Physiology · Renal, fluids and acid–base
IV fluids: tonicity and where they go !
- Concept
- Exam priority: medium
Each IV fluid ends up in a different part of the body depending on what is in it: in the plasma, across the extracellular space or inside cells.
Updated 2026-11-01
Also called: Crystalloids, Colloids, Normal saline, Lactated Ringer’s
What it is
Where an infused fluid goes is decided by its effective tonicity, not by its osmolarity in the bag. Solutions with sodium at plasma concentration stay in the extracellular space (plasma plus interstitium). Dextrose in water looks isosmotic, but the glucose is metabolised, leaving free water that spreads through the whole body, cells included. Colloids (albumin) keep more fluid in the plasma through oncotic pressure. This page explains composition and destination; it does not say how much to give or how fast.
Key facts
- Reference plasma: Na⁺ ≈140, Cl⁻ ≈100, HCO₃⁻ ≈24 mmol/L, osmolality ≈290 mOsm/kg.
- The strong ion difference (Na⁺ − Cl⁻) falls with chloride-rich fluids: that is the mechanism of hyperchloraemic acidosis.
- With a damaged endothelial glycocalyx (sepsis), even colloids leak more into the interstitium.
- Glucose solutions provide few calories and no sodium: they give free water, they do not expand volume.
Approximate composition per litre
| Solution | Na⁺ (mmol/L) | Cl⁻ (mmol/L) | Other | Osmolarity (mOsm/L) | Main destination |
|---|---|---|---|---|---|
| 0.9 % NaCl (normal saline) | 154 | 154 | — | ≈308 | Extracellular (isotonic) |
| Lactated Ringer’s (Hartmann’s) | ≈130 | ≈109 | K⁺ ≈4, Ca²⁺ ≈1.4–2 mmol/L, lactate ≈28 | ≈273 | Extracellular (near-isotonic) |
| Balanced acetate/gluconate solutions | ≈140 | ≈98 | K⁺, Mg²⁺, acetate, gluconate | ≈295 | Extracellular (isotonic) |
| 5 % dextrose in water | 0 | 0 | Glucose 50 g/L | ≈278 | Total body water (effectively hypotonic) |
| 0.45 % NaCl | 77 | 77 | — | ≈154 | Hypotonic: part enters cells |
| 3 % NaCl | ≈513 | ≈513 | — | ≈1027 | Hypertonic: draws water out of cells |
| Albumin 5 % / 20–25 % | variable | variable | Albumin | ≈isosmotic | Plasma (colloid); concentrated albumin draws fluid from the interstitium |
Clinical notes
- In hyponatraemia, a fluid with less sodium than the patient’s urine can make it worse (SIADH): what matters is the balance between what goes in and what comes out.
- Hypertonic saline is used in severe symptomatic hyponatraemia and cerebral oedema for its osmotic effect; its use and rate are set by the clinical team.
Test yourself
Think of the answer, then tap the question to see it.
Why is 5 % dextrose a poor choice for restoring intravascular volume even though its osmolarity is ≈278 mOsm/L?
The glucose enters cells and is metabolised, leaving free water that spreads through total body water; only a tiny fraction remains in the plasma.
Why can large amounts of 0.9 % NaCl cause metabolic acidosis?
It supplies Cl⁻ at a much higher concentration than plasma and no bicarbonate or metabolisable anion: the strong ion difference falls and a normal anion gap, hyperchloraemic acidosis appears.
What fraction of an isotonic crystalloid stays in plasma after equilibration, and why?
About ¼, because it spreads through the whole extracellular space and plasma is ≈¼ of that compartment; the rest moves to the interstitium following Starling forces.
Why does the lactate in Ringer’s act as an alkali rather than an acid?
It is infused as sodium lactate (the conjugate base); the liver oxidises it or turns it into glucose, consuming H⁺, which is equivalent to generating bicarbonate.
Related topics
- Fluid shifts between compartments
- Osmosis, osmolarity and tonicity
- Body fluid compartments
- Dehydration and fluid overload
- 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); 26.2 Water Balance (OpenStax (Rice University), 2022); Hypovolemia and Hypovolemic Shock (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.