The diaphragm is a double-domed, musculotendinous partition separating the thoracic and abdominal cavities. Its mainly convex superior surface faces the thoracic cavity, and its concave inferior surface faces the abdominal cavity (Fig. 2.91A & B). The diaphragm is the chief muscle of inspiration (actually, of respiration altogether, because expiration is largely passive). It descends during inspiration; however, only its central part moves because its periphery, as the fixed origin of the muscle, attaches to the inferior margin of the thoracic cage and the superior lumbar vertebrae.
Figure 2.91. Attachments, disposition, and features of the abdominal aspect of the diaphragm.
A.The thoracic wall and cage have been removed to demonstrate the attachments and convexity of the right dome of the diaphragm. B.The fleshy sternal, costal, and lumbar parts of the diaphragm (outlined with broken lines) attach centrally to the trefoil-shaped central tendon, the aponeurotic insertion of the diaphragmatic muscle fibers.
The pericardium, containing the heart, lies on the central part of the diaphragm, depressing it slightly (Fig. 2.92A). The diaphragm curves superiorly into right and left domes; normally the right dome is higher than the left dome owing to the presence of the liver. During expiration, the right dome reaches as high as the 5th rib and the left dome ascends to the 5th intercostal space. The level of the domes of the diaphragm varies according to the:
- Phase of respiration (inspiration or expiration).
- Posture (e.g., supine or standing).
- Size and degree of distension of the abdominal viscera.
Figure 2.92. Blood vessels of diaphragm.
A.The arteries and veins of the superior surface of the diaphragm. B.The arteries and veins of the inferior surface of the diaphragm.
The muscular part of the diaphragm is situated peripherally with fibers that converge radially on the trifoliate central aponeurotic part, the central tendon (see Fig. 2.91). The central tendon has no bony attachments and is incompletely divided into three leaves, resembling a wide cloverleaf (Fig. 2.91B). Although it lies near the center of the diaphragm, the central tendon is closer to the anterior part of the thorax.
The caval opening (vena caval foramen), through which the terminal part of the IVC passes to enter the heart, perforates the central tendon. The surrounding muscular part of the diaphragm forms a continuous sheet; however, for descriptive purposes it is divided into three parts, based on the peripheral attachments:
- Sternal part: consisting of two muscular slips that attach to the posterior aspect of the xiphoid process; this part is not always present.
- Costal part: consisting of wide muscular slips that attach to the internal surfaces of the inferior six costal cartilages and their adjoining ribs on each side; the costal parts form the right and left domes.
- Lumbar part: arising from two aponeurotic arches, the medial and lateral arcuate ligaments, and the three superior lumbar vertebrae; the lumbar part forms right and left muscular crura that ascend to the central tendon.
The crura of the diaphragm are musculotendinous bands that arise from the anterior surfaces of the bodies of the superior three lumbar vertebrae, the anterior longitudinal ligament, and the IV discs. The right crus, larger and longer than the left crus, arises from the first three or four lumbar vertebrae. The left crus arises from the first two or three lumbar vertebrae. Because it lies to the left of the midline, it is surprising to find that the esophageal hiatus is a formation in the right crus; however, if the muscular fibers bounding each side of the hiatus are traced inferiorly, it will be seen that they pass to the right of the aortic hiatus.
The right and left crura and the fibrous median arcuate ligament, which unites them as it arches over the anterior aspect of the aorta, form the aortic hiatus. The diaphragm is also attached on each side to the medial and lateral arcuate ligaments. The medial arcuate ligament is a thickening of the fascia covering the psoas major, spanning between the lumbar vertebral bodies and the tip of the transverse process of L1. The lateral arcuate ligament covers the quadratus lumborum muscles, continuing from the L12 transverse process to the tip of the 12th rib.
The superior aspect of the central tendon of the diaphragm is fused with the inferior surface of the fibrous pericardium, the strong, external part of the fibroserous pericardial sac that encloses the heart.
Vessels and Nerves of Diaphragm
The arteries of the diaphragm form a branch-like pattern on both its superior (thoracic) and inferior (abdominal) surfaces. The arteries supplying the superior surface of the diaphragm (Fig. 2.92; Table 2.13) are the pericardiacophrenic and musculophrenic arteries, branches of the internal thoracic artery, and the superior phrenic arteries, arising from the thoracic aorta. The arteries supplying the inferior surface of the diaphragm are the inferior phrenic arteries, which typically are the first branches of the abdominal aorta; however, they may arise from the celiac trunk.
