under-stretched (low perfusion), the macula densa senses too little salt flowing past, or sympathetic nerves fire on their β1 receptors. Follow the cascade in the diagram:
- Renin (gold) enters the blood. It is an enzyme, not a hormone that acts on tissue — its only job is to cut something.
- The liver is constantly pouring angiotensinogen (grey) into the plasma. Renin snips it into angiotensin I (orange) — a 10-amino-acid peptide that does almost nothing on its own.
- Blood carries angiotensin I to the lungs, whose capillary lining is carpeted with ACE (angiotensin-converting enzyme). ACE clips off two more amino acids, leaving the 8-amino-acid angiotensin II (red) — one of the most powerful vasoconstrictors your body makes.
- Angiotensin II then does two visible things at once. It squeezes the arterioles (watch the vessel narrow and the resistance meter climb), and it drives the adrenal cortex to release aldosterone (purple).
- Aldosterone travels to the distal nephron and collecting duct and switches on sodium channels (ENaC) and sodium pumps. Sodium is pulled back into the blood — and water follows sodium. Blood volume rises, the tank fills, stroke volume rises, and pressure climbs.
Notice the timescale. The nerve loop works in seconds; aldosterone works by switching genes on, so it takes hours, and its full effect on volume takes days. That is why blood-pressure changes from diet or medication are slow, and why one salty meal does not give you hypertension — but a salty decade might.
ADH: the water dial
Angiotensin II also makes you thirsty and triggers ADH (antidiuretic hormone, also called vasopressin) from the posterior pituitary. ADH opens water channels (aquaporin-2) in the collecting duct so water is reabsorbed without sodium — pure volume. At the very high levels seen in haemorrhage it also constricts vessels directly. Aldosterone saves salty water; ADH saves plain water. Together they defend the volume the heart has to work with.
Sodium and potassium: the balance that actually matters
Sodium raises pressure by the most boring mechanism imaginable: it holds on to water. Retaining roughly 140 mmol of sodium (about 3 g) keeps about one extra litre of fluid in the body. More volume → more stroke volume → more pressure. Switch the animation to High salt and watch the tank fill and the pressure climb.
Watch the renin bar at the same time — it collapses. That is not a bug. A body flooded with salt correctly switches RAAS off. Pressure still rises, because the volume is already there. This is why blood pressure can be high while renin is low, and it explains a lot about who responds to which drug.
Potassium does the opposite, and it is not simply “the anti-sodium.” A high-potassium intake flips what researchers call the potassium switch: it turns down the NCC sodium transporter in the distal tubule, so sodium is passed along and excreted instead of reclaimed. The result is natriuresis — salt leaves in the urine (watch the orange sodium particles divert from the blood into the urine when you select High K⁺ / DASH). Potassium also relaxes vascular smooth muscle and improves the endothelium’s ability to make nitric oxide.
The numbers are real and modest: raising potassium intake lowers systolic pressure by roughly 3–5 mmHg, more in people who already have hypertension. The full DASH eating pattern (fruit, vegetables, legumes, nuts, low-fat dairy — high in potassium, magnesium and calcium) lowered blood pressure by about 5.5/3.0 mmHg overall in the original trial, and about 11.4/5.5 mmHg in participants who were hypertensive. Cutting sodium on top of that lowers it further. Magnesium adds a smaller effect of its own — on the order of 2 mmHg.
What matters most is the ratio, shown as the orange/green bar. A typical Western diet runs about 3.4 g sodium against 2.6 g potassium — a molar ratio above 2. The WHO targets are under 2 g of sodium and at least 3.5 g of potassium a day, which brings the ratio below 1. Most people fix that not with a supplement but with food: potatoes, beans, lentils, leafy greens, bananas, avocado, yoghurt, fish.
One important safety note: potassium is not universally safe to load. If you have chronic kidney disease, or take an ACE inhibitor, ARB or spironolactone, your kidneys may not clear potassium well, and high-potassium salt substitutes can push blood potassium to dangerous levels. That is a conversation to have with your prescriber before you change anything.
When the system runs high: chronic hypertension
Turn on Chronic hypertension and three things change together. The arterioles are thickened and remodelled, so resistance runs high. The large arteries are stiff, so the same stroke volume produces a much bigger pressure swing — the pulse pressure widens from ~40 to over 60 mmHg, which is itself a cardiovascular risk marker. And the pressure sits above the 130/80 mmHg threshold that now defines high blood pressure.
Why doesn’t the baroreflex simply fix it? Because it resets. Within days, the brainstem starts treating the new, higher pressure as the number to defend. The fast loop is a stabiliser, not a thermostat — it protects you from sudden swings, not from a slow drift. Meanwhile renin is often normal, which sounds reassuring but is actually the problem: at a pressure that high, renin should have been switched off completely.
Where the medicines cut in
Almost every blood-pressure drug interrupts one arrow in this diagram.
- ACE inhibitors (lisinopril, ramipril — the “-prils”) block the lung step. Press the ACE inhibitor button: the red X lands on the ACE node, angiotensin I piles up, angiotensin II collapses, the arteriole relaxes and pressure falls. Renin actually rises, because the angiotensin II that normally tells the kidney “enough” is gone.
- ACE also breaks down bradykinin, a vasodilator. Blocking ACE lets bradykinin accumulate — which helps lower pressure but also causes the classic dry cough in roughly 1 in 10 people. ARBs (the “-sartans”) block the angiotensin II receptor instead and do not cause that cough.
- Now try the ACE inhibitor on top of High salt. It barely works. With renin already switched off there is little angiotensin II to block — which is why salt-sensitive, low-renin hypertension usually responds better to a diuretic or a calcium-channel blocker, and why reducing salt makes these drugs work better.
- Now try it on Dehydration. Pressure crashes. In a volume-depleted person, angiotensin II is the only thing holding pressure and kidney filtration up — block it and you risk hypotension and acute kidney injury. This is why many prescribers give “sick-day rules” to pause these drugs during a vomiting or diarrhoeal illness.
- Spironolactone blocks aldosterone at the receptor. Thiazide diuretics attack the volume side directly. Beta-blockers cut the sympathetic arrow to the heart and to renin release.
None of this replaces medical advice, and blood pressure should be measured, not guessed. But once you can see the loop, the logic of every treatment — less salt, more potassium, weight loss, exercise, and the specific drug your doctor chose — stops being a list of rules and becomes one picture.