How ACE Inhibitors Work: The RAAS System Explained

ACE inhibitors are among the most prescribed medications in the world — and among the most misunderstood. Millions of people take lisinopril, enalapril, or ramipril every day for blood pressure, heart failure, or kidney protection, often without a clear picture of what these drugs are actually doing. The mechanism is elegantly simple once you understand the cascade they interrupt: the renin-angiotensin-aldosterone system, or RAAS.

This guide walks through the RAAS cascade step by step, explains why blocking a single enzyme has downstream effects on blood pressure, fluid balance, and kidney health, and covers the most clinically important aspects of the class — including the dry cough, the angioedema risk, and why these drugs are the first choice in diabetic kidney disease even when blood pressure is already controlled. No dosing information is included — that conversation belongs with your prescriber.

What Are ACE Inhibitors?

Angiotensin-converting enzyme (ACE) inhibitors are a class of medications that block a specific enzyme — ACE — that plays a central role in regulating blood pressure and fluid balance. By blocking this enzyme, the entire downstream hormonal cascade it drives is interrupted, reducing blood pressure, decreasing fluid retention, and relieving pressure on the kidneys and heart.

The class was pioneered with captopril (Capoten), the first ACE inhibitor approved by the FDA in 1981 — developed after researchers studying the venom of a Brazilian pit viper found a peptide that blocked the same enzyme. Since then, a full generation of longer-acting, better-tolerated agents followed. The drugs in common use today include:

• Lisinopril (Zestril, Prinivil) — the most widely prescribed ACE inhibitor in the United States
• Enalapril (Vasotec) — a prodrug, converted to its active form (enalaprilat) in the liver
• Ramipril (Altace) — strong cardiovascular outcome data from the HOPE trial
• Benazepril (Lotensin)
• Quinapril (Accupril)
• Perindopril (Aceon)
• Fosinopril — unique for its dual hepatic/renal elimination; useful when kidney function is reduced
• Captopril (Capoten) — the original; shorter-acting, now rarely used for chronic therapy

What They Treat

ACE inhibitors are used across a range of conditions — all connected by the common thread of RAAS overactivation playing a harmful role:

• Hypertension — a first-line option, particularly in patients with diabetes or kidney disease
• Heart failure with reduced ejection fraction (HFrEF) — reduce mortality and hospitalizations
• Post-myocardial infarction (post-MI) — preserve cardiac function after a heart attack
• Left ventricular dysfunction — even without overt symptoms
• Diabetic nephropathy — slow progression of kidney damage regardless of blood pressure level
• Chronic kidney disease (CKD) protection — reduce intraglomerular hypertension

The RAAS System: How the Cascade Works

The renin-angiotensin-aldosterone system is the body's primary hormonal mechanism for regulating blood pressure and fluid balance. Under normal conditions, it functions as a precise feedback loop — when blood pressure drops or blood flow to the kidneys falls, the system activates to raise it back up. The problem arises when this system is chronically overactivated, as happens in hypertension, heart failure, and kidney disease, where its pressure-raising effects become destructive rather than protective.

The cascade proceeds in four steps:

Why the Dry Cough?

Between 10% and 20% of patients on ACE inhibitors develop a persistent, dry, non-productive cough. In Asian populations — particularly of Chinese descent — the rate can reach 40%. The cough is not a sign of infection or allergy. It has a specific pharmacological cause.

ACE does not have just one substrate. In addition to converting angiotensin I to angiotensin II, ACE is the primary enzyme responsible for breaking down bradykinin — a peptide that causes vasodilation and promotes inflammation. When ACE is blocked, bradykinin accumulates throughout the body, including in the airways and lung tissue, where it stimulates sensory C-fibers, triggering the cough reflex.

This distinction — ACE inhibitors affect bradykinin; ARBs do not — is also why ARBs were developed in the first place. The cough is one of the most common reasons for switching drug classes. It typically resolves within 1–4 weeks of stopping the ACE inhibitor.

ACE Inhibitors vs. ARBs: Different Mechanisms, Same System

Both ACE inhibitors and ARBs interrupt the RAAS and produce similar clinical benefits: lower blood pressure, reduced cardiovascular events, and kidney protection. They differ in where along the pathway they act and in their side effect profiles.

