Final answer:
Chloride is the most abundant anion in the extracellular fluid, contributing to osmotic pressure and electrical neutrality by balancing positively charged cations.
Step-by-step explanation:
The most abundant anion in the extracellular fluid is chloride. Chloride ions play a critical role in the body, helping to maintain the osmotic pressure gradient between the intracellular fluid (ICF) and extracellular fluid (ECF), ensuring proper hydration. They balance the positive charges of cations, particularly sodium ions, within the ECF to preserve the electrical neutrality. Furthermore, chloride ions are closely involved in the renal system where their paths of secretion and reabsorption follow those of sodium ions.