Excretion of dilute urine requires:
A.
impermeability of the collecting duct to water.
B.
transport of sodium and chloride ions out of the descending loop of Henle.
C.
relative permeability of the distal tubule to water.
D.
the presence of ADH.
The Correct Answer and Explanation is:
Correct Answer: A. Impermeability of the collecting duct to water.
Explanation
The ability of the kidneys to produce dilute urine is essential for maintaining the body’s fluid and electrolyte balance. This process is regulated by the nephron, the functional unit of the kidney, which filters blood and selectively reabsorbs or secretes various substances to concentrate or dilute urine as needed.
To excrete dilute urine, the kidneys must reabsorb solutes (such as sodium and chloride ions) while retaining water in the tubule, ultimately leading to the elimination of a fluid that has low osmolarity (i.e., is more dilute than plasma).
The collecting duct plays a crucial role in this process. When the body needs to conserve water, antidiuretic hormone (ADH) is released, increasing the water permeability of the collecting duct, allowing water to be reabsorbed back into the bloodstream. However, when the body needs to eliminate excess water (i.e., produce dilute urine), ADH secretion is suppressed. As a result, the collecting duct becomes impermeable to water, and water remains in the tubular fluid, leading to dilute urine.
Let’s examine the other options:
- B. Transport of sodium and chloride out of the descending loop of Henle is incorrect. The descending limb is permeable to water, not solutes. Solute reabsorption occurs mainly in the ascending limb.
- C. Permeability of the distal tubule to water would result in water reabsorption, concentrating the urine—opposite of what is required for dilute urine.
- D. The presence of ADH increases water reabsorption and leads to concentrated urine, not dilute. For dilute urine, absence of ADH is needed.
Thus, the correct answer is A, because the impermeability of the collecting duct to water, due to lack of ADH, is necessary to retain water in the filtrate and excrete it as dilute urine.
