Water, electrolyte and acid-base balance

Published on 02/03/2015 by admin

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Last modified 02/03/2015

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11 Water, electrolyte and acid-base balance

Salt and volume distribution

Na+ retention and excretion control extracellular volume (water will passively follow salt). Systemic baro-receptors (carotid sinus, aortic arch) ‘sense’ changes in arterial tone (a function of cardiac output and systemic vascular resistance). A fall in effective arterial blood volume (EABV) activates the sympathetic nervous system, leading to Na+ retention by the kidney. With an expanded EABV, increased renal Na+ loss occurs.


Disorders of sodium are more accurately disorders of water than of sodium, as disturbances of sodium concentration are mainly caused by a disturbance of water balance. The danger posed by a low or high serum Na+ comes from both the electrolyte abnormality and its correction; the skull does not allow the brain to change in volume, so movement of water into the CNS risks increased intracranial pressure (and eventually tentorial herniation). Equally, movement of water out of the brain can lead to sudden and irreversible osmotic injury (central pontine myelinolysis (CPM)).


Hyponatraemia (plasma Na+ < 135 mmol/L) is common in hospitalized patients, but symptomatic hyponatraemia is very much less common — and more dangerous.

Hyponatraemia with a normal extracellular volume

Excess body water results from an abnormally high intake.