A water central heating system is a closed loop system that is under pressure. This means the water in it is circulated around and around the system and is cut off from other water supplies under normal operation. Naturally, slow leaks happen and gas can enter the system in various ways so occasionally this needs to be released from the system. Any gas in the system naturally collects at the highest points along the path - which tend to be the radiators.

When you bleed a radiator you are opening the system to the outside and hopefully where the gas has accumulated. Since the system is under pressure it forces the gas out of the system to equalize the pressure with the outside. This will cause the pressure of the system to drop and eventually it will stop.

However there should be a control valve somewhere, typically on/near the boiler that connects the central heating system up to the mains water supply. You can open this valve to cause water to flow into the central heating system and pressurize it and really this should be done every time you bleed the radiator a significant amount.

In apartments though you might find that you are on a building wide circuit, or you might have one isolated for your apartment. If you have a boiler in your apartment then you are likely on a closed system and should be able to equalize the pressure yourself. If it is building wide you need to talk with your building manager.

Note that you should not need to bleed your radiators that often. Once every several years should be more than enough. If you are doing it frequently then you likely have a large leak in your system and likely want to get someone to check that out.