How Blood Pressure Medications Work

How Blood Pressure Is Regulated

Blood pressure (BP) is the force that blood exerts against artery walls. It is determined by two main factors:

Cardiac output: How much blood the heart pumps per minute (heart rate × stroke volume).
Peripheral vascular resistance: How narrow or constricted the blood vessels are.

The body regulates BP through the autonomic nervous system, the renin-angiotensin-aldosterone system (RAAS), and fluid balance managed by the kidneys. Hypertension (chronically high BP) can arise from problems in any of these systems — which is why so many different drug classes can treat it.

ACE Inhibitors and ARBs

The renin-angiotensin-aldosterone system (RAAS) is a hormonal cascade that raises blood pressure when the kidneys sense reduced blood flow:

The kidneys release renin.
Renin converts angiotensinogen (a protein) to angiotensin I.
ACE (angiotensin-converting enzyme) converts angiotensin I to angiotensin II.
Angiotensin II constricts blood vessels and tells the adrenal glands to release aldosterone, which causes the kidneys to retain sodium and water — raising BP.

ACE inhibitors (lisinopril, enalapril, ramipril) block step 3 — preventing the production of angiotensin II. Blood vessels relax, aldosterone is suppressed, and the kidneys excrete more sodium and water. The result: lower blood pressure.

A side effect of ACE inhibitors is a dry, persistent cough — caused by accumulation of bradykinin, another substance ACE normally breaks down. This affects about 10–15% of patients, particularly those of Asian descent.

ARBs (angiotensin receptor blockers) — losartan, valsartan, candesartan — bypass step 3 entirely and instead block the receptor where angiotensin II docks. The end result is similar to ACE inhibition, but because bradykinin is not affected, ARBs rarely cause cough.

Beta-Blockers

The sympathetic nervous system speeds up the heart and constricts blood vessels in response to stress or exertion. It does this by releasing adrenaline (epinephrine) and noradrenaline, which bind to beta-adrenergic receptors in the heart.

Beta-blockers (metoprolol, atenolol, carvedilol, bisoprolol) are antagonists at beta-adrenergic receptors. By blocking these receptors, they: - Slow the heart rate - Reduce the force of each heartbeat - Decrease cardiac output

The net effect is lower blood pressure. Beta-blockers are particularly useful in patients with concurrent conditions: heart failure, prior heart attack, atrial fibrillation, and migraines (some beta-blockers are also approved for migraine prevention).

Beta-1 vs. Beta-2 selectivity: Beta-1 receptors are primarily in the heart; beta-2 receptors are in the lungs (where they promote bronchodilation). Non-selective beta-blockers (propranolol, carvedilol) block both, which can worsen breathing in patients with asthma or COPD. Cardioselective beta-blockers (metoprolol, atenolol) preferentially block beta-1, reducing this risk — though selectivity is not absolute at high doses.

Calcium Channel Blockers

Calcium enters muscle cells through channels in the cell membrane, triggering contraction. In blood vessels, muscle contraction means vasoconstriction (narrowing). In the heart, calcium drives both contraction force and heart rate.

Calcium channel blockers (CCBs) block these channels, preventing calcium entry.

Two main subclasses:

Dihydropyridines (amlodipine, nifedipine, felodipine): Primarily affect blood vessels, causing vasodilation and reducing peripheral resistance. They have minimal direct effect on heart rate. Common side effects: ankle swelling (from capillary dilation), flushing, and headache.
Non-dihydropyridines (diltiazem, verapamil): Affect both blood vessels and the heart's electrical conduction system, slowing heart rate. Useful when rate control is also needed (e.g., atrial fibrillation + hypertension). Should not be combined with beta-blockers due to excessive slowing of the heart.

Diuretics (Water Pills)

Diuretics increase the kidneys' excretion of sodium and water, reducing the volume of blood circulating through the vessels. Less fluid means less pressure.

Thiazide diuretics (hydrochlorothiazide, chlorthalidone) are the most common choice for hypertension. They work on the distal tubule of the kidney, blocking sodium reabsorption. Potassium is often lost along with sodium, which is why potassium levels should be monitored.

Loop diuretics (furosemide, bumetanide) are more potent and work on the loop of Henle in the kidney. They're preferred in heart failure or kidney disease, where more aggressive fluid removal is needed.

Potassium-sparing diuretics (spironolactone, eplerenone, amiloride) reduce sodium retention without the potassium loss — useful additions when patients are at risk for low potassium or for patients with heart failure.

Dose-Response and Combination Therapy

The dose-response curveDose-Response Curve

A graphical representation of the relationship between drug dose and the magnitude of its effect. The curve typically has a sigmoidal (S-shaped) form and is used to determine the effective dose range,

describes how increasing the dose of a drug produces increasing effect — up to a maximum (Emax). Most blood pressure medications have a reasonably flat dose-response curve above moderate doses: doubling the dose does not double the BP reduction. Instead, doubling the dose often doubles the side effects.

This is why combination therapy using low-to-moderate doses of two different drug classes is usually more effective (and better tolerated) than maximizing the dose of one drug. Each drug classDrug Class

A group of medications that share a similar chemical structure, mechanism of action, or therapeutic use. Drugs within the same class often have similar effects, side effects, and drug interactions, th

addresses a different mechanism:

RAAS inhibitor (ACE inhibitor or ARB) + CCB: An evidence-based combination (e.g., the ACCOMPLISH trial showed this outperformed RAAS + thiazide for cardiovascular events).
RAAS inhibitor + thiazide diuretic: Classic first-line combination, widely used and cost-effective.
Beta-blocker + CCB (dihydropyridine only, not verapamil/diltiazem): Useful when rate control and vasodilation are both needed.

Key Takeaways

Blood pressure is determined by cardiac output and peripheral vascular resistance; different drug classes act on different components of this system.
ACE inhibitors block angiotensin II production; ARBs block its receptor — both suppress the RAAS cascade that raises blood pressure.
Beta-blockers are antagonists at beta-adrenergic receptors, slowing the heart and reducing its output.
Calcium channel blockers prevent calcium entry into muscle cells, relaxing blood vessel walls (dihydropyridines) or slowing the heart (non-dihydropyridines).
Diuretics reduce blood volume by increasing kidney excretion of sodium and water.
Combination therapy at moderate doses is usually preferred over maximizing one drug, because the dose-response curve flattens while side effects continue rising.