Alicia thinks of the heart as a mechanical pump. Tricia adds electrical tissue. Kai Kai notices secretory granules inside atrial cardiomyocytes and asks why a pump cell would store hormones. The answer expands the organ’s job: the heart also reports its loading state to the rest of the body.
The heart works as an endocrine organ by sensing mechanical and neurohumoral conditions and releasing signalling peptides—most prominently atrial natriuretic peptide, ANP, and B-type natriuretic peptide, BNP—that act on kidneys, blood vessels and other targets involved in pressure and volume regulation. Stretch can increase peptide release and synthesis. ANP and BNP activate natriuretic-peptide receptor A, raise cyclic GMP in target cells and promote responses that oppose excessive sodium retention, volume expansion and vasoconstrictor signalling.
This article supports How the Heart Works. How the Frank-Starling Mechanism Works owns the immediate mechanical response to filling. Here the reader job is slower chemical communication: how stretch becomes a circulating hormonal signal that feeds back on the volume-pressure system.
The discussion explains normal physiology and measurement concepts. It is not a guide to interpreting an individual’s BNP or NT-proBNP result or diagnosing heart or kidney disease.
1. The discovery changed the definition of the heart
For much of physiology, the heart was described primarily as muscle, valves and conducting tissue. In 1981, experiments by de Bold and colleagues showed that atrial extracts could produce rapid natriuresis and diuresis when administered to animals.
This finding established that cardiac tissue contained a circulating factor capable of altering kidney function and body-fluid balance.
The review The heart as an endocrine organ traces how that observation led to the identification of cardiac natriuretic peptides and to a broader endocrine view of the heart.
2. ANP is stored prominently in atrial cardiomyocytes
Atrial natriuretic peptide is synthesised as a larger precursor and stored in secretory granules, especially in atrial cardiomyocytes.
Mechanical stretch of the atrial wall can trigger a rapid increase in peptide secretion from this stored pool.
This is why the atrium is not merely a receiving chamber. Its wall contains endocrine machinery capable of converting loading information into a circulating chemical signal.
3. BNP is B-type natriuretic peptide, not simply a brain hormone
BNP was first isolated from brain tissue, which explains its historical name. In cardiovascular physiology it is now called B-type natriuretic peptide.
Cardiomyocytes synthesise BNP, and ventricular production becomes especially important when ventricular wall stress and neurohumoral stimulation increase.
The name therefore records discovery history rather than the main organ context in which BNP is usually discussed clinically and physiologically.
4. Stretch-secretion coupling converts mechanics into endocrine output
When atrial or ventricular myocardium experiences increased wall stretch, mechanosensitive cellular pathways influence natriuretic-peptide release and gene expression.
Acute atrial stretch can produce a rapid burst of ANP release from stored granules. More sustained haemodynamic load can increase synthesis of natriuretic-peptide precursors.
The heart therefore contains two responses to filling: an immediate mechanical response through Frank-Starling behaviour and a chemical response through stretch-sensitive endocrine signalling.
5. Wall stress matters more than volume alone
Cardiomyocytes respond to their mechanical environment, which includes pressure, chamber radius, wall thickness and regional deformation.
The same chamber volume can generate different wall stress if pressure or geometry differs. Endocrine signalling therefore should not be reduced to a simple rule that more millilitres automatically mean more hormone.
Mechanical state, neurohumoral signals and timescale all influence peptide synthesis and secretion.
6. ANP is made first as a larger precursor
The NPPA gene encodes preproANP, which is processed to proANP and stored in atrial granules.
At secretion, the membrane serine protease corin cleaves proANP to generate biologically active ANP and an amino-terminal fragment.
This processing chain illustrates a common endocrine strategy: cells synthesise an inactive or less active precursor, store it and then cleave it into the circulating active form when release is triggered.
7. BNP also emerges from prohormone processing
The NPPB gene encodes preproBNP, which becomes proBNP. Processing separates biologically active BNP from the amino-terminal fragment commonly called NT-proBNP.
BNP and NT-proBNP are therefore related products of the same precursor but are not identical molecules and do not have identical clearance kinetics.
