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.
Material can be expelled from the airway by forced expiration using the abdominal muscles, as in coughing or sneezing.
Contraction of abdominal wall muscles can dramatically increase intraabdominal pressure when the diaphragm is in a fixed position (Fig. 4.4). Air is retained in the lungs by closing valves in the larynx in the neck. Increased intra-abdominal pressure assists in voiding the contents of the bladder and rectum and in giving birth.
The abdominal wall consists partly of bone but mainly of muscle (Fig. 4.5). The skeletal elements of the wall (Fig. 4.5A) are: the five lumbar vertebrae and their intervening intervertebral discs, the superior expanded parts of the pelvic bones, and bony components of the inferior thoracic wall, including the costal margin, rib XII, the end of rib XI, and the xiphoid process.
Muscles make up the rest of the abdominal wall (Fig. 4.5B):
Lateral to the vertebral column, the quadratus lumborum, psoas major, and iliacus muscles reinforce the posterior aspect of the wall. The distal ends of the psoas major and iliacus muscles pass into the thigh and are major flexors of the hip joint.
Lateral parts of the abdominal wall are predominantly formed by three layers of muscles, which are similar in orientation to the intercostal muscles of the thorax—transversus abdominis, internal oblique, and external oblique.
Anteriorly, a segmented muscle (the rectus abdominis) on each side spans the distance between the inferior thoracic wall and the pelvis.
Structural continuity between posterior, lateral, and anterior parts of the abdominal wall is provided by (aponeuroses) derived from muscles of the lateral wall. A fascial layer of varying thickness separates the abdominal wall from the peritoneum, which lines the abdominal cavity.
The general organization of the abdominal cavity is one in which a central gut tube (gastrointestinal system) is suspended from the posterior abdominal wall and partly from the anterior abdominal wall by thin sheets of tissue (mesenteries; Fig. 4.6): a ventral (anterior) mesentery for proximal regions of the gut tube; a dorsal (posterior) mesentery along the entire length of the system.
Different parts of these two mesenteries are named according to the organs they suspend or with which they are associated.
Major viscera, such as the kidneys, that are not suspended in the abdominal cavity by mesenteries are associated with the abdominal wall.
The abdominal cavity is lined by peritoneum, which consists of an epithelial-like single layer of cells (the mesothelium) together with a supportive layer of connective tissue. Peritoneum is similar to the pleura and serous pericardium in the thorax.
The peritoneum reflects off the abdominal wall to become a component of the mesenteries that suspend the viscera.
Parietal peritoneum lines the abdominal wall.
Visceral peritoneum covers suspended organs.
Normally, elements of the gastrointestinal tract and its derivatives completely fill the abdominal cavity, making the peritoneal cavity a potential space, and on the adjacent abdominal wall slide freely against one another.
Abdominal viscera are either intraperitoneal or retroperitoneal:
Intraperitoneal structures, such as elements of the gastrointestinal system, are suspended from the abdominal wall by mesenteries;
Structures that are not suspended in the abdominal cavity by a mesentery and that lie between the parietal peritoneum and abdominal wall are retroperitoneal in position.