Countercurrent mechanism operates in loop of Henle/vasa rectae/Both of these.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Countercurrent Mechanism**: The countercurrent mechanism is a process that occurs in the kidneys, specifically in the nephron, which is the functional unit of the kidney. It plays a crucial role in the concentration of urine.

2. **Identifying the Structures Involved**: The two main structures involved in the countercurrent mechanism are the Loop of Henle and the Vasa Recta. The Loop of Henle is responsible for the reabsorption of water and salts, while the Vasa Recta helps maintain the concentration gradient in the medulla of the kidney.

3. **Function of the Loop of Henle**: The Loop of Henle consists of a descending limb and an ascending limb. The descending limb is permeable to water but not to salts, allowing water to be reabsorbed into the bloodstream. The ascending limb is impermeable to water but allows salts to be reabsorbed, which helps to create a high concentration of solutes in the medulla.

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