The abdomen is the part of the trunk between the thorax and the pelvis (Fig. 2.1). 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 musculo-aponeurotic 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 (the inferior margin of the thoracic skeleton superiorly and the pelvic girdle inferiorly) 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.
Figure 2.1. Overview of viscera of thorax and abdomen in situ.
Through voluntary or reflexive contraction, its muscular roof, anterolateral walls, and floor can raise internal (intra-abdominal) pressure to aid 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.
Overview: Walls, Cavities, Regions, and Planes
The dynamic, multi-layered, musculo-aponeurotic abdominal walls not only contract to increase intra-abdominal pressure, but also distend considerably, accommodating expansions caused by ingestion, pregnancy, fat deposition, or pathology.
The anterolateral abdominal wall and several organs lying against the posterior wall are covered on their internal aspects with a serous membrane or peritoneum (serosa) that reflects (turns sharply and continues) onto the abdominal viscera (L., soft parts, internal organs), such as the stomach, intestine, liver, and spleen. Thus, a bursal sac or lined potential space (peritoneal cavity) is formed between the walls and the viscera that normally contains only enough extracellular (parietal) fluid to lubricate the membrane covering most of the surfaces of the structures forming or occupying the abdominal cavity. Visceral movement associated with digestion occurs freely, and the double-layered reflections of peritoneum passing between the walls and the viscera provide passage for the blood vessels, lymphatics, and nerves. Variable amounts of fat may also occur between the walls and viscera and the peritoneum lining them.
The abdominal cavity:
- forms the superior and major part of the abdominopelvic cavity (Fig. 2.2), the continuous cavity that extends between the thoracic diaphragm and pelvic diaphragm.
- has no floor of its own because it is continuous with the pelvic cavity. The plane of the pelvic inlet (superior pelvic aperture) arbitrarily, but not physically, separates the abdominal and the pelvic cavities.
- extends superiorly into the osseocartilaginous thoracic cage to the 4th intercostal space (Fig. 2.1). Consequently, the more superiorly placed abdominal organs (spleen, liver, part of the kidneys, and stomach) are protected by the thoracic cage. The greater pelvis (expanded part of the pelvis superior to the pelvic inlet) supports and partly protects the lower abdominal viscera (part of the ileum, cecum, appendix, and sigmoid colon).
- is the location of most digestive organs, parts of the urogenital system (kidneys and most of the ureters), and the spleen.
Figure 2.2. Abdominopelvic cavity.
The body has been sectioned in the median plane to show the abdominal and pelvic cavities as subdivisions of the continuous abdominopelvic cavity.
Nine regions of the abdominal cavity are used to describe the location of abdominal organs, pains, or pathologies (Table 2.1A & B). The regions are delineated by four planes: two sagittal (vertical) and two transverse (horizontal) planes. The two sagittal planes are usually the midclavicular planes that pass from the midpoint of the clavicles (approximately 9 cm from the midline) to the midinguinal points, midpoints of the lines joining the anterior superior iliac spine (ASIS) and the pubic tubercles on each side.
Table 2.1. Abdominal Regions (A), Reference Planes (B), and Quadrants (C)
Most commonly, the transverse planes are the subcostal plane, passing through the inferior border of the 10th costal cartilage on each side, and the transtubercular plane, passing through the iliac tubercles (approximately 5 cm posterior to the ASIS on each side) and the body of the L5 vertebra. Both of these planes have the advantage of intersecting palpable structures.
Some clinicians use the transpyloric and interspinous planes to establish the nine regions. The transpyloric plane, extrapolated midway between the superior borders of the manubrium of the sternum and the pubic symphysis (typically the L1 vertebral level), commonly transects the pylorus (the distal, more tubular part of the stomach) when the patient is recumbent (supine or prone) (Fig. 2.1). Because the viscera sag with the pull of gravity, the pylorus usually lies at a lower level when the individual is standing erect. The transpyloric plane is a useful landmark because it also transects many other important structures: the fundus of the gallbladder, neck of the pancreas, origins of the superior mesenteric artery (SMA) and hepatic portal vein, root of the transverse mesocolon, duodenojejunal junction, and hila of the kidneys. The interspinous plane passes through the easily palpated ASIS on each side (Table 2.1B).
For more general clinical descriptions, four quadrants of the abdominal cavity (right and left upper and lower quadrants) are defined by two readily defined planes: (1) the transverse transumbilical plane, passing through the umbilicus (and the intervertebral [IV] disc between the L3 and L4 vertebrae), dividing it into upper and lower halves, and (2) the vertical median plane, passing longitudinally through the body, dividing it into right and left halves (Table 2.1C).
It is important to know what organs are located in each abdominal region or quadrant so that one knows where to auscultate, percuss, and palpate them (Table 2.1), and to record the locations of findings during a physical examination.