Overview
Water is the solvent of life. It bathes cells, transports compounds in blood, separates charged molecules, dissipates heat and participates directly in chemical reactions. The body maintains total water at roughly 60% of body weight despite daily fluctuations in intake and output.
This lesson covers the distribution of body water across fluid compartments, the ionic composition of extracellular and intracellular fluid, the concepts of osmolarity, osmolality and tonicity, disorders of fluid volume, and the buffer systems and organs (lungs, kidneys) that maintain acid-base balance — all essential background for interpreting electrolyte and blood gas results.
Learning Objectives
After this lesson you will be able to…- Describe the distribution of total body water across intracellular and extracellular compartments.
- Compare the ionic composition of extracellular and intracellular fluid.
- Differentiate osmolarity, osmolality and tonicity, and calculate plasma osmolality.
- Classify disorders of fluid volume — isotonic, hypotonic and hypertonic dehydration/expansion.
- Explain the buffer systems of the body and the roles of the lungs and kidneys in acid-base balance.
Clinical Story
Why This MattersA 3-year-old child is brought to the emergency department with three days of severe diarrhoea and vomiting, sunken eyes and reduced skin turgor. The physician orders serum electrolytes to assess dehydration and guide IV fluid replacement. The lab technologist must understand fluid compartments and osmolality to help interpret whether this is isotonic, hypotonic or hypertonic dehydration.
Core Concepts
Total body water is ~50–60% of body weight in adults (~75% in children), roughly 42 L. Approximately 1/3 (14 L) is intracellular fluid (ICF) and 2/3 (28 L) is extracellular fluid (ECF). ECF is further divided into plasma (~25% of ECF, 3.5 L), interstitial fluid (~75% of ECF, 10.5 L), and a small transcellular compartment (GI secretions, urine, sweat, ~0.5 L).
ECF's major cation is Na⁺ (~145 mmol/L) and major anions are Cl⁻ (~105 mmol/L) and HCO₃⁻ (~25 mmol/L). ICF's major cation is K⁺ (~150 mmol/L) and major anion is inorganic phosphate (~100 mmol/L). This unequal distribution is maintained by active transport (Na⁺/K⁺-ATPase) and is fundamental to nerve and muscle excitability.
Osmolarity is osmoles per litre of solution; osmolality is osmoles per kilogram of solvent. Normal plasma osmolality is 280–295 mOsm/kg H₂O, calculated as Plasma osmolality = 2 × Plasma Na⁺ (mmol/L), since sodium and chloride contribute ~90% of plasma osmolality.
Isotonic fluid has the same osmolality as plasma and does not change cell volume (e.g. 0.9% NaCl). Hypotonic fluid has lower osmolality and causes cells to swell. Hypertonic fluid has higher osmolality and causes cells to shrink.
Iso-osmotic dehydration occurs with loss of isotonic fluid (diarrhoea, vomiting, haemorrhage). Hypo-osmotic dehydration occurs with loss of salt in excess of water (adrenocortical insufficiency). Hyperosmotic dehydration occurs with loss of water in excess of solutes (diabetes insipidus, excessive sweating). Corresponding volume expansion states occur with excess isotonic, hypotonic (water excess) or hypertonic (hypertonic saline) fluid gain.
Normal blood pH is maintained between 7.35 and 7.45 by four buffer systems: the bicarbonate-carbonic acid system (most important in plasma), the plasma protein system (mainly albumin, 95% of non-bicarbonate plasma buffer), the phosphate system (important intracellularly), and the haemoglobin system (major buffer in RBCs, 85%, aided by 2,3-DPG).
The lungs regulate the volatile acid load by adjusting ventilation to remove CO₂. The kidneys regulate the fixed acid load through HCO₃⁻ reabsorption (80% in PCT), excretion of H⁺ as titratable acid (phosphate buffer) and excretion of H⁺ as NH₄⁺ (with new bicarbonate generation). Metabolic acidosis shows decreased pH and bicarbonate (e.g. diabetes mellitus); metabolic alkalosis shows increased pH and bicarbonate (e.g. vomiting); respiratory acidosis shows decreased pH with decreased CO₂ elimination; respiratory alkalosis shows increased pH with increased CO₂ elimination (e.g. hyperventilation).
Laboratory Principle
Plasma osmolality is measured directly by freezing-point depression osmometry, which detects the lowering of a solution's freezing point in proportion to the number of dissolved particles. Electrolytes such as sodium, potassium and chloride are commonly measured using Ion-Selective Electrodes (ISE), which generate a voltage proportional to the activity of the specific ion in the sample, allowing rapid, accurate quantification.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| ISE reference/internal filling solution | Manufacturer-specific | Electrode function for Na⁺/K⁺/Cl⁻ | Room temperature, sealed |
| Osmometer calibration standards | Traceable, multi-level | Osmometer calibration | 2–8 °C |
| Blood gas quality control ampoules | Two/three-level | Blood gas analyzer QC | 2–8 °C |
| Heparinised syringes/capillary tubes | Lithium heparin | Arterial/capillary blood gas collection | Room temperature, use before expiry |
Step-by-Step Procedure
Collect blood anaerobically (blood gas) or in a plain/lithium-heparin tube (electrolytes) as appropriate; avoid prolonged tourniquet use and excess anticoagulant.
