The abdomen is the part of the trunk between the thorax and the pelvis. It is a flexible, dynamic container, housing most of the organs of the alimentary system and part of the urogenital system. Containment of the abdominal organs and their contents is provided by musculoaponeurotic walls anterolaterally, the diaphragm superiorly, and the muscles of the pelvis inferiorly.
The anterolateral musculo-aponeurotic walls are suspended between and supported by two bony rings linked by a semirigid lumbar vertebral column in the posterior abdominal wall. Interposed between the more rigid thorax and pelvis, this arrangement enables the abdomen to enclose and protect its contents while providing the flexibility required by respiration, posture, and locomotion. Through voluntary or reflexive contraction, its muscular roof, anterolateral walls, and floor can raise internal (intraabdominal) pressure to aid expulsion from the abdominopelvic cavity or from the adjacent thoracic cavity, expulsion of air from the thoracic cavity (lungs and bronchi) or of fluid (e.g., urine or vomitus), flatus, feces, or fetuses from the abdominopelvic cavity.