Viscera not under the domes of the diaphragm are supported and protected predominantly by the muscular walls of the abdomen.
One of the most important roles of the abdominal wall is to assist in breathing:
It relaxes during inspiration to accommodate expansion of the thoracic cavity and the inferior displacement of abdominal viscera during contraction of the diaphragm (Fig. 4.3).
During expiration, it contracts to assist in elevating the domes of the diaphragm, thus reducing thoracic volume.