Division of Chemical Pathology David Haarburger
Fluid compartments Human body is approximately 60% water Total body water 42ℓ Transcellular fluid (7%) 1ℓ Plasma (23%) 3ℓ Interstitial fluid (70%) 10ℓ Intracellular fluid (⅔) 28ℓ Extracellular fluid (⅓) 14ℓ
Osmotic concentration The total concentration of solutes in a solution Represents the number of particles Measured in osmoles per litre Colligative properties Lowering of vapour pressure Elevation of boiling point Depression of freezing point Osmotic pressure
Sodium Primary cation of extracellular fluid Primary determinant of extracellular osmolarity Intimately related to fluid balance Hypernatraemia causes cerebral dehydration Lethargy, weakness, irritability Twitching, seizures, and coma Hyponatraemia causes cerebral oedema Nausea, malaise, lethargy Obtundation, seizures, coma
Sodium Hypernatraemia caused by Hyponatraemia caused by Increased sodium intake Decreased sodium intake Drinking seawater Increased sodium loss Intravenous hypertonic Diarrhoea saline Diuretics Decreased free water Increased free water intake intake Hypodypsia Polydypsia Increased free water loss Exercise-associated Sweating, fever hyponatraemia Diabetes insipidus Decreased free water loss Osmotic diuresis (glucose, SIADH mannitol) Advanced renal failure
Sodium and water loss Normonatraemic hypovolaemia Normal Loss of sodium and water Haemorrhage Burns Effusion of ECF in body spaces (ascites) Free Prone to circulatory collapse water Hypernatraemic loss hypovolaemia Loss of low sodium water Sweating Diabetes insipidus Prone to cerebral Isotonic dehydration fluid loss
Sodium and fluid homeostasis Renin-angiotensin-aldosterone system Low renal perfusion Increased renin secretion Angiotensinogen → Angiotensin I Vasoconstriction Angiotensin I → Angiotensin II Increased ADH releaseVaso- constriction Increased sodium reabsorption Increased aldosterone secretion Increased sodium (water) absorption
Sodium and fluid homeostasis Arginine vasopressin High osmolarity / Low plasma volume Increased ADH secretion Increased thirst Increased (free) water reabsoption Vasoconstriction
SodiumClinical conditions associated with Clinical conditions associated withhypernatraemia hyponatraemia Sodium excess Water excess Inappropriate ADH secretion High sodium intake Glucocorticoid deficiency Administration of high sodium Hypothyroidism containing fluids Psychogenic polydypsia Condition associated with increased total body sodium Primary hyperaldosteronsism Heart failure Water deficiency Liver disease Renal failure Burns Nephrotic syndrome Hyperventilation Sodium deficiency GIT losses (vomiting, diarrhoea) Diabetes insipidus Burns Decreased fluid intake Diuretic therapy Adrenal insufficiency Conditions associated with a decreased Salt-losing nephropathy total body sodium Renal tubular acidosis Osmotic diuresis Osmotic diuresis Diabetes mellitus, mannitol infusion Bicarbonaturia, ketonuria Excessive sweating Transcellular movement Adrenal insufficiency Exercise, fever Sick cell syndrome GIT losses (vomiting, diarrhoea) Pseudohyponatraemia Hyperlipidaemia, hyperglobulinaemia
Potassium Predominant intracellular cation Only 2% of potassium is extracellular [K+] = 4 [K+] = 150 Major role of K+ is to create a membrane K+ potential in excitable cells (nerve, Na+ muscle, β-cells of pancreas) Plasma potassium negatively regulated by aldosterone ― + CELL Hypokalaemia hyperpolarises cells -90mV Muscle weakness Decreased cardiac excitability, cardiac arrest Decreased insulin secretion Hyperkalaemia depolarises cells voltage-gated Na channel, Cardiac arrhythmias, ventricular opens once fibrillation membrane potential falls to -60mV
Potassium Predominant intracellular cation Only 2% of potassium is extracellular Plasma potassium is a poor indicator of body potassium Major role of K+ is to create a membrane potential in excitable cells (nerve, muscle, β-cells of pancreas) Distal convoluted tubule Hypokalaemia hyperpolarises cells Muscle weakness Na+ Decreased cardiac excitability, cardiac ATP Na+ arrest K+ K+ Decreased insulin secretion Hyperkalaemia depolarises cells H+ Cardiac arrhythmias, ventricular fibrillation Plasma potassium negatively regulated by aldosterone Tubular lumen
Chloride Primary anion of extracellular fluid Intimately associated with sodium No symptoms directly associated to hyperchloraemia or hypochloraemia Hyperchloraemia caused by Causes of hypernatraemia Metabolic acidosis Hypochloraemia caused by Causes of hyponatraemia Metabolic alkalosis
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