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Q50 — Countercurrent System and Nephron Function

Theoretical A Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Comparative Osmoregulation & Excretion

The countercurrent system is a common biological pattern, classified into countercurrent multipliers and exchangers. In vertebrates, these systems often involve a specialized network of arteries and veins lying close together, known as the rete mirabile. In humans, an example of this system is found in the kidneys, where the nephron and vasa recta together form a countercurrent multiplier. This arrangement allows for efficient exchange and concentration of solutes and water, which are crucial in urine concentration and maintaining the body’s fluid balance.

Diagram of a countercurrent osmotic multiplier with a semipermeable membrane and applied pressure, alongside a nephron diagram showing the glomerulus, descending and ascending limbs, collecting duct, and vasa recta with osmolarity values labeled at successive depths. Figure 1. A Countercurrent osmotic multiplier: solute passes through a reservoir separated by a semipermeable membrane; black arrows show water movement driven by pressure P. B Nephron diagram: 1 - descending limb, 2 - ascending limb, 3 - collecting duct. Numbers indicate osmolarity (mOsmol/kg).

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. Water movement across the membrane in the countercurrent multiplier (Figure 1A) and loop of Henle is primarily driven by the similar forces.
B. Sodium reabsorption explains the difference in osmolarity between the descending limb and the collecting duct.
C. The osmolarity in region Q in the tissues surrounding the tubules is between 100 and 290 mOsmol/kg.
D. Blood entering the vasa recta in region Q has higher osmolarity than blood leaving the vasa recta in region Q.
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Question reproduced from IBO 2024, Theoretical Exam Part A, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)