Which of the following statements correctly describes the osmolarity of mammalian urine?

• A. The osmolarity of mammalian urine can be higher than that of blood plasma due to the kidney's ability to concentrate solutes.
• B. Mammalian urine always has a lower osmolarity than blood plasma.
• C. The osmolarity of mammalian urine is constant and does not change with hydration status.
• D. Mammalian urine contains no solutes, so its osmolarity is zero.

Answer: (A)

Verified step by step guidance

1. Understand the concept of osmolarity, which is a measure of the total concentration of solute particles in a solution, expressed in osmoles per liter (Osm/L). In biological systems, osmolarity helps describe how concentrated a fluid is with dissolved substances.
2. Recognize that blood plasma typically has a relatively constant osmolarity, around 290 mOsm/L, which is tightly regulated by the body to maintain homeostasis.
3. Know that the kidneys play a crucial role in regulating the osmolarity of urine by either concentrating or diluting it depending on the body's hydration status. This is achieved through mechanisms like the countercurrent multiplier system in the nephron.
4. Realize that when the body is dehydrated, the kidneys conserve water by producing urine with a higher osmolarity than blood plasma, meaning urine can be more concentrated with solutes.
5. Conclude that the osmolarity of mammalian urine is variable and can be higher than that of blood plasma, which explains why the correct statement is that the kidney can concentrate solutes, leading to urine osmolarity exceeding that of blood plasma.