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

# Dehydration and fluid overload

- Mechanism
- Exam priority: medium

Too little or too much fluid changes blood pressure, pulse, urine and skin; the kidney tries to fix it by keeping or shedding salt and water.

Updated 2026-11-01

Also called: Volume depletion, Hypovolaemia, Hypervolaemia

## Key points

- Volume depletion = loss of Na⁺ and water (diarrhoea, vomiting, haemorrhage, burns); pure dehydration = loss of water (hypernatraemia).
- Signs of low volume: tachycardia, orthostatic hypotension, flat neck veins, oliguria, dry mucosa.
- A healthy kidney facing low volume: urine Na⁺ 20, concentrated urine.
- Low effective arterial volume with increased body fluid: heart failure, cirrhosis, nephrotic syndrome. The kidney retains sodium and worsens the oedema.
- In chronic kidney disease sodium retention is primary: hypertension and oedema with high arterial volume.
- Weight: an acute change of ≈1 kg is ≈1 L of fluid.

## What it is

Two ideas should be kept apart: volume depletion (loss of extracellular salt and water, with normal sodium) and dehydration in the strict sense (loss of water, with high sodium). In overload, the extracellular compartment is expanded by sodium and water retention; the excess collects in the interstitium (oedema) according to Starling forces. What the kidney “sees” is effective arterial blood volume, which can be low even when the body holds too much fluid.

## How it works, step by step

- Loss of extracellular fluid lowers venous return, stroke volume and blood pressure.
- Baroreceptors raise sympathetic tone: tachycardia, vasoconstriction and renin release.
- Angiotensin II and aldosterone retain Na⁺; ADH is released when the volume fall is large, even with normal osmolality.
- The kidney makes little urine, concentrated and almost sodium-free (unless diuretics or tubular damage are present).
- In overload, high venous pressure pushes fluid into the interstitium; lymphatics are overwhelmed and oedema appears.
- If the heart or liver fails, effective arterial volume stays low: the kidney keeps retaining sodium despite the oedema.

### Starling forces along a capillary

**Arteriolar NFP** +4 mmHg **Venular NFP** −13 mmHg **Filtration** 0.5 **Reabsorption** 5.0 **Net absorption** 4.5 a.u.

Dehydration: the blood is concentrated (↑ πc) and Pc falls: absorption wins and fluid moves from the interstitium into the plasma (the same happens after a hemorrhage).

Tap a force (Pc, πc, Pif, πif), the lymph or the net pressure value.

- Filtration (fluid out)
- Absorption (fluid in)
- Net filtration pressure
- Normal net pressure
- Lymphatic (returns the excess)
- Edema

Illustrative Durgical schematic of the classic Starling model; values are approximate and flow units arbitrary. The revised Starling principle (glycocalyx) shows that most capillaries at steady state absorb very little: the lymph returns almost all that is filtered.

### Starling forces along a capillary

**Arteriolar NFP** +14 mmHg **Venular NFP** +6 mmHg **Filtration** 10.0 **Reabsorption** 0.0 **Net filtration** 10.0 a.u. (lymph capacity 7.5)

Heart failure: high venous pressure raises Pc along the whole capillary; there is almost no reabsorption and filtration outruns the lymph (peripheral edema in right-sided failure, pulmonary edema in left-sided).

Edema: net filtration (10.0 a.u.) is more than the lymph can return (7.5 a.u.), so fluid collects in the interstitium.

Tap a force (Pc, πc, Pif, πif), the lymph or the net pressure value.

- Filtration (fluid out)
- Absorption (fluid in)
- Net filtration pressure
- Normal net pressure
- Lymphatic (returns the excess)
- Edema

Illustrative Durgical schematic of the classic Starling model; values are approximate and flow units arbitrary. The revised Starling principle (glycocalyx) shows that most capillaries at steady state absorb very little: the lymph returns almost all that is filtered.

## How the two states look

| Finding | Low volume | High volume |
| --- | --- | --- |
| [Pulse and pressure](https://durgical.com/en/conditions/hypovolaemic-shock/) | Tachycardia, orthostasis, hypotension | Normal or hypertension |
| Neck veins | Flat | Distended |
| [Lungs and skin](https://durgical.com/en/conditions/heart-failure/) | Dry mucosa, slow capillary refill | Crackles, pitting oedema, ascites |
| Weight | Falls | Rises |
| [Urine](https://durgical.com/en/conditions/cirrhosis/) | Scant, low Na⁺ (if the kidney is healthy) | Variable; low Na⁺ if effective arterial volume is low |
| Labs | High BUN/creatinine, haemoconcentration | Haemodilution; hyponatraemia in heart failure or cirrhosis |

## Clinical notes

- Gastroenteritis in young children: more body water per kilogram, higher daily turnover and dependence on others to drink; dehydration progresses fast.
- Dengue (common in Colombia): in the critical phase plasma leaks from capillaries, causing haemoconcentration and hypovolaemia even when effusions are present.
- In hot climates and during physical work sweat can exceed 1 L per hour; older adults feel less thirst.

## Memory aid

“The kidney doesn’t see body water, it sees arterial pressure”: that is why it retains salt in heart failure even when fluid is in excess.

## Test yourself

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

## Why does a patient with heart failure and oedema have a low urine sodium, as if dehydrated?

Low cardiac output reduces effective arterial volume; baroreceptors and kidney respond as to hypovolaemia (RAAS, sympathetic, ADH) and retain sodium and water although total body fluid is increased.

## Why does the BUN/creatinine ratio rise in volume depletion?

With slow tubular flow and high ADH, the tubule reabsorbs more urea (which follows water), whereas creatinine is not reabsorbed: BUN rises more than creatinine.

## What is the physiological difference between “volume depletion” and “dehydration”?

Depletion is loss of extracellular sodium and water with normal serum sodium; pure dehydration is loss of water, with high sodium and osmolality and cell shrinkage.

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## Related topics

- [Fluid shifts between compartments](https://durgical.com/en/physiology/fluid-shifts-between-compartments/)
- [IV fluids: tonicity and where they go](https://durgical.com/en/physiology/iv-fluids-tonicity-and-where-they-go/)
- [Sodium and extracellular volume regulation (with natriuretic peptides)](https://durgical.com/en/physiology/sodium-and-extracellular-volume-regulation/)
- [How edema forms](https://durgical.com/en/physiology/how-edema-forms/)
- [Shock physiology](https://durgical.com/en/physiology/shock-physiology/)
- [Starling forces at the capillary](https://durgical.com/en/physiology/starling-forces-at-the-capillary/)

## Related conditions

- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)
- [Heart failure](https://durgical.com/en/conditions/heart-failure/)
- [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/)
- [Cirrhosis](https://durgical.com/en/conditions/cirrhosis/)
- [Dengue](https://durgical.com/en/conditions/dengue/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)

## In Biochemistry

- [Urea and blood urea nitrogen (BUN) as lab tests](https://durgical.com/en/biochemistry/topics/urea-and-blood-urea-nitrogen-as-lab-tests/)
- [Albumin](https://durgical.com/en/biochemistry/enzymes/albumin/)

[← 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: [26.1 Body Fluids and Fluid Compartments](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-1-body-fluids-and-fluid-compartments) (OpenStax (Rice University), 2022); [Hypovolemia and Hypovolemic Shock](https://www.ncbi.nlm.nih.gov/books/NBK513297/) (StatPearls (NCBI Bookshelf), 2024); [Dehydration: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000982.htm) (MedlinePlus, US National Library of Medicine, 2024); [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. 25). 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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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/dehydration-and-fluid-overload/