The Next Evolution: Sacubitril/Valsartan (Entresto)

Sacubitril/valsartan (Entresto) represents the latest advance in RAAS-targeting therapy for heart failure. It combines valsartan (an ARB) with sacubitril, a neprilysin inhibitor. Neprilysin is an enzyme that, like ACE, degrades bradykinin and other vasoactive peptides — including natriuretic peptides, which promote fluid excretion and vasodilation. By blocking neprilysin, sacubitril raises natriuretic peptide levels, amplifying the beneficial vasodilatory and diuretic effects. In patients with heart failure with reduced ejection fraction, Entresto has shown superior outcomes compared to ACE inhibitor therapy alone, and has substantially replaced ACE inhibitors in this indication in patients who can tolerate it.

Kidney Protection: The Intraglomerular Pressure Story

One of the most important — and counterintuitive — aspects of ACE inhibitor therapy is how they protect the kidneys, and why they are prescribed for kidney disease even in patients whose blood pressure is already at goal.

The kidney's filtering unit, the glomerulus, operates under pressure. Blood enters through the afferent arteriole and exits through the efferent arteriole. Angiotensin II preferentially constricts the efferent arteriole — the outgoing vessel — which is like partially closing the drain: the pressure inside the glomerulus rises, forcing more plasma through the filter. This raises the glomerular filtration rate (GFR) in the short term, but the sustained high pressure damages the delicate filtration membrane over years, leading to protein leakage into the urine and, eventually, irreversible scarring.

The Initial Creatinine Rise

When an ACE inhibitor is started, serum creatinine typically rises within the first 1–2 weeks. This alarms many patients — and some clinicians. A creatinine rise of up to 30% above baseline is expected and acceptable. It reflects a real reduction in intraglomerular filtration pressure, which temporarily decreases the GFR as the kidney adjusts to operating at lower pressure. This is the therapy working as intended. The rise typically stabilizes, and the long-term kidney trajectory is far better than it would be without the drug. A rise beyond 30%, a sharply rising creatinine, or a dramatic rise paired with hyperkalemia warrants re-evaluation — particularly to rule out bilateral renal artery stenosis, a contraindication to ACE inhibitor use.

Hyperkalemia

Because ACE inhibitors reduce aldosterone, the kidneys retain more potassium than usual. Mild hyperkalemia is common and often manageable with dietary adjustments. More significant hyperkalemia becomes a concern when ACE inhibitors are combined with potassium-sparing diuretics (spironolactone, amiloride), potassium supplements, or in patients with already-impaired kidney function who cannot excrete excess potassium efficiently. Potassium levels should be checked after initiation and after dose adjustments.

Angioedema: The Rare but Serious Risk

Angioedema is the most feared adverse effect of ACE inhibitors. It is rare — affecting approximately 0.1–0.7% of patients — but potentially life-threatening. It presents as rapid, asymmetric swelling of the lips, tongue, face, and, critically, the throat and larynx. Laryngeal angioedema can obstruct the airway and is a medical emergency.

The mechanism is bradykinin-mediated. Accumulating bradykinin causes local vasodilation and increased vascular permeability — fluid leaks out of blood vessels into surrounding tissue, causing the characteristic swelling. ACE inhibitor angioedema is distinct from allergic (IgE-mediated) angioedema in that it does not typically respond well to antihistamines or epinephrine, though these are still administered in the acute setting.

Risk Factors

• Black patients — 3–5 times higher risk compared to white patients; the mechanism is not fully understood but may involve genetic differences in bradykinin metabolism
• Prior history of angioedema of any cause
• History of ACE inhibitor angioedema — absolute contraindication to rechallenge with any ACE inhibitor
• Concomitant use of mTOR inhibitors, neprilysin inhibitors, or DPP-4 inhibitors — all raise bradykinin levels and compound the risk

After angioedema attributable to an ACE inhibitor, the drug must be permanently stopped. The question of whether an ARB is safe to use afterward is complex — ARBs do not affect bradykinin, but a small risk of angioedema has been reported, and some guidelines recommend caution in patients with severe or recurrent ACE inhibitor angioedema. This decision requires individual clinical judgment.

Available Strengths

ACE inhibitors are available in a range of tablet strengths to allow precise titration. The following are approved strengths — dosing decisions are made by your prescriber based on the specific indication, kidney function, and individual response.

Common Pill Imprints

If you are trying to identify a pill, the imprint on the tablet is the most reliable identifier. Common imprints for ACE inhibitors include:

Frequently Asked Questions