Helium is in the contained in the main envelope.
Helium is seven times less dense then air, So therefore to ascend the pilot has to reduce the ship's weight. The only way to do this is to release air from the ballonets. So if you pump the air back in, the ship descend.
This picture shows an airship
An airship is the airborne equivalent of a submarine. Most of the airship's body is filled with Helium which is a light gas. This will reduce its weight bringing more upthrust from the air around the craft. The airship uses its buoyancy and propellers to ascend, and descend, and also to manoeuvre with great precision.