Fluid and electrolyte

• Total Body Water (TBW)


  In a 70 kg man is about 42 L and contributes to 60% of total body weight
• Sodium (Na)


  There is an ≈ 3,000 mmol of sodium, mainly in the ECF
• Daily intake of water

  In an adult approx. 1.5 L
• Daily intake of sodium

  In an adult approx. 60 – 150 mmol
• Distribution of water in body compartments in a 70 kg man

  • Intracellular fluid compartment (ICF) - 24 L
  • Extracellular fluid compartment (ECF) - Interstitial (13 L), Intravascular (5 L) - 18 L
  • Total Body Water - 42 L
• Water composition of tissues and organs by weight

  • Adapted from Pivarnik and Palmer 1994
• Internal water balance


  Maintenance in its distribution between the different body compartments
• Three important factors that influence internal water balance distribution

  • Osmolality
  • Hydrostatic pressure
  • Colloid osmotic pressure
• External water balance


  Matches input and output of water from the body
• Osmotic gradients across cell membranes in the osmoreceptor centers in the hypothalamus


  Control both the intake and loss of water by stimulating thirst and Antidiuretic hormone (ADH) release by the pituitary gland
• ADH or arginine vasopressin


  Synthesized in supraoptic and paraventricular nuclei of the hypothalamus and transported to the posterior pituitary gland and secreted
• ADH


  Enhances water reabsorption in excess of solute from the collecting ducts of the kidney
• Secretion of ADH


  Stimulated by the flow of water in and out of cerebral cells caused by a relatively high ECF osmolality
• Stretch receptors in the left atrium and baroreceptors in the aortic arch and carotid sinus


  Influence ADH in response to hypovolemia
• Body fluid compartments


  • Intracellular - K+ is the predominant cation
  • Extracellular - Na+ is the predominant cation
  • Interstitial space with low pr- concentration
  • Intravascular (plasma) space with relatively high pr- concentration
• Na+/K+ pumps


  The differential concentrations of Na+ and K+ between intracellular and extracellular compartments are maintained by the energy dependent pumps
• Distribution of water across biological membranes

  Depends on the balance between hydrostatic pressure and osmotic pressure differences on each side of the membrane
• Colloid osmotic pressure together with hydrodynamic factors

  Affects the movement of water and low molecular mass solutes (predominantly NaCl) between the intravascular and extravascular compartments
• Mechanisms of fluid movement

  • Osmosis
  • Diffusion
  • Filtration
  • Active transport
• Fluid imbalances


  • Isotonic loss of water and electrolytes (fluid volume deficit)
  • Isotonic gain of water and electrolytes (fluid volume excess)
  • Hyperosmolar loss of only water (dehydration)
  • Hypo-osmolar gain of only water (overhydration)
• Regulating body fluids


  • Fluid intake - Thirst
  • Fluid output - Urine, Insensible loss, Feces
  • Maintaining homeostasis - Kidneys, ADH, Renin-angiotensin-aldosterone system, Atrial natriuretic system
• Fluid and electrolyte balance

  • Volume imbalance: Net volume gain - hypervolemia, Net volume loss - hypovolemia
  • Water intoxication - overhydration
• Fluid volume excess (FVE)


  • Intake exceeds output - Weight gain, Pitting edema, Cough, Dyspnea, Cardiac palpitations, Decreased urinary output, Mental status changes
• Fluid volume deficit (FVD)


  • Output exceeds intake - Diarrhea, Diuretics, Vomiting, Gastric suction, Anorexia, Increasing fatigue and weakness, Weight loss, Fever, Excess urine output, Change in mental status, Traumatic injury (burn), Blood loss
• Factors affecting body fluid, electrolyte, and acid-base balance

  • Age
  • Gender
  • Body size
  • Environmental temperature
  • Lifestyle
• Risk factors for fluid, electrolyte, and acid-base imbalances


  • Chronic diseases
  • Acute conditions
  • Medications
  • Treatments
  • Extremes of age
  • Inability to access food and fluids
• Specific illnesses


  • COPD, Asthma, Cystic Fibrosis
  • CHF
  • Kidney disease
  • Diabetes Mellitus
  • Cushing's or Addison's disease
  • Cancer
  • Malnutrition, anorexia nervosa, bulimia
  • Ileostomy
  • Gastroenteritis
  • Bowel obstruction
  • Head injury
  • Fever, draining wounds, fistulas
  • Surgery
• Medications to monitor


  • Diuretics
  • Corticosteroids
  • NSAIDS/Opoiods
• Treatments that affect fluid balance

  • Chemotherapy
  • IV therapy or TPN
  • Nasogastric suction
  • Enteral Feedings
  • Mechanical Ventilation
• Electrolyte imbalances


  • Hyponatraemia
  • Hypernatremia
  • Hypokalaemia
  • Hyperkalaemia
  • Hypocalcaemia
  • hypercalcaemia
  • Hypochloraemia
  • Hyperchloraemia
  • Hypophosphataemia
  • Hyperphosphataemia
  • Hypomagnesaemia
  • Hypermagnesaemia
• Dietary intake of Na+ (and Cl-)

  Can vary globally, but total body Na+ can be maintained even if < 5 or > 750 mmol is taken in 24 hrs
• Urinary losses of Na+


  Normally closely match intake, and there is little loss through skin or faeces. However, GIT can be a major source of Na+ loss in disease
• Amount of Na+ excreted in urine

  Controls the ECF volume since, when osmoregulation is normal, the amount of ECF water is controlled to maintain a constant concentration of ECF NA+
• 2 major factors that control sodium balance

  • Aldosterone
  • Renal blood flow
• Water and sodium deficiency

  Water and sodium are usually lost together in the body and the imbalance in the degrees of their deficiency is due to the composition of the fluid lost or used in replacement
• Losses or gains of pure water


  Distributed across all fluid compartments
• Losses or gains of Na+ and water

  Borne by the much smaller ECF compartment
• Loss of isotonic fluid


  Needs more urgent replacement than losses of water
• Circulatory overload


  More likely with excessive administration of isotonic Na+ -containing solutions than isotonic dextrose
• Causes of water depletion

  • Inadequate intake - Infants, Coma, Very sick patients, nausea, dysphagia etc
  • Abnormal losses via Lungs - Mechanical ventilation, Skin - fever, Hot climate, Renal tract - Diabetes insipidus, Lithium therapy