What is the primary role of sodium (Na+) in the body?

Sodium (Na+) plays a critical role in maintaining fluid balance, transmitting nerve impulses, and muscle contraction. As the major extracellular cation, sodium is primarily found outside cells and is essential for various bodily functions. The kidneys regulate sodium levels through filtration and reabsorption, which helps to control blood pressure and blood volume. This regulation is crucial because sodium affects osmotic balance and influences fluid retention.

The options describing sodium as a major intracellular cation or a minor electrolyte in blood do not accurately represent its primary function or distribution in the body. Sodium is predominantly found in the extracellular space and acts as the primary cation there, making it vital for cellular communication and functions. Thus, the identification of sodium as the major extracellular cation controlled by the kidneys highlights its essential role in homeostasis and physiological processes.