A Architecture and wall structure of the abdominal and pelvic cavities
Whereas the thoracic and abdominal cavities are separated by the diaphragm, the abdominal and pelvic cavities are continuous with each other. They are divided topographically by the linea terminalis. Thus, they form a single functional unit (see p. 2). Bones (vertebral column, thorax, and pelvis) as well as muscles (diaphragm, abdominal, and pelvic floor muscles) along with their fasciae and aponeuroses form the walls of this space. It is bounded by the following structures:
• Superiorly (see Ca): diaphragm with right and left domes and central tendon
• Inferiorly (see Cb): bony pelvis, muscles of the pelvic wall (iliacus, obturator internus, piriformis, and coccygeus) and pelvic floor muscles (mainly the levator ani forming most of the pelvic diaphragm)
• Posteriorly (see Cc): lumbar vertebral column, deep muscles of the abdominal wall (quadratus lumborum and psoas major), and intrinsic back muscles
• Anteriorly and laterally (see Cd): anterior and lateral muscles of the abdominal wall together with their aponeuroses (rectus abdominis and transversus abdominis as well as internal and external abdominal obliques)
B Functional aspects of abdominal and pelvic wall structure: abdominal press
Abdominal press is made possible by the structure of the abdominal and pelvic walls, which plays an important role in their elastic properties. “Abdominal press” describes the voluntary contraction of the diaphragm, and abdominal and pelvic muscles. When the muscles contract they reduce the volume of the abdominal cavity thereby significantly raising the intra-abdominal pressure: pressure in the standing position is approximately 1.7 kPa (2.75 mmHg), when lying down it is approximately 0.2 kPa (1.5 mmHg), and under strain including coughing or squeezing it is 10–20 kPa (75–150 mmHg).
Abdominal press is important in
• Emptying of the rectum (defecation), of the bladder (micturition), and of the stomach (vomiting),
• Uterine contractions during the expulsive phase of labor (“expulsive pains”),
• Stabilizing the spinal column (mainly the lumbar spine) and the trunk (the wall stiffens like the wall of an inflated ball), for example when lifting heavy loads, but also in the standing posture (hydrostatic effect of abdominal press).
Hernias occur when the pressure load is greater than the strength of the complex myofascial network. They develop either in the anterior abdominal wall or more commonly in the groin region because the weight of the pelvic and abdominal organs puts increasing strain on the wall structures, which increases from superior to inferior. Additionally, the pelvic floor muscles in particular are much less able to withstand the increased abdominal pressure than the abdominal wall muscles or the diaphragm. During abdominal press, closure of the glottis and the retaining of air in the lungs gives support to the diaphragm; there is no such compensatory mechanism in the pelvic floor muscles making it a characteristic weak spot. After excessive stretching (e.g., caused by vaginal delivery) the pelvic floor is unable to maintain the pelvic organs in their normal position (pelvic floor descent) and provides inadequate support for the abdominal press. The results are urinary and fecal incontinence.
C Boundaries of the abdominal and pelvic cavities
a Diaphragm, inferior view; b Pelvic floor, superior view; c (d) Anterior (posterior) view of the posterior (anterior) trunk wall.
A Midsagittal section through the abdomen and pelvis, viewed from the left side