This is why laboratory measurements of BNP and NT-proBNP should not be treated as interchangeable numbers with one common numerical scale.
8. ANP and BNP share a principal receptor pathway
ANP and BNP bind natriuretic-peptide receptor A, also called NPR-A or guanylyl cyclase-A.
The receptor has an extracellular peptide-binding domain and an intracellular guanylyl-cyclase domain. Ligand binding increases production of cyclic GMP from GTP.
Cyclic GMP then regulates downstream protein kinases, phosphodiesterases and ion-handling pathways in target cells. One circulating peptide signal is thereby converted into several tissue-specific physiological effects.
9. The kidney is a major endocrine target
Natriuretic peptides promote sodium excretion—natriuresis—and can increase water excretion through both direct renal actions and interactions with other hormonal systems.
They influence glomerular haemodynamics and tubular sodium handling and oppose sodium-retaining signals such as the renin–angiotensin–aldosterone system.
The review The renal and cardiovascular effects of natriuretic peptides summarises these kidney and vascular actions.
10. Natriuresis changes the circulation on a slower timescale
If the kidneys excrete more sodium, extracellular water tends to follow over time. Circulating volume and venous filling conditions can then fall.
This endocrine feedback acts more slowly than the Frank-Starling response occurring on the next beat and the baroreflex response occurring over seconds.
The same underlying disturbance—excess stretch—can therefore recruit several controls on different timescales: beat-to-beat mechanics, neural reflexes and renal-endocrine volume regulation.
11. Natriuretic peptides influence vascular tone
NPR-A/cGMP signalling can relax vascular smooth muscle and oppose vasoconstrictor pathways.
Reducing vascular tone can lower the pressure load against which the heart ejects and can alter venous capacitance and distribution.
The endocrine heart therefore can influence both sides of the pressure-volume problem: the volume returning to the heart and the vascular load presented to it.
12. The system opposes the renin–angiotensin–aldosterone axis
Renin–angiotensin–aldosterone signalling tends to support sodium retention, vasoconstriction and volume maintenance when effective circulating volume is threatened.
Natriuretic-peptide signalling generally pushes in the opposite direction by promoting sodium excretion and vasodilatory effects.
The circulation therefore contains opposing hormonal systems whose relative activity depends on current pressure, volume, kidney perfusion and neurohumoral state.
13. Natriuretic peptides interact with sympathetic control
Natriuretic-peptide signalling can reduce sympathetic activity through central and peripheral pathways, while sympathetic and renin–angiotensin systems can promote conditions that increase cardiac load.
This creates cross-talk between the neural and endocrine control layers.
The heart therefore participates in a distributed regulatory network rather than sending one hormone to one target organ in isolation.
14. Endocrine feedback helps turn stretch into reduced future stretch
Consider the sign of the feedback. Greater cardiac filling and wall stress can increase natriuretic-peptide signalling. That signalling promotes sodium loss and vasodilatory effects.
Over time, reduced extracellular volume and vascular load tend to oppose the original stretch.
This is negative feedback: the response acts in a direction that tends to reduce the initiating mechanical disturbance.
15. The endocrine heart links the cardiovascular and renal systems
The heart detects mechanical consequences of circulating volume. The kidneys control sodium and water excretion, which strongly influence that volume.
Natriuretic peptides provide one information channel from heart to kidney, while renal and vascular hormones provide signals in the opposite direction.
Blood-volume regulation is therefore a conversation among organs, not a property owned by the kidneys or heart alone.
16. ANP and BNP are not the only cardiac signals
Modern cardiac endocrinology describes other heart-derived signalling molecules, including factors associated with inflammation, growth and systemic metabolism.
The review Cardiac Endocrinology: Heart-Derived Hormones in Physiology and Disease surveys this broader field.
ANP and BNP remain the clearest physiological examples for this article because their link among mechanical stretch, kidney function and vascular regulation is well established.
17. CNP belongs mainly to a different signalling context
C-type natriuretic peptide, CNP, belongs to the same peptide family but is produced importantly by endothelial and other tissues and signals mainly through NPR-B.
It has important local vascular and tissue effects but is not simply a third interchangeable cardiac-volume hormone.
