Final answer:
Bicarbonate (HCO₃⁻) is the second most abundant extracellular anion, critical for maintaining the body's acid-base balance as part of buffer systems, second to chloride which is the most abundant.
Step-by-step explanation:
The second most abundant extracellular anion is bicarbonate (HCO₃⁻). While chloride (Cl⁻) is the most prevalent extracellular anion, bicarbonate plays a crucial role in maintaining the acid-base homeostasis in the body. It functions within buffer systems to help stabilize pH levels in the blood. Our body fluids are kept neutral with a balance of cations and anions; sodium (Na⁺) being the main extracellular cation that complements the role of chloride and bicarbonate anions in maintaining this delicate balance and proper hydration.
The kidneys regulate the reabsorption and secretion of these ions to preserve the necessary physiological conditions. It helps prevent the blood from becoming too acidic or too alkaline. Bicarbonate is found in the blood and other body fluids.