L'équilibre hydro-électrolytique est fondamental pour le fonctionnement cellulaire. Les électrolytes (sodium, potassium, calcium, magnésium, phosphore) régulent la transmission nerveuse, la contraction musculaire et l'équilibre acido-basique. Les déséquilibres peuvent engager le pronostic vital et nécessitent une reconnaissance rapide.
- Compartiments liquidiens : intracellulaire (2/3 du volume total), extracellulaire (1/3 : plasma + liquide interstitiel)
- Osmose : déplacement de l'eau des zones hypotoniques vers hypertoniques à travers une membrane semi-perméable
- Sodium (Na⁺ 135–145 mEq/L) : principal cation extracellulaire, régule l'osmolarité et la volémie
- Potassium (K⁺ 3,5–5,0 mEq/L) : principal cation intracellulaire, essentiel à la conduction cardiaque
- Solutions IV : isotonique (NaCl 0,9%, sérum glucosé 5%), hypotonique (NaCl 0,45%), hypertonique (NaCl 3%)
Fluids & Electrolytes Foundations
Understand body fluid compartments, osmotic principles, electrolyte roles in normal physiology, fluid shifts, and acid-base foundations, all at the conceptual level without disease states.
Body Fluid Compartments
Where the body's water lives
Approximately 60% of adult body weight is water, distributed between two main fluid compartments. Intracellular fluid (ICF) is inside cells (~40% body weight, ~2/3 of total body water). Extracellular fluid (ECF) is outside cells (~20% body weight, ~1/3 of total body water). ECF is further divided into intravascular (plasma, ~5%) and interstitial (between cells, ~15%). The distribution matters because each compartment has different electrolyte compositions that must be maintained for normal function.
Intracellular (ICF)
~2/3 of total body water. Primary cation: K⁺. Primary anion: HPO₄²⁻. Contains most of the body's potassium and phosphate. Cell function depends on this environment being tightly regulated.
Interstitial
Fluid between cells (~15% body weight). Similar electrolyte composition to plasma but with very little protein. Bathes cells and allows nutrient/waste exchange. Excess accumulation = edema.
Intravascular (Plasma)
Fluid within blood vessels (~5% body weight). Primary cation: Na⁺. Contains plasma proteins (albumin) that create oncotic pressure. This is the only compartment directly accessible for IV fluid administration.
The Na⁺/K⁺ Gradient
Na⁺ is concentrated OUTSIDE cells, K⁺ is concentrated INSIDE cells. This gradient is maintained by the Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in per cycle). This concentration difference is essential for nerve impulse transmission, muscle contraction, and maintaining cell volume. Disrupting this gradient has immediate physiological consequences.
Osmosis & Tonicity
How water moves between compartments
Water moves by osmosis, the net movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. Water follows solute. This is a passive process requiring no energy. It always moves toward higher solute concentration. This principle governs fluid distribution between compartments and is the basis for understanding IV fluid therapy.
Tonicity of Solutions
Osmotic vs Oncotic Pressure
Osmotic pressure is created by ALL solutes (electrolytes, glucose, urea). Oncotic (colloid osmotic) pressure is the portion of osmotic pressure created specifically by plasma proteins (mainly albumin). Oncotic pressure keeps fluid inside blood vessels. When albumin is low (malnutrition, liver disease, nephrotic syndrome), oncotic pressure drops and fluid leaks into interstitial spaces → edema.
Electrolyte Roles in Normal Function
What each major electrolyte does
Each electrolyte has specific physiological roles. Understanding their normal functions helps you appreciate why imbalances cause predictable symptoms.
Sodium (Na⁺), Normal: 135–145 mEq/L
Primary ECF cation. Regulates water distribution (water follows sodium). Drives nerve impulse conduction. Major determinant of plasma osmolarity. Changes in sodium primarily affect water balance and neurological function.
Potassium (K⁺), Normal: 3.5–5.0 mEq/L
Primary ICF cation. Critical for cardiac electrical conduction (resting membrane potential), skeletal muscle contraction, and nerve transmission. Even small changes outside the narrow normal range affect cardiac rhythm. The most dangerous electrolyte to get wrong.
