Maintenance of pH and ionic balance of blood takes place by the selective secretion of certain ions takes place in
PCT
DCT
Collecting duct
All the above

Options

A. PCT
B. DCT
C. Collecting duct
D. All the above

Answer

To solve the question regarding the maintenance of pH and ionic balance of blood through selective secretion of certain ions, we can analyze each of the options provided:

1. **Understanding the Role of the Urinary System**:
- The urinary system plays a crucial role in maintaining homeostasis, which includes the regulation of pH and ionic balance in the blood. This is achieved through processes such as filtration, reabsorption, and secretion of various ions.

2. **Analyzing the Proximal Convoluted Tubule (PCT)**:
- In the PCT, approximately 70-80% of electrolytes are filtered and reabsorbed. Additionally, selective secretion of ions such as sodium and hydrogen occurs here. This contributes to the regulation of pH and ionic balance.

3. **Examining the Distal Convoluted Tubule (DCT)**:
- The DCT is responsible for the reabsorption of ions such as sodium, water, and bicarbonate (HCO3). It also involves the selective secretion of certain ions. This further assists in maintaining the ionic balance and pH of the blood.

4. **Looking at the Collecting Duct**:
- The collecting duct plays a significant role in urinary concentration by regulating the amount of water reabsorbed. It also selectively secretes ions like hydrogen (H+) and potassium (K+), which are important for maintaining the pH and ionic balance of the blood.

5. **Conclusion**:
- Since all three structures (PCT, DCT, and collecting duct) contribute to the maintenance of pH and ionic balance through selective secretion and reabsorption of ions, the correct answer to the question is "All the above".

### Final Answer:
**All the above**