Physiology · Renal, fluids and acid–base
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
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 mmHgVenular NFP −13 mmHgFiltration 0.5Reabsorption 5.0Net 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
Starling forces along a capillary
Arteriolar NFP +14 mmHgVenular NFP +6 mmHgFiltration 10.0Reabsorption 0.0Net 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
How the two states look
| Finding | Low volume | High volume |
|---|---|---|
| Pulse and pressure | Tachycardia, orthostasis, hypotension | Normal or hypertension |
| Neck veins | Flat | Distended |
| Lungs and skin | Dry mucosa, slow capillary refill | Crackles, pitting oedema, ascites |
| Weight | Falls | Rises |
| Urine | 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.
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.
Related topics
- Fluid shifts between compartments
- IV fluids: tonicity and where they go
- Sodium and extracellular volume regulation (with natriuretic peptides)
- How edema forms
- Shock physiology
- Starling forces at the capillary
Related conditions
In Biochemistry
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 (OpenStax (Rice University), 2022); Hypovolemia and Hypovolemic Shock (StatPearls (NCBI Bookshelf), 2024); Dehydration: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2024); Heart Failure (Congestive Heart Failure) (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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