Final answer:
Sodium (Na+) is the ion most abundant in extracellular fluid, playing a key role in fluid balance and osmotic pressure regulation across cell membranes.
Step-by-step explanation:
The ion most abundant in extracellular fluid is sodium (Na+). Sodium is the major cation of the extracellular fluid, responsible for roughly half of the osmotic pressure gradient that exists between the interior of cells and the extracellular environment. This ion is essential for maintaining the balance of fluids in the body and is closely regulated. While chloride (Cl-) is the predominant extracellular anion that balances cations like sodium, sodium itself has a higher concentration in the extracellular fluid compared to other cations such as calcium (Ca+2), potassium (K+), and magnesium (Mg+2). The kidneys play a vital role in excreting excess sodium, which enters the body primarily through dietary intake of sodium chloride (NaCl).