Final answer:
Bicarbonate is the second most abundant anion in the extracellular fluid, playing a key role in acid-base balance. Chloride is the most prevalent extracellular anion.
Step-by-step explanation:
The second most abundant anion in the extracellular fluid (ECF) is bicarbonate (HCO3-). While chloride is the predominant extracellular anion, bicarbonate plays a critical role in maintaining the body's acid-base balance as a constituent of buffer systems. It is essential to note that the concentration and distribution of ions like sodium, potassium, and chloride are tightly regulated within the ECF and intracellular fluid (ICF), ensuring electrical neutrality and contributing to physiological functions such as nerve impulse transmission and muscle contraction.