Final answer:
The sodium ion is at the highest concentration in extracellular fluid due to active transport by sodium-potassium pumps.
Step-by-step explanation:
The sodium ion is at the highest concentration in extracellular fluid primarily due to active transport by sodium-potassium pumps. These pumps actively move sodium ions out of the cell and potassium ions into the cell, creating a higher concentration of sodium ions outside the cell. This concentration gradient allows sodium ions to move into the cell through channels and secondary active transport.
The sodium-potassium pump actively moves 3 sodium ions out of the cell for every 2 potassium ions it brings in, which leads to a net expulsion of positive charges and thus, makes the interior of the cell negatively charged. This is how the cell maintains a polarized state, which is necessary for the transmission of nerve impulses and muscle contractions.