In proximal convoluted tubule H* is exchanged for?
**Core Concept**
In the proximal convoluted tubule (PCT), a key process called secondary active transport helps to reabsorb essential nutrients and ions from the filtrate back into the bloodstream. This process involves the exchange of hydrogen ions (H+) for other ions.
**Why the Correct Answer is Right**
The correct answer involves the movement of sodium ions (Na+) in exchange for hydrogen ions (H+) in the PCT. This exchange is mediated by the sodium-hydrogen exchanger (NHE3) in the luminal membrane of the PCT cells. The NHE3 exchanger uses the electrochemical gradient of sodium ions to drive the exchange of H+ ions for Na+ ions, allowing for the reabsorption of sodium and the secretion of hydrogen ions into the lumen. This process is crucial for maintaining acid-base balance and electrolyte homeostasis.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because sodium ions are exchanged for hydrogen ions, not chloride ions, in the PCT.
* **Option B:** This option is incorrect because potassium ions are not exchanged for hydrogen ions in the PCT. Potassium is instead secreted into the lumen in exchange for sodium ions in the distal convoluted tubule.
* **Option D:** This option is incorrect because calcium ions are not exchanged for hydrogen ions in the PCT. Calcium reabsorption occurs primarily in the proximal convoluted tubule, but it is mediated by a different mechanism involving the calcium-sodium exchanger (NCX1).
**Clinical Pearl / High-Yield Fact**
The sodium-hydrogen exchanger (NHE3) is a critical transporter in the PCT that helps to maintain acid-base balance and electrolyte homeostasis. Inhibition of NHE3 can lead to metabolic acidosis and hypokalemia.
**Correct Answer: C. Sodium ions**