Question

in addition to their other functions, red blood cells (rbcs) have the capacity to carry water. the surface of the cell contains structures called aquaporins, which transport water across cell membranes. due to their unique cell structure, rbcs are able to expand up to 74% or shrink up to 40% compared to the original cell size.

1. which statement provides the best explanation for how red blood cells contribute to a feedback mechanism that maintains homeostasis?

(1) the aquaporins in the red blood cells facilitate the exchange of water in environments of different concentrations and help the kidneys more efficiently regulate salt and water balance.
(2) the aquaporins in the red blood cells prevent the exchange of water in environments of different concentrations and help the kidneys more efficiently regulate salt and water balance.
(3) the red blood cells change their shape to move through the kidney but have minimal effect on salt and water balance in the kidneys.
(4) the red blood cells change their shape to move through the kidney but do not regulate any feedback mechanisms.

Explanation:

To solve this, we analyze each option:

• Option 1: Aquaporins facilitate water exchange (as per the passage: "aquaporins... transport water across cell membranes"). Kidneys regulate salt - water balance, and RBCs with aquaporins helping water exchange would aid kidneys in this, supporting homeostasis.
• Option 2: Says aquaporins "prevent" water exchange, which contradicts the passage's statement that aquaporins transport water.
• Option 3: Claims RBCs have "minimal effect" on salt - water balance, but the passage implies their water - carrying function (via aquaporins) is related to homeostasis, so this is incorrect.
• Option 4: States RBCs "do not regulate any feedback mechanisms", but the question is about how they contribute to a feedback mechanism for homeostasis, so this is wrong.

Answer:

(1) The aquaporins in the red blood cells facilitate the exchange of water in environments of different concentrations and help the kidneys more efficiently regulate salt and water balance.