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[Physiology](https://durgical.com/en/physiology/) · [Renal, fluids and acid–base](https://durgical.com/en/physiology/#s-renal)

# Sodium and extracellular volume regulation (with natriuretic peptides)

- Regulation
- Exam priority: high

The body's total sodium decides how much fluid sits outside the cells. The kidney adjusts it with hormones that keep or shed salt.

Updated 2026-11-01

Also called: Effective circulating volume, Atrial natriuretic peptide, Pressure natriuresis

**See it in 3D:** [Kidneys](https://durgical.com/en/anatomy/kidney/) · [Right atrium](https://durgical.com/en/anatomy/right-atrium/) · [Left adrenal gland](https://durgical.com/en/anatomy/suprarenal-gland/) · [Carotid bifurcation](https://durgical.com/en/anatomy/carotid-bifurcation/)

## Key points

- Total sodium → extracellular volume; Na⁺ concentration → water (ADH). Do not mix them up.
- Sensors: baroreceptors (carotid sinus, aortic arch), atria, afferent arteriole and macula densa.
- Na⁺ retainers: angiotensin II (proximal tubule), aldosterone (collecting duct), sympathetic system (renin and tubule).
- Na⁺ removers: atrial (ANP) and brain (BNP) natriuretic peptides, pressure natriuresis, less aldosterone.
- In heart failure and cirrhosis total volume is high but 'effective' volume is low, so the kidney keeps retaining sodium and edema forms.
- High blood BNP signals distended ventricles (used in heart failure).

## What it is

Total body sodium sets extracellular volume, because water follows it through ADH and thirst. The body does not measure sodium directly: it senses effective circulating volume with pressure and stretch sensors. When volume falls, the renin–angiotensin–aldosterone system and the sympathetic system retain Na⁺; when it rises, natriuretic peptides and pressure natriuresis remove it.

## How it works, step by step

- Effective circulating volume falls (bleeding, vomiting, diarrhea) and pressure drops in arteries and atria.
- Baroreceptors activate the sympathetic system, which constricts vessels and stimulates renin through β1 receptors.
- Lower afferent arteriole perfusion and less NaCl at the macula densa also release renin.
- Angiotensin II constricts the efferent, stimulates proximal NHE3 and releases aldosterone, ADH and thirst.
- Aldosterone increases ENaC in the collecting duct: Na⁺ is retained, and water with it.
- As volume recovers, the atria stretch, ANP is released, renin falls and the kidney excretes sodium again.
- Over the long term, high arterial pressure increases Na⁺ excretion (pressure natriuresis) until balance returns.

### Hormone axes and feedback

Renin–angiotensin–aldosterone system **Lesion** None (normal)

Tap a box to see the hormone, or choose a lesion to see the lab pattern.

- Stimulates (+)
- Inhibits / negative feedback (−)
- Level ↑
- Level ↓

Illustrative schematic drawn by Durgical: simplified axes and classic lab patterns; not for clinical decisions.

## Reference values

| Parameter | Value | Note |
| --- | --- | --- |
| Plasma sodium | 135–145 mEq/L | |
| Extracellular water | ≈ 1/3 of body water | ≈ 14 L in 70 kg |
| Na⁺ filtered per day | ≈ 25,000 mEq |
- *+* **Angiotensin II**: Retains Na⁺ in the proximal tubule, releases aldosterone and ADH.
- *+* **Aldosterone**: Retains Na⁺ (ENaC) in the collecting duct in exchange for K⁺ and H⁺.
- *+* **Renal sympathetic nerves**: Release renin, constrict the afferent and increase tubular reabsorption.
- *−* **ANP and BNP**: Dilate the afferent, raise GFR, inhibit renin, aldosterone and collecting duct reabsorption.
- *−* **Pressure natriuresis**: High arterial pressure → less reabsorption → more Na⁺ in urine.
- *−* **Renal dopamine and prostaglandins**: Promote local natriuresis.

