Relationship between intra-abdominal volume, abdominal wall compliance and intra-abdominal pressure. Intra-abdominal volume (IAV) versus intra-abdominal pressure (IAP). The direction of the movement associated with the sole action of the rib cage inspiratory muscles, abdominal expiratory muscles and the diaphragm are shown. The direction of the latter depends on abdominal compliance (Cab) but is constrained within the sector shown. Reproduced with permission from .