Sodium is considered the most important cation in the extracellular fluid. Normal sodium levels in the extracellular compartment are 135-148 mEq/L, compared with 10-14 mEq/L inside the cell. Sodium has several different functions. Primarily, it is responsible for the osmolality of the body’s vascular fluids. Doubling the serum sodium gives one a rough estimate of the body’s osmolality (see Chapter 3). It also has an affinity for chloride and helps maintain acid-base balance when combined with HCO₃.

The cellular membrane is impermeable to sodium, making the electrolyte dependent on the sodium-potassium pump for transportation in and out of the cell (see Chapter 2). Sodium is known for its ability to assist with the conduction of impulses with muscle and nerve fibers, and this is accomplished via the sodium-potassium pump. As sodium shifts
into the cell potassium shifts out, resulting in depolarization of the cell membrane. When sodium shifts back out of the cell potassium shifts into the cell, and the cell is considered to be repolarized. The sodium-potassium pump not only assists in maintaining fluid balance, it also assists in maintaining neuromuscular activity. Sodium plays a major role in maintaining homeostasis within the body. When serum sodium levels become elevated or fall below normal values, problems with fluid and acid-base balance as well as impulse conduction may occur.