## Volume states

| State | Total sodium | Effective volume | Renal response | Example |
| --- | --- | --- | --- | --- |
| [Hypovolemia](https://durgical.com/en/conditions/hypovolaemic-shock/) | ↓ | ↓ | Retains Na⁺ (FENa
- In heart failure, low output activates RAAS and the sympathetic system; Na⁺ and water retention worsen congestion. That is why ACE inhibitors, ARBs and aldosterone antagonists help.
- In cirrhosis, splanchnic vasodilation lowers effective volume: ascites with very low urine sodium.
- Aldosterone excess causes hypertension with little edema: natriuretic 'escape' (ANP, pressure natriuresis) limits expansion.

## Memory aid

'The kidney doesn't count sodium, it counts pressure': if it feels low pressure it keeps salt, even in a swollen body.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does a patient with heart failure and edema keep retaining sodium?

Total volume is high, but the output and pressure sensed by baroreceptors and the kidney are low: effective volume is low, so RAAS and the sympathetic system stay active.

## What does atrial natriuretic peptide do to lower volume?

Released when the atria stretch, it dilates the afferent (raising GFR), inhibits renin and aldosterone and reduces Na⁺ reabsorption in the medullary collecting duct.

## Why does primary hyperaldosteronism cause hypertension but rarely edema?

After some Na⁺ retention, expansion raises pressure and ANP: pressure natriuresis 'escapes' aldosterone's sodium effect, although K⁺ loss continues.

## What does a FENa below 1% mean in an oliguric patient?

The tubules work and avidly reabsorb sodium because the kidney senses a low effective volume (a prerenal cause), if no diuretics were given.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/sodium-and-extracellular-volume-regulation/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Kidneys](https://durgical.com/en/anatomy/kidney/)
- [Right atrium](https://durgical.com/en/anatomy/right-atrium/)
- [Left adrenal gland](https://durgical.com/en/anatomy/suprarenal-gland/)
- [Carotid bifurcation](https://durgical.com/en/anatomy/carotid-bifurcation/)

## Related topics

- [The renin–angiotensin–aldosterone system](https://durgical.com/en/physiology/the-renin-angiotensin-aldosterone-system/)
- [Long-term blood pressure control by the kidney](https://durgical.com/en/physiology/long-term-blood-pressure-control-by-the-kidney/)
- [ADH, thirst and water balance](https://durgical.com/en/physiology/adh-thirst-and-water-balance/)
- [The baroreceptor reflex](https://durgical.com/en/physiology/the-baroreceptor-reflex/)
- [Distal tubule and collecting duct](https://durgical.com/en/physiology/distal-tubule-and-collecting-duct/)
- [Dehydration and fluid overload](https://durgical.com/en/physiology/dehydration-and-fluid-overload/)
- [Body fluid compartments](https://durgical.com/en/physiology/body-fluid-compartments/)

## Related conditions

- [Hyponatraemia](https://durgical.com/en/conditions/hyponatraemia/)
- [Heart failure](https://durgical.com/en/conditions/heart-failure/)
- [Cirrhosis](https://durgical.com/en/conditions/cirrhosis/)
- [Arterial hypertension](https://durgical.com/en/conditions/arterial-hypertension/)
- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)

## In Biochemistry

- [Aldosterone](https://durgical.com/en/biochemistry/molecules/aldosterone/)
- [Angiotensin-converting enzyme (ACE)](https://durgical.com/en/biochemistry/enzymes/angiotensin-converting-enzyme/)

## In Pharmacology

- [ACE inhibitors](https://durgical.com/en/pharmacology/classes/ace-inhibitors/)
- [Angiotensin II receptor blockers (ARBs)](https://durgical.com/en/pharmacology/classes/angiotensin-ii-receptor-blockers/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [25.9 Regulation of Fluid Volume and Composition](https://openstax.org/books/anatomy-and-physiology-2e/pages/25-9-regulation-of-fluid-volume-and-composition) (OpenStax (Rice University), 2022); [26.3 Electrolyte Balance](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-3-electrolyte-balance) (OpenStax (Rice University), 2022); [Heart Failure (Congestive Heart Failure)](https://www.ncbi.nlm.nih.gov/books/NBK430873/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 30). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/physiology/sodium-and-extracellular-volume-regulation/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/sodium-and-extracellular-volume-regulation/