MCAT Biology Question 144: Answer and Explanation
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Question: 144
9. Under normal physiological circumstances, the primary function of the nephron is to create urine that is:
- A. hypertonic to the blood.
- B. hypotonic to the blood.
- C. isotonic to the filtrate.
- D. hypotonic to the vasa recta.
Correct Answer: A
Explanation:
The kidneys function to eliminate wastes such as urea, while reabsorbing various important substances such as glucose and amino acids for reuse by the body. Generation of a solute concentration gradient from the cortex to the medulla allows a considerable amount of water to be reabsorbed. Excretion of concentrated urine serves to limit water losses from the body and helps to preserve blood volume. Thus, the primary function of the nephron is to create urine that is hypertonic to the blood, making choice (A) the correct answer and eliminating choices (B) and (D). Water should be reabsorbed from the filtrate, so urine should be hypertonic to the filtrate, eliminating choice (C).