Which of the following equation is correct ?

Options

A. `KHbO_(2)+H^(+)underset(RBC)iffHb+K+HO_(2)`
B. `Hb-O_(2)underset("Dissociation in lungs")overset("Association in tissues")iffHbHO_(2)`
C. `Na^(+)+HCO_(3) underset("Erythrocyte")iffNaHCO_(3)`
D. `HbO_(2) underset("Association in lungs")overset("Dissociation in tissues")iffHb+O_(2)`

Answer

To determine which of the given equations is correct regarding the transportation of respiratory gases in the body, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Hemoglobin's Role**:
- Hemoglobin (Hb) is a respiratory pigment that transports oxygen (O2), carbon dioxide (CO2), and carbon monoxide (CO) in the blood.
- It forms oxyhemoglobin (HbO2) when it binds with oxygen, carbaminohemoglobin when it binds with carbon dioxide, and carboxyhemoglobin when it binds with carbon monoxide.

2. **Analyzing the First Equation**:
- The first option states: \( K + HbO2 + H^+ \rightleftharpoons Hb + K^+ + HO2 \).
- In this equation, the presence of \( K \) (potassium) is irrelevant to the transportation of oxygen. Therefore, this equation is incorrect.

3. **Analyzing the Second Equation**:
- The second option states: \( Hb + O2 \rightarrow HbO2 \).
- This equation suggests that hemoglobin binds with oxygen to form oxyhemoglobin. However, the equation is not written correctly in terms of directionality (it should be reversible). Thus, this equation is also incorrect.

4. **Analyzing the Third Equation**:
- The third option states: \( Na + HCO3 \rightarrow NaHCO3 \).
- This equation indicates that sodium (Na) combines with bicarbonate (HCO3) to form sodium bicarbonate (NaHCO3). While this reaction can occur, it is important to note that this process does not take place inside red blood cells (RBCs) but rather in the blood plasma. Therefore, this equation is misleading and incorrect in the context provided.

5. **Analyzing the Fourth Equation**:
- The fourth option states: \( HbO2 \rightleftharpoons Hb + O2 \).
- This equation correctly represents the dissociation of oxyhemoglobin into hemoglobin and oxygen, which occurs at the tissues where oxygen is released. Additionally, at the lungs, hemoglobin binds with oxygen to form oxyhemoglobin. Thus, this equation is correct.

### Conclusion:
The correct equation among the options provided is the fourth one: \( HbO2 \rightleftharpoons Hb + O2 \).