A self-regulating gastric band apparatus for adjusting stoma size. The apparatus includes an adjustable gastric band that has an inner ring expanding with injected fluid. A band adjustment assembly is provided for implanting with the gastric band that includes a sensor for sensing fluid pressure in the inner ring. The band adjustment assembly further includes a pump assembly connected to the expandable inner ring and to a controller that can operate the pump assembly to adjust the volume of the fluid in the band based on the sensed fluid pressure. The band adjustment assembly includes memory storing an operating range relative to a target fluid pressure, and the pump assembly is operated to maintain the sensed band pressure within the operating range. The target pressure being set to maintain pressure variations below a predefined variation limit generally corresponding with satiated fill volumes for a particular patient and implanted band. An elongated fluid reservoir may extend along a substantial part of a fill tube. A balloon-like expandable fluid reservoir in fluid communication with the pump assembly may store a volume of the fluid for adjusting the volume of fluid in the lumen. A protective outer sheath may be provided around the exterior of an expandable fluid reservoir in both a first, deflated state, and a second, inflated state of the reservoir.