Table 2.13. Neurovascular Structures of Diaphragm
|Vessels and Nerves||Superior Surface of Diaphragm||Inferior Surface of Diaphragm|
|Arterial supply||Superior phrenic arteries from thoracic aorta
Musculophrenic and pericardiophrenic arteries from internal thoracic arteries
|Inferior phrenic arteries from abdominal aorta|
|Venous drainage||Musculophrenic and pericardiacophrenic veins drain into internal thoracic veins; superior phrenic vein (right side) drains into IVC||Inferior phrenic veins; right vein drains into IVC; left vein is doubled and drains into IVC and suprarenal vein|
|Lymphatic drainage||Diaphragmatic lymph nodes to phrenic nodes, then to parasternal and posterior mediastinal nodes||Superior lumbar lymph nodes; lymphatic plexuses on superior and inferior surfaces communicate freely|
|Innervation||Motor supply: phrenic nerves (C3–C5)
Sensory supply: centrally by phrenic nerves (C3–C5), peripherally by intercostal nerves (T5–T11) and subcostal nerves (T12)
The veins draining the superior surface of the diaphragm are the pericardiacophrenic and musculophrenic veins, which empty into the internal thoracic veins and, on the right side, a superior phrenic vein, which drains into the IVC. Some veins from the posterior curvature of the diaphragm drain into the azygos and hemi-azygos veins (see Chapter 1). The veins draining the inferior surface of the diaphragm are the inferior phrenic veins. The right inferior phrenic vein usually opens into the IVC, whereas the left inferior phrenic vein is usually double, with one branch passing anterior to the esophageal hiatus to end in the IVC and the other, more posterior branch usually joining the left suprarenal vein. The right and left phrenic veins may anastomose with each other.
The lymphatic plexuses on the superior and inferior surfaces of the diaphragm communicate freely (Fig. 2.93A). The anterior and posterior diaphragmatic lymph nodes are on the superior surface of the diaphragm. Lymph from these nodes drains into the parasternal, posterior mediastinal, and phrenic lymph nodes. Lymphatic vessels from the inferior surface of the diaphragm drain into the anterior diaphragmatic, phrenic, and superior lumbar (caval/aortic) lymph nodes. Lymphatic capillaries are dense on the inferior surface of the diaphragm, constituting the primary means for absorption of peritoneal fluid and substances introduced by intraperitoneal (I.P.) injection.
Figure 2.93. Lymphatic vessels, lymph nodes, and nerves of diaphragm.
A.Lymphatic vessels are formed in two plexuses, one on the superior surface of the diaphragm and the other on its inferior surface; the plexuses communicate freely. B.The phrenic nerves supply all of the motor and most of the sensory innervation to the diaphragm. The lower six or seven intercostal and subcostal nerves provide sensory innervation peripherally.
The entire motor supply to the diaphragm is from the right and left phrenic nerves, each of which arises from the anterior rami of C3–C5 segments of the spinal cord and is distributed to the ipsilateral half of the diaphragm from its inferior surface (Fig. 2.93B). Sensory innervation (pain and proprioception) to the diaphragm is also mostly from the phrenic nerves. Peripheral parts of the diaphragm receive their sensory nerve supply from the intercostal nerves (lower six or seven) and the subcostal nerves.
The diaphragmatic apertures (openings, hiatus) permit structures (vessels, nerves, and lymphatics) to pass between the thorax and abdomen (Figs. 2.91, 2.92, and 2.94). There are three large apertures for the IVC, esophagus, and aorta and a number of small ones.
Figure 2.94. Apertures of diaphragm.
“8-10-12” is a convenient memory device, referring to the thoracic vertebral levels at which the inferior vena cava, esophagus, and aorta penetrate the diaphragm.
The caval opening is an aperture in the central tendon primarily for the IVC. Also passing through the caval opening are terminal branches of the right phrenic nerve and a few lymphatic vessels on their way from the liver to the middle phrenic and mediastinal lymph nodes. The caval opening is located to the right of the median plane at the junction of the central tendon’s right and middle leaves. The most superior of the three large diaphragmatic apertures, the caval opening lies at the level of the IV disc between the T8 and T9 vertebrae. The IVC is adherent to the margin of the opening; consequently, when the diaphragm contracts during inspiration, it widens the opening and dilates the IVC. These changes facilitate blood flow through this large vein to the heart.
