Fluid, Electrolyte & Acid-Base Balance Vocabulary

Fluid, Electrolyte & Acid-Base Balance

Link Between the Cell and the Surrounding Environment

• The cell membrane is selectively permeable, controlling exchange between intracellular fluid (ICF) and extracellular fluid (ECF).

• Movement of water, nutrients, and waste occurs through:

  • Diffusion (passive).

  • Osmosis (water movement).

  • Facilitated diffusion (via proteins).

  • Active transport (requires ATP).

Fluid Compartments in the Body

• ICF:

  • ~66% of total body water.

  • Located inside cells.

• ECF:

  • ~33% of total body water.

  • Located outside cells.

  • Splits into:

    • Interstitial fluid (~75% of ECF).

    • Plasma/intravascular fluid (~25% of ECF).

• Other minor compartments: CSF, lymph, synovial fluid.

Fluid and Electrolyte Balance

• Key Electrolytes:

  • Na+ (Sodium):

    • Location: ECF.

    • Functions: Fluid balance, nerve transmission.

  • K+ (Potassium):

    • Location: ICF.

    • Functions: Muscle and nerve function.

  • Ca2+ (Calcium):

    • Location: ECF.

    • Functions: Muscle contraction, clotting.

  • Cl- (Chloride):

    • Location: ECF.

    • Functions: Acid-base balance.

  • HCO3- (Bicarbonate):

    • Location: ECF.

    • Functions: pH buffering.

Regulation of Fluid Levels

• Input: food, fluids, IV therapy.

• Output: urine, faeces, sweat, breathing.

• Organs involved: kidneys, lungs, GI tract, skin.

Control of Fluid Levels

• Hormones:

  • ADH: Water reabsorption in kidneys.

  • Aldosterone: Retains Na+ and water, excretes K+.

  • ANP: Promotes excretion of Na+ and water.

  • RAAS: Increases BP and volume (vasoconstriction, Na+ retention).

Diffusion and Osmosis

• Diffusion:

  • Moves: Solutes (O2, CO2).

  • Direction: High → Low.

  • Energy Required?: No.

• Facilitated diffusion:

  • Moves: Glucose, ions.

  • Direction: High → Low (via proteins).

  • Energy Required?: No.

• Osmosis:

  • Moves: Water.

  • Direction: Low → High solute concentration.

  • Energy Required?: No.

• Active transport:

  • Moves: Ions.

  • Direction: Low → High.

  • Energy Required?: Yes (ATP).

Molarity, Osmolarity, and Tonicity

• Molarity: Moles/L of solute (chemistry-focused).

• Osmolarity: Particles/L (considers ion dissociation).

• Tonicity: Effect on cell volume (clinical application).

Solution Effects on Cells

• Isotonic: No change (0.9% NaCl, Hartmann's).

• Hypertonic: Cells shrink (3% NaCl, mannitol).

• Hypotonic: Cells swell (0.45% NaCl, D5W).

Acid-Base Balance

Acids vs Bases

• Acid:

  • Role: Donates H+.

  • pH: < 7.

  • Examples: HCl, carbonic acid.

• Base:

  • Role: Accepts H+ / donates OH-.

  • pH: > 7.

  • Examples: Bicarbonate, ammonia.

Definition and Importance

• Acid-base balance: Regulation of pH within a narrow range to maintain physiological function.

• Normal Blood pH: 7.35 – 7.45

  • < 7.35 = Acidosis

  • > 7.45 = Alkalosis

• Importance: Enzyme activity, electrolyte balance, oxygen delivery, muscle/nerve function.

Body's Buffer Systems

• Bicarbonate:

  • Location: ECF.

  • Role: Main pH buffer.

• Protein (e.g., haemoglobin):

  • Location: ICF/RBCs.

  • Role: Binds/releases H+.

• Phosphate:

  • Location: ICF, urine.

  • Role: Cellular/renal buffering.

• Ammonia:

  • Location: Kidneys.

  • Role: Removes excess H+.

• Respiratory:

  • Location: Lungs.

  • Role: Controls CO2 levels.

Pathophysiology and Pharmacology of a Haemodynamically Unstable Patient

Key Mechanisms & Concepts

• Starling’s Principle: Fluid movement across capillary membranes depends on the balance between hydrostatic pressure (pushes fluid out) and oncotic pressure (pulls fluid in).

• If blood hydrostatic pressure > oncotic pressure, fluid moves into interstitial space → oedema.

Fluid Compartments Calculations

• Total body water = 60% body weight (adult male), 50–55% (female)

• ECF = 1/3 of TBW; ICF = 2/3 of TBW

• For TBW of 75 L:

  • ICF = 50 L

  • ECF = 25 L

Drugs to Raise BP

• Noradrenaline (norepinephrine)

• Phenylephrine

Fluid Replacement

• Crystalloids (e.g., 0.9% NaCl, Hartmann's)

• Colloids (e.g., albumin)

• Blood products (packed RBCs, FFP, platelets)

Pharmacology

• Tranexamic acid: antifibrinolytic, used to prevent excessive bleeding, esp. within 3 hours of trauma.

Adrenaline Concentrations

• 1:1000 = 1 \text{ mg/mL}

• 1:10000 = 0.1 \text{ mg/mL}

Central Venous Access

• PICC, midline, CVL: each has advantages/disadvantages for long-term use, infection risk, and ease of access.

• Common veins: subclavian, internal jugular

• Documentation post-insertion includes date, site, number of lumens, any complications

Blood Loss & Haemodynamics

• Shock: MAP < 65 \text{ mmHg}, reduced tissue perfusion

• Target Hb = >70 \text{ g/L}, MAP = >65 \text{ mmHg}

• Blood group compatibility for A-: A- or O-

Hormonal Response to Low BP

• ADH increases water reabsorption

• RAAS system activates vasoconstriction and Na+ retention

Disseminated Intravascular Coagulation (DIC)

• Uncontrolled clotting → consumption of clotting factors → bleeding

• Triggered by trauma, sepsis, malignancy

This guide provides foundational knowledge to apply in clinical settings, such as understanding IV fluids, interpreting ABGs, managing haemodynamic instability, and choosing appropriate pharmacological interventions.