Here's how it works:

1. Active Transport: The sodium-potassium pump actively transports sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. This process requires energy, which is provided by ATP.

2. Sodium Gradient: This pumping action creates a concentration gradient of sodium ions across the cell membrane, with a higher concentration of sodium outside the cell.

3. Water Movement: Water follows the sodium gradient by osmosis. This means water moves from an area of high water concentration (inside the cell) to an area of low water concentration (outside the cell).

4. Reabsorption: The movement of water out of the cell helps to reabsorb water from the filtrate back into the bloodstream.

Specifically, in the kidneys:

* The sodium-potassium pump is present in the cells of the proximal convoluted tubule (PCT), a segment of the nephron where most water reabsorption occurs.

* By pumping sodium ions out of the PCT cells, the pump creates an osmotic gradient that draws water from the filtrate into the bloodstream.

In summary, the sodium-potassium pump plays a crucial role in water reabsorption by creating a sodium gradient that drives water movement.