How do air get in to hydronic heating system?

True hydronic baseboard heat has an opening at both the top and the bottom of the wall unit with a finned copper tube running down its length. Because hot air is less dense than cold air, the hot air naturally rises. As heat is absorbed in the air surrounding the finned copper tube, it rises and flows out of the opening at the top. (convection) As this warm air leaves the heater it draws cooler air into the bottom opening which is then heated and flows out the top continuously as long as there is heat in the finned copper tube.