Keeping the peptide sources and receptor preferences distinct prevents the word natriuretic from collapsing several related systems into one pathway.
18. Clearance controls how long the signal persists
Circulating natriuretic peptides are removed through receptor-mediated clearance and enzymatic degradation, including by neprilysin.
Secretion rate therefore does not uniquely determine circulating concentration. Production, processing, distribution and clearance all contribute.
This is a general endocrine rule: a high measured concentration can reflect increased production, reduced clearance or both.
19. BNP concentration is not a direct pressure gauge
BNP-related concentrations are influenced by wall stress, synthesis, processing, clearance, age, kidney function and other physiological or clinical conditions.
A blood concentration therefore cannot be converted into one exact intracardiac pressure or chamber volume by a universal formula.
The signal is informative because it reflects biological response to load, not because it acts as a miniature pressure transducer floating in plasma.
20. Exercise can alter natriuretic-peptide signalling without making exercise pathological
Exercise changes chamber filling, wall stress, sympathetic activity and circulating hormones. Natriuretic-peptide concentrations can change during and after exercise depending on intensity, duration and individual characteristics.
A transient physiological increase therefore should not be interpreted outside context as equivalent to chronic haemodynamic stress.
Timescale and operating state again determine meaning.
21. Worked problem: the pump reports its own loading state
Imagine a model circulation in which venous return rises for several hours, increasing atrial stretch. ANP release rises and renal sodium excretion increases.
If extracellular fluid volume then falls toward the original state, atrial stretch decreases and the stimulus for ANP release also falls.
The heart has acted as both a pump experiencing the volume load and a sensor-endocrine organ reporting that load to the kidney.
22. Worked problem: identical peptide concentration, different causes
Model A doubles peptide secretion while clearance also doubles. Model B keeps secretion unchanged while clearance falls by half.
Depending on kinetics, both models could produce similar circulating concentrations despite very different underlying mechanisms.
A measured hormone level is therefore an output of production and clearance, not a direct readout of secretion alone.
23. Worked problem: mechanical and endocrine responses operate on different clocks
A sudden rise in venous return stretches the ventricle on the next beat and immediately recruits Frank-Starling behaviour. Atrial and ventricular endocrine responses also begin, but renal sodium loss alters body volume over a much longer interval.
Both responses oppose overload, yet they do so on different timescales.
Calling them redundant would miss the value of fast local compensation paired with slower systemic correction.
24. Worked problem: concentration and biological effect can diverge
Suppose two systems have equal ANP concentration, but one has fewer functional NPR-A receptors in a target tissue.
The circulating signal matches, yet the downstream cGMP response can differ.
Endocrine physiology therefore requires three levels: hormone concentration, receptor response and whole-organ effect.
25. The endocrine-heart mechanism in one causal chain
Circulating volume, pressure and chamber geometry create myocardial wall stress. Stretch-sensitive cardiomyocytes increase release and, over longer intervals, synthesis of natriuretic-peptide precursors. ProANP and proBNP are processed into active peptides and related fragments. ANP and BNP circulate to kidneys, vessels and other tissues. NPR-A activation raises cyclic GMP. Renal sodium handling shifts toward natriuresis, vascular tone can fall and sodium-retaining neurohumoral systems are opposed. Over time, extracellular volume and cardiac loading tend to decrease, reducing the original stretch signal.
Alicia redraws the heart with an outgoing hormone arrow as well as an outgoing blood-flow arrow. Tricia connects that hormone arrow to the kidneys. Kai Kai closes the loop by returning from renal sodium handling to blood volume, venous return and chamber stretch.
The deeper lesson is that the heart does more than create pressure. It also reports pressure-volume stress chemically to the wider organism, helping coordinate the future conditions under which it will have to pump.
Evidence trail and connected reading
For the endocrine-organ framework, see The heart as an endocrine organ. For kidney and vascular effects, see The renal and cardiovascular effects of natriuretic peptides. For the wider catalogue of heart-derived hormones, see Cardiac Endocrinology.
Return to the parent: How the Heart Works. Continue to How the Frank-Starling Mechanism Works, How Venous Return Works and How the Baroreflex Works.