Final answer:
Secondary active transport utilizes the electrochemical gradient to transport substances into the cell with the help of symport proteins. It does not always rely on sodium ions and is not limited to the kidney.
Step-by-step explanation:
Secondary active transport is a process that brings sodium ions and other compounds into the cell. It utilizes the electrochemical gradient created by primary active transport to transport other substances through the membrane. Symport proteins are often involved in secondary active transport, as they move two or more substances in the same direction. However, it does not always use the influx of sodium ions and is not exclusively important in the kidney.