Final answer:
In the countercurrent exchange model, equilibration between interstitial osmolarity and tubular osmolarity occurs in the loop of Henle. The descending limb allows water to move out of the filtrate, while the ascending limb reabsorbs solutes. The vasa recta exchanges solutes and water between the tubules and the capillaries.
Step-by-step explanation:
In the countercurrent exchange model, equilibration between interstitial osmolarity and tubular osmolarity occurs in the loop of Henle of the kidneys. The loop of Henle is divided into a descending limb and an ascending limb, with the descending limb being permeable to water and the ascending limb being impermeable to water but permeable to solutes, such as sodium and chloride ions.
As the filtrate flows through the descending limb, water moves out of the filtrate to the surrounding interstitial fluid, causing the osmolarity inside the limb to increase. This creates a concentration gradient. In the ascending limb, sodium and chloride ions are actively reabsorbed, further increasing the osmolarity of the interstitial fluid.
The vasa recta, a network of capillaries around the loop of Henle, acts as the countercurrent exchanger. It allows the exchange of solutes and water between the renal tubules and the surrounding capillaries, helping to maintain the osmotic gradient in the medulla.
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