30.7 : Roles of Electrolytes: Sodium and Potassium

Sodium plays a crucial role in maintaining fluid and electrolyte balance and overall bodily homeostasis. Sodium balance is primarily regulated by kidney function, which adjusts sodium elimination to match dietary intake and maintain proper electrolyte levels. Sodium is the most abundant cation in the extracellular fluid (ECF) and is found in salts such as sodium chloride (NaCl) and sodium bicarbonate (NaHCO₃). Although cellular plasma membranes are relatively impermeable to sodium, its role in regulating ECF volume and water distribution is vital, as water tends to follow sodium. Variations in plasma sodium levels influence plasma volume, blood pressure, and the volumes of intracellular fluid (ICF) and interstitial fluid (IF). Sodium also plays a critical role in producing bodily secretions, such as gastric, intestinal, and pancreatic juices, as well as saliva and bile.

Potassium, the primary intracellular cation, is essential for neuromuscular function and various metabolic processes. Changes in ECF potassium levels significantly affect neurons and muscle fibers due to potassium's direct influence on the resting membrane potential. For example, an increase in ECF potassium levels causes depolarization, which may initially enhance cellular excitability but can lead to reduced excitability if sustained. Conversely, a decrease in ECF potassium levels results in hyperpolarization, making cells less responsive and reducing excitability.

The heart is particularly sensitive to potassium imbalances. Fluctuations in potassium levels can disrupt electrical conduction, potentially causing arrhythmias or sudden cardiac death. Potassium also plays a pivotal role in maintaining the body's pH balance. Changes in hydrogen ion (H⁺) concentrations trigger shifts in potassium to preserve the body's cation balance. While these shifts may not alter the total potassium content in the body, they can profoundly affect the functionality of excitable cells.