As the lungs and thorax move during ventilation, the movement and displacement of structures such as the lungs, abdominal organs, rib cage, and diaphragm create resistance to breathing. Tissue resistance remains constant under most circumstances. For example, an obese patient or one with fibrosis has increased tissue resistance, but it usually does not change significantly when these patients are mechanically ventilated. On the other hand, if a patients develops ascites, or fluid build-up in the peritoneal cavity, tissue resistance increases.