What is the distribution of sodium in the body?

Sodium is the major extracellular cation of the body

50% is found in the extracellular fluid

45% is found in the bone

5% in the intracellular compartment

The vast majority (∼70%) is found in the readily exchangeable form.

What are the major physiological roles of sodium?

Because of the content of sodium in the body, it exerts significant osmotic forces, and so important for internal water balance between the intra and extracellular compartments. It also has a role in determining external water balance and the extracellular fluid volume. The other important role of sodium is in generating the action potential of excitable cells.

What is the daily sodium requirement?

The daily requirement is about 1 mmol/kg/day.

What is the normal plasma concentration?

The normal is 135–145 mmol/l.

Give a simple classification of the causes of hyponatraemia.

Water excess

Increased intake: polydipsia, iatrogenic, e.g. TURP syndrome, excess dextrose administration

Retention of water: SIADH

Retention of water and salt: nephrotic syndrome, cardiac and hepatic failure

Water loss (with even greater sodium loss)

Renal losses: diuretics, Addison's disease, relief of chronic urinary obstruction

Gut losses: diarrhoea, vomiting

Pseudohyponatraemia: in the presence of hyperlipidaemia

What is pseudohyponatraemia?

This is hyponatraemia that occurs as a peculiarity of the way in which the sodium concentration of the plasma is measured and expressed. In the presence of hyperlipidaemia or hyper–proteiaemia, the sodium concentration may be falsely low if it is expressed as the total volume of plasma, and not just the aqueous phase (which…