4th July 2024

Electrolyte imbalance

Closely linked to renal function, electrolyte balance is vital to normal physiology. The management of electrolytes clinically can quickly become complicated, particularly in the context of an acutely unwell patient with multi-morbidity and polypharmacy.

Sodium

Sodium is vital in maintaining extracellular fluid volume and membrane potential regulation. Regulation occurs in the kidneys where the majority of reabsorption takes place in the proximal convoluted tubule. Owing to the isotonic nature of reabsorption at this point in the nephron, water is also reabsorbed here. Further sodium reabsorption occurs in the distal ascending portion of the loop of Henle, with only 5%-10% of reabsorption taking place in the distal convoluted tubule and cortical and medullary collecting ducts. Nevertheless, the reabsorption along the distal nephron remains vital and highly regulated.

Potassium

With its haemostasis closely linked to sodium, potassium is the vital intracellular ion. Filtration occurs in the glomerulus, with reabsorption in the proximal convoluted tubule and thick ascending loop of Henle. Aldosterone increases its secretion.

Calcium

Diet is the predominant source of this predominantly extracellular cation. It is highly regulated owing to its role in skeletal mineralisation, muscle contraction and neurological transmission, as well as vascular haemostasis. Absorption in the intestine is primarily controlled by 1,23-dihydroxy vitamin D3 (the active form of vitamin D). Parathyroid hormone regulates its secretion in the distal tubule of the kidney and calcitonin reduces plasma levels through its action on bones.

Bicarbonate

Acid-base balance is regulated by the kidneys and drives bicarbonate levels. Filtered bicarbonate is reabsorbed by kidneys, whilst diarrhoea results in its loss. Renal disease will often result in an imbalance and, therefore, excess bicarbonate levels.

Phosphate

Eighty-five percent of total body phosphate is found in bones and teeth as hydroxyapatite, with soft tissue containing the remaining 15%. It is regulated by Vitamin D3, parathyroid hormone and calcitonin, in conjunction with calcium, with excretion primarily by kidneys. Imbalance is often a result of either poor dietary intake, gastrointestinal disorders or defective renal excretion.

References

• Shrimanker I, Bhattarai S. Electrolytes. 2023 Jul 24. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan–. PMID: 31082167
• Palmer LG, Schnermann J. Integrated control of Na transport along the nephron. Clin J Am Soc Nephrol. 2015 Apr 7;10(4):676-87. doi: 10.2215/CJN.12391213. Epub 2014 Aug 6. PMID: 25098598; PMCID: PMC4386267