Corticopapillary gradient is maintained by all BUT which of the following:
a. Active transport of sodium ions out of the ascending limb of the loop of Henle
b. Urea recycling from the collecting duct via the urea transporter
c. Water reabsorption from the collecting duct via aquaporins
d. Countercurrent exchange
The correct answer and explanation is:
The correct answer is c. Water reabsorption from the collecting duct via aquaporins.
Explanation:
The corticopapillary gradient refers to the osmotic gradient within the kidney, primarily in the renal medulla, which allows for the concentration of urine. This gradient is essential for the kidneys’ ability to conserve water and excrete waste efficiently. It is maintained by several processes in the nephron, each contributing to the gradient in specific ways.
- Active transport of sodium ions out of the ascending limb of the loop of Henle:
This process is a key component in the maintenance of the corticopapillary gradient. The thick ascending limb (TAL) of the loop of Henle actively transports sodium ions (Na+) from the filtrate into the surrounding interstitial fluid. This segment is impermeable to water, which ensures that water does not follow the sodium ions, creating a high osmolarity in the medullary interstitium. This helps to establish the gradient that allows water reabsorption downstream. - Urea recycling from the collecting duct via the urea transporter:
Urea plays a significant role in the corticopapillary gradient. Urea is passively reabsorbed in the inner medullary collecting ducts through urea transporters (UT-A1 and UT-A3). It diffuses back into the medullary interstitium, contributing to the osmotic gradient. Urea recycling helps to maintain a high osmolality in the medulla, further aiding in water reabsorption. - Water reabsorption from the collecting duct via aquaporins:
While water reabsorption via aquaporins is crucial for conserving water in the kidneys, it is not a direct mechanism for maintaining the corticopapillary gradient. Aquaporins allow water to be reabsorbed in response to the osmotic gradient created by the active transport of sodium and urea recycling. However, the presence of aquaporins is a result of the established gradient rather than a mechanism for creating it. - Countercurrent exchange:
This is a process that helps maintain the gradient by facilitating the exchange of solutes and water between the vasa recta (the blood vessels surrounding the loop of Henle) and the surrounding interstitial fluid. As blood flows through the vasa recta, it exchanges solutes and water with the medullary interstitium. This process helps to preserve the osmotic gradient by preventing washout of solutes in the medulla.
Thus, while water reabsorption via aquaporins is essential for water conservation, it does not directly maintain the corticopapillary gradient, making it the correct answer in this case.