Important role of distal tubule of the kidney in acid-base balance –
**Core Concept**
The distal tubule of the kidney plays a crucial role in acid-base balance by regulating the reabsorption of bicarbonate ions and the excretion of hydrogen ions, thus maintaining the body's acid-base equilibrium.
**Why the Correct Answer is Right**
The distal tubule contains alpha-intercalated cells, which secrete hydrogen ions into the urine, and beta-intercalated cells, which reabsorb bicarbonate ions. The alpha-intercalated cells express the H+/K+-ATPase enzyme, which pumps hydrogen ions into the urine, while the beta-intercalated cells express the HCO3- cotransporter, which reabsorbs bicarbonate ions back into the bloodstream. This mechanism helps to maintain the body's acid-base balance by adjusting the pH of the urine and the blood.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the proximal tubule, not the distal tubule, is primarily responsible for the reabsorption of bicarbonate ions.
* **Option B:** This option is incorrect because the collecting duct, not the distal tubule, is primarily responsible for the regulation of potassium ion excretion.
* **Option C:** This option is incorrect because the loop of Henle, not the distal tubule, is primarily responsible for the reabsorption of calcium ions.
**Clinical Pearl / High-Yield Fact**
The distal tubule's role in acid-base balance is crucial for maintaining the body's acid-base equilibrium, and disturbances in this process can lead to conditions such as metabolic acidosis or alkalosis.
**Correct Answer: D**