What role does sodium play in the body?

Sodium primarily plays a crucial role in regulating body fluids and blood pressure. It helps maintain the balance of fluids in and around cells, as well as within the bloodstream. Sodium works by attracting and holding water, which is essential for maintaining proper hydration levels. In terms of blood pressure, sodium's effect on fluid balance can influence the volume of blood circulating in the vessels; higher sodium intake can lead to increased blood volume, which may elevate blood pressure.

While sodium does play a part in muscle contraction through its involvement in nerve impulse transmission, the primary function highlighted in the context of this question pertains to fluid and blood pressure regulation. Building strong bones is not a role associated with sodium, as this function is primarily attributed to calcium and phosphorus. Therefore, the focus on sodium's significant impact on body fluids and blood pressure makes the first choice the most relevant answer.