Answer: Sodium and chloride ions.
- A Glucose dissolved in the filtrate
- B Water
- C Urea, but not other ions
- D Sodium and chloride ions
Correct answer: D. Sodium and chloride ions
Explanation: The descending limb is permeable to water but largely impermeable to Na+ and Cl-, while the ascending limb is impermeable to water but actively transports out Na+ and Cl-, together creating the medullary osmotic gradient.
Filtrate flows through four functionally distinct stages of the nephron: Bowman's capsule filters blood, the PCT reabsorbs most useful substances unconditionally, the Loop of Henle builds a concentration gradient via counter-current exchange, the DCT secretes selected substances, and the collecting duct performs the final ADH-controlled water reabsorption.
Concept context
Nitrogenous waste, kidney structure, urine formation in the nephron, and hormonal regulation of excretion.
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