Sodium - Metabolism
Digestive absorption
Sodium intake from food, primarily in the form of chloride, is 1 to 4 g per day. The digestive absorption of sodium is very fast and almost complete.
Tissue distribution
In blood
Almost the total amount of sodium in blood is in the plasma, there is very little in blood cells. Its normal plasma concentration is 140 mmol/L, corresponding to 3.2 g per liter; that of the extravascular fluid is approximately the same.
Hyponatremia, characterized by a plasma concentration lower than 136 mmol/L can result from either a decrease of sodium with normal aqueous volume, or from an expansion of the aqueous volume and dilution of sodium. This last type known as of dilution hyponatremia is often drug-induced, it is observed especially with antidepressants.
Hypernatremia, characterized by a sodium concentration higher than144 mmol/L, can result from an increase of sodium or a decrease of the aqueous volume.
In tissues
The cells contain little sodium whereas they are rich in potassium. Bones contain a large amount of sodium: more than 40% of total sodium, that is to say approximately 40 g , is present in bones, in non exchangeable and exchangeable forms.
Elimination
The elimination of sodium is primarily urinary. Its digestive elimination is very low because, although present in exocrine digestive secretions, it can be reabsorbed. Sweat, tears contain sodium but play a negligible part in its elimination.
In the kidney, after glomerular filtration, sodium is actively reabsorbed at the level of the proximal convoluted tubule, the ascending limb of Henle's loop and finally the distal convoluted tubule and collecting duct where its reabsorption is controlled by aldosterone. The regulation of sodium excretion is carried out by the glomerulotubular balance whose mechanism is poorly understood.
Aldosterone decreases the urinary excretion of sodium, whereas atrial natriuretic peptide increases it.
Disorders of sodium metabolism
The metabolism of sodium is closely linked to that of water. One can observe hyponatremia and hypernatremia. It is not necessarily a question of lack or excess of sodium, but often the consequences of dissociated variations of water and sodium, a water excess, for example, causes a hyponatremia of dilution. Hyponatremia can be observed in patients with a mediastinal tumor or a neurological disease or more frequently in patients treated with certain drugs (antidepressants, carbamazepine, neuroleptic agents. See Antidiuretic hormone, ADH or vasopressin.
A sufficient intake of water or electrolytes by oral route each time it is possible avoids and corrects the anomalies, while reducing the risk of overloading linked to parenteral administration. However, when the intake by oral route is impossible or when there is an important imbalance, they should be administered by intravenous route.
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