The Heart as an Endocrine Organ: Atrial and B-type Natriuretic Peptides

The heart, long regarded solely as a muscular pump, also functions as an endocrine organ. Through the secretion of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), the heart participates directly in blood pressure regulation, sodium balance and fluid homeostasis. These hormones are released in response to stretch within the cardiac chambers, allowing the heart to communicate real-time information about circulating volume to the kidneys, vasculature and adrenal glands. This endocrine function is essential for preventing fluid overload and maintaining stable cardiovascular physiology.

What You Need to Know

Although traditionally viewed as a mechanical pump, the heart also functions as an endocrine organ by releasing hormones that regulate blood volume, vascular tone, and sodium balance. The key cardiac hormones are atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), which are synthesised and released in response to myocardial stretch caused by increased intravascular volume or pressure. ANP is produced mainly by atrial myocytes, while BNP is produced predominantly by ventricular myocytes, particularly under conditions of ventricular strain.

ANP and BNP act to counterbalance systems that increase blood pressure and fluid retention, particularly the renin–angiotensin–aldosterone system. By promoting sodium and water excretion and reducing vascular resistance, these peptides protect the heart from volume overload and excessive wall stress. Their actions are rapid and hormonally mediated, allowing the heart to participate directly in circulatory homeostasis rather than responding passively to changes imposed by other organs.

The principal physiological effects of ANP and BNP include:

• increased renal sodium excretion and urine output

• vasodilation of arterial and venous vessels, reducing preload and afterload

• inhibition of renin, angiotensin II, and aldosterone secretion

• suppression of sympathetic nervous system activity

Through these combined effects, natriuretic peptides reduce circulating blood volume and lower blood pressure, acting as natural antagonists to aldosterone and angiotensin II. They therefore serve as a critical protective mechanism during states of volume expansion, such as heart failure, renal impairment, or excessive sodium intake.

In clinical settings, BNP and its inactive fragment NT-proBNP are widely used as biomarkers of cardiac stress and heart failure severity. Elevated levels reflect increased ventricular wall tension and correlate with disease severity, prognosis, and response to treatment.

Beyond the Basics

Endocrine signalling from cardiac muscle

Cardiac myocytes synthesise atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) as pre-prohormones that are stored within intracellular granules. When myocardial stretch occurs, mechanical deformation activates intracellular signalling pathways involving stretch-activated ion channels, increased intracellular calcium, and cyclic GMP. These signals trigger cleavage and release of the active peptides into the circulation.

This mechanosensitive hormone release allows the heart to respond directly to changes in preload and wall tension. Rather than relying solely on neural or renal feedback, the myocardium can initiate endocrine signalling that reduces volume and pressure, functioning as a self-regulating component of cardiovascular homeostasis.

Physiological effects on the kidneys

ANP and BNP exert potent effects on renal haemodynamics and tubular function. They increase glomerular filtration rate by dilating the afferent arteriole and constricting the efferent arteriole, which raises glomerular capillary pressure and promotes filtration. At the tubular level, they inhibit sodium reabsorption in the collecting ducts and suppress renin release from juxtaglomerular cells.

The combined effect is enhanced natriuresis and diuresis, leading to reduced intravascular volume and decreased venous return. By lowering preload in this way, natriuretic peptides directly reduce cardiac workload during states of volume expansion.

Effects on the vasculature and adrenal glands

ANP and BNP also act on the vasculature by binding to guanylyl cyclase–coupled receptors on vascular smooth muscle cells, increasing cyclic GMP and promoting vasodilation. This reduces systemic vascular resistance and arterial pressure, further unloading the heart.

In addition, natriuretic peptides inhibit aldosterone secretion from the adrenal cortex. Reduced aldosterone limits renal sodium and water retention, reinforcing the volume-lowering effects initiated at the kidney. Through these coordinated renal, vascular, and adrenal actions, ANP and BNP form a powerful counter-regulatory system opposing the renin–angiotensin–aldosterone axis.

Ventricular BNP and heart failure

BNP is produced in significantly greater quantities during ventricular stretch, making it a sensitive indicator of myocardial strain. Elevated BNP reflects increased preload, reduced ventricular compliance, and heightened neurohormonal activation rather than isolated volume overload alone.

In clinical practice, BNP levels correlate with heart failure severity, treatment response, and prognosis. Persistent elevation suggests ongoing haemodynamic stress and maladaptive neuroendocrine activation, while falling levels indicate improved ventricular loading conditions. This close relationship between myocardial stretch and hormone release explains why BNP is such a valuable biomarker in the assessment and management of heart failure.

Clinical Connections

ANP and BNP are central to both the pathophysiology and clinical management of cardiovascular disease because they reflect the heart’s endocrine response to volume and pressure overload. Inadequate natriuretic peptide activity contributes to sodium retention, increased vascular tone, and hypertension, while excessive stretch-induced release is a hallmark of heart failure and reflects maladaptive neurohormonal activation rather than effective compensation.

Clinically, abnormalities in natriuretic peptide signalling are associated with a characteristic set of findings and applications:

• low or insufficient activity contributing to hypertension and volume expansion

• markedly elevated BNP levels in heart failure due to ventricular stretch and wall stress

• use of BNP and NT-proBNP as biomarkers to assess severity, prognosis, and treatment response

• utility of BNP measurement in emergency settings to distinguish heart failure–related dyspnoea from pulmonary or non-cardiac causes

In heart failure, persistently elevated BNP indicates ongoing haemodynamic stress and poor ventricular compliance, while falling levels suggest effective unloading and therapeutic response. This makes natriuretic peptides valuable not only for diagnosis but also for monitoring disease progression and guiding treatment decisions.

Pharmacological strategies that enhance natriuretic peptide signalling have been explored to counteract maladaptive RAAS activation. Recombinant BNP and drugs that inhibit neprilysin, the enzyme responsible for natriuretic peptide degradation, aim to prolong vasodilation, natriuresis, and RAAS suppression. These approaches highlight the clinical importance of cardiac endocrine signalling in managing complex cardiovascular disease.

Concept Check

1. How does cardiac chamber stretch trigger the release of ANP and BNP?

2. Why do natriuretic peptides increase urine output and decrease blood pressure?

3. How do natriuretic peptides counteract the renin–angiotensin–aldosterone system?

4. Why are BNP levels clinically important in diagnosing heart failure?

5. How do the renal, vascular and adrenal effects of ANP and BNP work together to regulate blood volume?