Electrolytes

Electrolytes:
Osmotic pressure and water distribution are primarily a function of sodium (Na+), potassium (K+), chloride (Cl-), and bicarbonate (HCO₃-). Why is osmotic pressure and water distribution important? It’s important because it’s what keeps the natural balance in our cells and bodies. If the osmotic pressure and water distribution controlled by these electrolytes and bicarbonate are out of whack bad things happen! Let’s dive in to them!

Sodium (Na+):
The normal reference range for Na+ is 135-145 mmol/L. Sodium represents ~90% of all cations in the serum. The reference range refers to extracellular sodium because intracellular sodium is a much lower concentration (4-10 mmol/L). The gradient is maintained by active transport in the form of an ATP dependent sodium potassium pump (3 Na+ out of the cell, 2 K+ in).

Sodium is regulated mostly by water intake and excretion; 60-75% of filtered sodium is reabsorbed in the kidney proximal tubule (recycling and efficiency!). In addition, the adrenal steroid aldosterone regulates sodium. Aldosterone is a powerful transcription factor that will upregulate mechanisms related to sodium reabsorption. Aldosterone is signaled to increase when blood pressure decreases, this makes sense because sodium and blood pressure are correlated.

Why is sodium important?
Critically low sodium <120 mmol/L (hyponatremia), can cause immediate health problems. The obvious ways hyponatremia occurs is sodium loss in excess of water loss, and water gain in excess of sodium gain.

Nerdy note
There have been marathon runners who over the course of the marathon drank too much water in comparison to the amount of sodium they took in; some have collapsed and died. These are sad examples of how hyponatremia can effect the body quickly.

Other things that cause increased sodium loss are adrenal gland defects (decreased aldosterone), renal failure (Na+ cannot be reabsorbed and is lost in urine), vomiting, diarrhea, and more.

Conversely, critically high sodium >160 mmol/L (hypernatremia) can cause issues as well. Other than thirst, most of the conditions are central nervous system (CNS) oriented. Too much sodium pulls water out of cells and this can become dangerous. Increased fluid loss in diseases like diabetes insipidus can also cause hypernatremia.

Potassium (K+):
Next up is potassium. Potassium is a major intracellular ion whose serum level is much lower at 3.7-5.3 mmol/L. It somewhat runs opposite to Na+ in terms of intracellular and extracellular concentration (think back to the sodium potassium pump). Potassium is involved in heart contraction, cardiac rhythm, and a whole lot of other processes.

Hypokalemia, <2.8 mmol/L, can be caused by vomiting and diarrhea, increased insulin, alkalosis, hyperaldosteronism (Cushing’s syndrome), diuretics (from kidney disorder), and more.

Hyperkalemia, >5.0 mmol/L, can cause cardiac arrhythmias at >6.0 mmol/L, and neuromuscular issues such as confusion and tingling at >8.0 mmol/L. Hyperkalemia can be caused by decreased renal excretion of K+, excess intake of K+, insulin deficiency, acidosis, exercise, crush injuries, and other processes that release K+ from cells like hemolysis.

Chloride (Cl-):
Chloride is basically sodium’s buddy. The normal reference range is 95-105 mmol/L. It mainly functions in osmotic pressure and ionic balance.