Calcium (Ca²⁺), Normal: 8.5–10.5 mg/dL
Muscle contraction (including cardiac), bone structure (99% stored in bone), blood clotting cascade, nerve impulse transmission, enzyme activation. Only the ionized (free) fraction is physiologically active. Albumin level affects total calcium measurement.
Magnesium (Mg²⁺), Normal: 1.5–2.5 mEq/L
Cofactor for over 300 enzyme systems. Involved in energy production (ATP requires Mg²⁺), protein synthesis, neuromuscular function, and cardiac rhythm stability. Works in tandem with calcium and potassium, deficiency of one often accompanies deficiency of others.
Acid-Base Foundations
pH regulation at the conceptual level
The body maintains blood pH between 7.35 and 7.45. This narrow range is essential for enzyme function, protein structure, and cellular processes. pH below 7.35 = acidosis (excess H⁺). pH above 7.45 = alkalosis (deficit of H⁺). The body uses three systems to maintain this range: buffer systems (immediate), respiratory system (minutes), and renal system (hours to days). Three regulatory systems work together to maintain this balance.
Buffer Systems
Response: Immediate (seconds). Chemical buffers (bicarbonate, phosphate, protein) absorb or release H⁺ to resist pH changes. Limited capacity, buffers are consumed and must be regenerated.
Respiratory System
Response: Minutes. Controls CO₂ (which is acidic when dissolved). Hyperventilation blows off CO₂ → raises pH. Hypoventilation retains CO₂ → lowers pH. The lungs are the fast compensator.
Renal System
Response: Hours to days. Kidneys excrete H⁺ or reabsorb/generate HCO₃⁻ as needed. Most powerful but slowest compensator. Determines long-term acid-base balance.
The Bicarbonate Equation
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. This single equation is the key to understanding acid-base balance. The left side (CO₂) is controlled by the lungs. The right side (HCO₃⁻) is controlled by the kidneys. Normal ratio of HCO₃⁻ to CO₂ is 20:1, as long as this ratio is maintained, pH stays normal.
Match Fluid & Electrolyte Concepts
Terms
Definitions
Fluids & Electrolytes Quiz
1/20Which fluid compartment contains the most body water?
Nursing Responsibilities
Surveiller le bilan hydrique (entrées/sorties), peser le patient quotidiennement à la même heure, évaluer les signes de surcharge ou de déshydratation (turgor cutané, muqueuses, pression artérielle, diurèse). Contrôler les résultats biologiques avant d'administrer des suppléments électrolytiques, notamment le potassium IV. Éduquer le patient sur la restriction ou l'augmentation des apports selon la pathologie.
Clinical Pearls
L'hypokaliémie potentialise la toxicité de la digoxine — vérifier le K⁺ avant administration. L'hyponatrémie sévère ne doit pas être corrigée trop rapidement (risque de myélinolyse centropontine). Le potassium IV ne doit jamais être administré en bolus direct — toujours dilué et en perfusion lente avec monitoring cardiaque. La pesée quotidienne est plus fiable que le bilan entrées/sorties pour détecter une rétention hydrique.
Patient Education
Expliquez aux patients l'importance d'un apport hydrique adapté (environ 1,5–2 L/jour en l'absence de restriction), les signes de déshydratation (soif intense, urines foncées, étourdissements) et les aliments riches en potassium (bananes, oranges, pommes de terre) si une supplémentation est prescrite ou recommandée.
Key Takeaways
- L'eau suit toujours le sodium — comprendre la natrémie permet de prédire les mouvements liquidiens
- L'hypokaliémie et l'hyperkaliémie sont toutes deux dangereuses pour le rythme cardiaque — surveiller l'ECG
- Le potassium IV doit toujours être dilué et administré lentement avec monitoring — jamais en IVD
- Le bilan hydrique et la pesée quotidienne sont les outils de surveillance de base en soins infirmiers