The correct option is 2 only
Explanation
The collecting duct is a long tubule that extends from the cortex of the kidney to the inner parts of the medulla. It serves as the final site for the adjustment of urine volume and composition, playing a critical role in osmoregulation and the maintenance of the body's acid-base balance.
Statement-wise Analysis
- Statement 1 is Incorrect: The collecting duct allows for the reabsorption of large amounts of water from the filtrate. This process is primarily regulated by the Antidiuretic Hormone (ADH), which increases the permeability of the duct to water, thereby facilitating the production of concentrated urine.
- Statement 2 is Correct: The collecting duct is not completely impermeable to urea. While generally less permeable than other solutes, it allows small amounts of urea to diffuse into the medullary interstitium. This "urea recycling" is essential for maintaining the high osmolarity of the medullary interstitium, which drives water reabsorption.
- Statement 3 is Incorrect: The collecting duct plays a significant role in the maintenance of pH and ionic balance of the blood. It achieves this through the selective secretion of hydrogen ions (H+) and potassium ions (K+) into the filtrate.
Key Takeaway: The collecting duct is vital for concentrating urine via water reabsorption, maintaining medullary osmolarity through partial urea permeability, and regulating blood pH by secreting H+ ions.