Excretion of dilute urine requires,
the presence of ADH.
impermeability of the collecting tubule to water.
relative permeability of the distal tubule to water.
transport of sodium and chloride ions out of the descending nephron loop.
The Correct Answer and Explanation is:
Correct Answer: Impermeability of the collecting tubule to water.
Explanation
The kidneys play a crucial role in maintaining the body’s fluid and electrolyte balance by concentrating or diluting urine. The excretion of dilute urine occurs when the body needs to eliminate excess water while conserving solutes. This physiological response is essential for maintaining proper plasma osmolality.
The impermeability of the collecting tubule to water is key to producing dilute urine. This impermeability is regulated by the hormone antidiuretic hormone (ADH). In the absence of ADH, the cells of the distal convoluted tubule and collecting duct remain impermeable to water. As a result, even though the filtrate becomes increasingly diluted as it moves through the ascending limb of the nephron loop (loop of Henle), water is not reabsorbed in the distal nephron. Therefore, a larger volume of dilute urine is excreted.
Let’s analyze the other options:
- Presence of ADH: This hormone increases water reabsorption by making the distal tubule and collecting duct more permeable to water, which leads to the production of concentrated urine — not dilute. Thus, ADH is not required for dilute urine formation.
- Relative permeability of the distal tubule to water: This is influenced by ADH. Without ADH, the distal tubule is relatively impermeable to water, which aids in the excretion of dilute urine. However, the key factor is the impermeability of the collecting duct.
- Transport of sodium and chloride ions out of the descending nephron loop: This is incorrect because the descending limb is primarily permeable to water and not involved in active ion transport. It’s the ascending limb that actively transports Na⁺ and Cl⁻, helping to dilute the filtrate.
In summary, the excretion of dilute urine is primarily achieved through the impermeability of the collecting tubule to water, which prevents water reabsorption and leads to the elimination of hypotonic urine.
Correct Answer: Impermeability of the collecting tubule to water.
In summary, the excretion of dilute urine is primarily achieved through the impermeability of the collecting tubule to water, which prevents water reabsorption and leads to the elimination of hypotonic urine.
