The body has two main "fluid spaces." What are these spaces and how do the electrolytes Na+ and K+ influence the flow of water into and out of these spaces?

What will be an ideal response?

The two spaces include the intracellular and extracellular spaces. Water always follows sodium, so movement of sodium across a membrane causes movement of water molecules across that same membrane. Sodium and potassium are exchanged by way of the sodium-potassium pump that moves sodium from one side of the cell while moving potassium to the other side of the cell.