## **Core Concept**
The primary buffer system in interstitial fluid, which is the fluid surrounding cells in the body, is crucial for maintaining acid-base balance. Buffer systems help neutralize excess hydrogen or hydroxide ions, thereby maintaining the body's pH within a narrow range. The main buffer systems in the body include the bicarbonate buffering system, phosphate buffer system, and protein buffer system.

## **Why the Correct Answer is Right**
The correct answer, **Bicarbonate buffer system**, is the most important buffer in interstitial fluid. This system consists of bicarbonate ions (HCO3-) and carbonic acid (H2CO3), which is a weak acid. The bicarbonate buffer system is particularly effective in extracellular fluids, including interstitial fluid and blood plasma, because it can quickly respond to changes in pH by either releasing or absorbing hydrogen ions. The reaction is as follows: H+ + HCO3- ⇌ H2CO3, which then can dissociate to CO2 and H2O, facilitating the removal of excess hydrogen ions.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While phosphate buffer system (HPO4^2-/H2PO4^-) is crucial, especially in intracellular fluid and renal regulation of acid-base balance, it is not the most important buffer in interstitial fluid.
- **Option C:** Protein buffer system, although significant, particularly in blood (with hemoglobin and plasma proteins) and intracellularly, is not the primary buffer system in interstitial fluid.
- **Option D:** This option is not specified, but based on the elimination, it would not represent the primary buffer system in interstitial fluid.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the bicarbonate buffer system is crucial for acid-base balance and is closely regulated by respiratory (through CO2 levels) and renal (through HCO3- reabsorption and H+ secretion) mechanisms. This system is vital for maintaining the body's acid-base homeostasis.

## **Correct Answer:** B. Bicarbonate buffer system