--- module: 018-01 language: en chapter: 18 title: "Sodium, Water, Potassium, and Volume Regulation" module_title: "Foundations" source_sha256: 2c64fd063237bb4cb850701dfcf0439ad08d9dbfe86ebbbe89546802513e7136 --- # Sodium, water, potassium, and volume regulation ## Content versus concentration ### Sodium content sets extracellular fluid volume ### Water relative to solute sets plasma sodium ### Volume depletion with any plasma sodium value ### Potassium shifts change plasma without content ## Compartments and tonicity ### Intracellular fluid holds most body water ### Extracellular fluid: plasma, interstitial, transcellular ### Sodium outside cells, potassium salts inside ### Sodium-potassium pump maintains the gradients ### Measured against calculated gives an osmolar gap ### Tonicity counts only the effective osmoles #### Urea adds osmolality, little sustained tonicity #### Hyperglycaemia draws water out, dilutes sodium ## Water balance and antidiuretic hormone ### Osmoreceptors drive thirst and hormone release ### Hormone raises collecting-duct water permeability ### Low effective volume stimulates through baroreceptors #### Volume defence can override osmotic regulation ### Dilute urine: suppression, filtration, diluting segments ### Concentrated urine: gradient and responsive ducts ### Available solute limits free-water excretion ## Hyponatraemia ### Excess water relative to exchangeable cations ### First ask whether it is hypotonic #### Hyperglycaemia gives hypertonic hyponatraemia #### Lipid or protein gives pseudohyponatraemia ### Hypovolaemic: gastrointestinal or renal sodium loss ### Euvolaemic: inappropriate hormone, deficiency, low solute ### Hypervolaemic: heart failure, cirrhosis, kidney, nephrotic ### Acute severe disease causes cerebral oedema ### Chronic adaptation reduces brain swelling #### Rapid correction risks osmotic demyelination ### Hypertonic saline with closely controlled correction ## Hypernatraemia ### Water deficit relative to body sodium ### Poor access, failed thirst, concentrating defect ### Gastrointestinal or skin loss, osmotic diuresis ### Risk with impairment, infancy, frailty, dependence ### Diabetes insipidus: deficient hormone or resistance ### Free water plus volume, ongoing loss, cause ### Chronic cases corrected gradually ## Effective circulating volume and oedema ### Body senses effective arterial filling ### Heart failure and cirrhosis: oedema with underfilling ### Sympathetic, aldosterone, hormone and thirst activate ### Retention worsens oedema and dilutional hyponatraemia ### No single volume sign is definitive ### Venous congestion itself reduces kidney function ## Potassium distribution and renal handling ### Most potassium resides inside cells ### Insulin and beta two stimulation drive uptake ### Acidosis shift depends on acid and transport ### Lysis, exercise, hyperosmolality, low insulin release it ### Distal secretion is the variable step #### Raised by aldosterone, potassium, delivery, flow #### Reduced by kidney failure or channel blockade ## Potassium disorders ### Hypokalaemia: loss, shift, or low intake #### Diuretics, mineralocorticoid excess, vomiting, diarrhoea #### Weakness, ileus, arrhythmia, concentrating defect #### Flat T waves, S T depression, U waves #### Magnesium deficiency makes correction difficult ### Hyperkalaemia: release, shift, retention, intake #### Haemolysed sample can mimic it #### Peaked T waves, P-wave loss, Q R S widening #### Danger without classic electrocardiographic change #### Calcium stabilises, insulin and glucose shift #### Kidneys, binders or dialysis remove potassium ## Calcium, magnesium, and phosphate ### Ionised calcium is the active fraction #### Alkalosis raises binding, lowers ionised calcium ### Hypocalcaemia: vitamin D, parathyroid, kidney, pancreatitis ### Hypercalcaemia: hyperparathyroidism or malignancy ### Magnesium depletion: gut loss, diuretics, alcohol #### Refractory hypokalaemia and torsades risk ### Phosphate depletion impairs energy and muscle #### Refeeding drives phosphate into cells ## Fluid prescription and monitoring ### Maintenance supplies expected needs ### Replacement matches abnormal ongoing losses ### Resuscitation addresses impaired perfusion ### Weight, balance, examination, laboratory, urine output ### A positive charted balance prompts examination