Here's why:

* Concentration Gradient: Salt concentration is higher in the gland cells than in the blood cells. This means the salt naturally wants to move from the gland cells (high concentration) to the blood cells (low concentration) due to diffusion.

* Active Transport: The blood cells are actively moving salt *against* this concentration gradient, meaning they are using energy to do so. This is the defining characteristic of active transport.

Think of it like this: Imagine trying to push a ball uphill. You need to exert energy to move it against the natural force of gravity. Similarly, cells need energy to move substances against their concentration gradient.

Key points:

* Energy Requirement: Active transport requires energy, often supplied by ATP (adenosine triphosphate).

* Specific Proteins: Active transport involves specialized proteins embedded in the cell membrane that bind to the substance being transported and use energy to move it across the membrane.

Example: The sodium-potassium pump is a classic example of active transport. It moves sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients. This process is vital for maintaining cell function.