Transport blood gas samples on ice and analyze within 15–30 minutes to prevent pH drift from ongoing cell metabolism.
Centrifuge and separate plasma/serum promptly to prevent cellular efflux of potassium.
Load the sample onto the ISE analyzer, osmometer or blood gas analyzer as required; the instrument reports concentration or activity directly.
Calculate derived values (e.g. anion gap, calculated osmolality) where needed and report against reference ranges, flagging critical results immediately.
Flow Diagram
Quality Control
Run multi-level QC on ISE and blood gas analyzers per shift, monitor electrode drift, and recalibrate promptly if control values fall outside acceptable limits.
Participate in an EQAS scheme for electrolytes and blood gases, reviewing bias reports each cycle against peer laboratories using similar instrumentation.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Low pH, low HCO₃⁻ | Metabolic acidosis (e.g. diabetic ketoacidosis) | Calculate anion gap; correlate with glucose/ketones |
| High pH, high HCO₃⁻ | Metabolic alkalosis (e.g. prolonged vomiting) | Assess chloride and potassium status |
| Low pH, high pCO₂ | Respiratory acidosis (e.g. COPD, respiratory depression) | Assess ventilatory status urgently |
| High pH, low pCO₂ | Respiratory alkalosis (e.g. hyperventilation, anxiety, pain) | Identify and treat underlying cause |
| Elevated calculated osmolality with low measured osmolality gap | Consistent hydration status | Large osmolal gap suggests unmeasured osmoles (e.g. toxic alcohol ingestion) |
Common Errors & How to Avoid Them
Cause: Air bubbles equilibrate with the sample, falsely altering pCO₂ and pO₂.
Prevention: Collect anaerobically and expel any air bubbles immediately, capping the syringe.
Cause: Ongoing cellular metabolism and potassium efflux from cells alter results over time.
Prevention: Transport on ice and analyze within the recommended window (usually 15–30 minutes).
Cause: Causes local muscle activity that falsely raises potassium.
Prevention: Avoid fist clenching; release the tourniquet promptly.
Laboratory Tips from the Bench
Calculated plasma osmolality (2×Na) can be compared with measured osmolality — a large osmolal gap (>10) suggests unmeasured solutes such as ethanol, methanol or ethylene glycol.
Always check for haemolysis before releasing a potassium result — even mild haemolysis can significantly raise measured potassium.
Remember "ROME" — Respiratory Opposite, Metabolic Equal — in respiratory disorders pH and pCO₂ move in opposite directions; in metabolic disorders pH and HCO₃⁻ move in the same direction.
Important Notes
Because sodium and its accompanying anions contribute ~90% of plasma osmolality, serum sodium is the single best surrogate marker for overall plasma tonicity.
Unlike the bicarbonate buffer system itself, the phosphate and ammonium buffer systems in the kidney generate genuinely new bicarbonate, replenishing what is consumed buffering the daily acid load.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer.
Clinical Case Study
Apply Your KnowledgeThree days of severe diarrhoea and vomiting, with sunken eyes, dry mucous membranes and reduced skin turgor on examination.
Low pH with low bicarbonate indicates metabolic acidosis, consistent with bicarbonate loss from severe diarrhoea; the low potassium reflects gastrointestinal losses. Normal sodium suggests isotonic fluid loss.
- →Diarrhoea causes loss of bicarbonate-rich fluid, predisposing to metabolic acidosis.
- →Normal serum sodium with clinical dehydration signs suggests isotonic fluid loss.
- →Potassium should always be monitored in patients with significant GI fluid losses.
Frequently Asked Questions
Osmolarity is osmoles per litre of solution (volume-based), while osmolality is osmoles per kilogram of solvent (mass-based). In dilute aqueous solutions like plasma the two values are numerically very close, but osmolality is the value actually measured in the laboratory.
Sodium and its accompanying anions (chloride and bicarbonate) account for about 90% of plasma osmotically active particles, making it the dominant determinant, even though glucose, urea and proteins also contribute.
When H⁺ is excreted bound to phosphate (as titratable acid) or as ammonium (NH₄⁺), a new bicarbonate ion is generated in the tubular cell and returned to the blood, replenishing the buffer consumed by the daily acid load — unlike simple bicarbonate reabsorption, which generates no new bicarbonate.
Quick Revision
10-Minute ReviewKey Takeaways
- Body water is distributed between intracellular and extracellular compartments in a 1:2 ratio.
- ECF and ICF have distinct ionic profiles maintained by active transport.
- Osmolality, not osmolarity, is the value directly measured and clinically relevant.
- Fluid volume disorders are classified as iso-, hypo- or hyperosmotic dehydration/expansion.
- Four buffer systems and the lungs and kidneys together maintain blood pH within 7.35–7.45.
- Acid-base disorders are classified as metabolic or respiratory, acidosis or alkalosis.
Competency Checklist
Track Your MasteryReferences
- Vasudevan DM, Sreekumari S, Vaidyanathan K. Textbook of Biochemistry for Medical Students. 8th ed.
- Guyton AC, Hall JE. Textbook of Medical Physiology. 13th ed.
- NIOS Medical Laboratory Technology curriculum — Biochemistry Module, Lesson 14: Body Water, Osmolarity and Ionic Composition of Body Fluids.