The esophageal hiatus is an oval opening for the esophagus in the muscle of the right crus of the diaphragm at the level of the T10 vertebra. The esophageal hiatus also transmits the anterior and posterior vagal trunks, esophageal branches of the left gastric vessels, and a few lymphatic vessels. The fibers of the right crus of the diaphragm decussate (cross one another) inferior to the hiatus, forming a muscular sphincter for the esophagus that constricts it when the diaphragm contracts. The esophageal hiatus is superior to and to the left of the aortic hiatus. In most individuals (70%), both margins of the hiatus are formed by muscular bundles of the right crus. In others (30%), a superficial muscular bundle from the left crus contributes to the formation of the right margin of the hiatus.
The aortic hiatus is the opening posterior in the diaphragm for the descending aorta. Because the aorta does not pierce the diaphragm, movements of the diaphragm do not affect blood flow through the aorta during respiration. The aorta passes between the crura of the diaphragm posterior to the median arcuate ligament, which is at the level of the inferior border of the T12 vertebra. The aortic hiatus also transmits the thoracic duct and sometimes the azygos and hemi-azygos veins.
Small Openings in Diaphragm
In addition to the three main apertures, there is a small opening, the sternocostal triangle (foramen), between the sternal and costal attachments of the diaphragm. This triangle transmits lymphatic vessels from the diaphragmatic surface of the liver and the superior epigastric vessels. The sympathetic trunks pass deep to the medial arcuate ligament, accompanied by the least splanchnic nerves. There are two small apertures in each crus of the diaphragm; one transmits the greater splanchnic nerve and the other the lesser splanchnic nerve.
Actions of Diaphragm
When the diaphragm contracts, its domes are pulled inferiorly so that the convexity of the diaphragm is somewhat flattened. Although this movement is often described as the “descent of the diaphragm,” only the domes of the diaphragm descend. The diaphragm’s periphery remains attached to the ribs and cartilages of the inferior six ribs. As the diaphragm descends, it pushes the abdominal viscera inferiorly. This increases the volume of the thoracic cavity and decreases the intrathoracic pressure, resulting in air being taken into the lungs. In addition, the volume of the abdominal cavity decreases slightly and intra-abdominal pressure increases somewhat.
Movements of the diaphragm are also important in circulation because the increased intra-abdominal pressure and decreased intrathoracic pressure help return venous blood to the heart. When the diaphragm contracts, compressing the abdominal viscera, blood in the IVC is forced superiorly into the heart.
The diaphragm is at its most superior level when a person is supine (with the upper body lowered, the Trendelenburg position). In this position, the abdominal viscera push the diaphragm superiorly in the thoracic cavity. When a person lies on one side, the hemidiaphragm rises to a more superior level because of the greater push of the viscera on that side. Conversely, the diaphragm assumes an inferior level when a person is sitting or standing. For this reason, people with dyspnea (difficult breathing) prefer to sit up, not lie down; non-tidal (reserve) lung volume is increased, and the diaphragm is working with gravity rather than opposing it.
Posterior Abdominal Wall
The posterior abdominal wall (Figs. 2.95,2.96–2.97) is mainly composed of the:
- Five lumbar vertebrae and associated IV discs (centrally).
- Posterior abdominal wall muscles, including the psoas, quadratus lumborum, iliacus, transversus abdominis, and oblique muscles (laterally).
- Diaphragm, which contributes to the superior part of the posterior wall.
- Fascia, including the thoracolumbar fascia.
- Lumbar plexus, composed of the anterior rami of lumbar spinal nerves.
- Fat, nerves, vessels (e.g., aorta and IVC), and lymph nodes.
Figure 2.95. Fascia and aponeuroses of abdominal wall at level of renal hila.
A.The relationships of the muscles, aponeurotic muscle sheaths, and fascia of the abdominal wall are demonstrated in transverse section. The three flat abdominal muscles forming the lateral walls span between complex anterior and posterior aponeurotic formations that ensheath vertically disposed muscles. The thin anterolateral walls (appearing disproportionately thick here) are distensible. Although flexible, the posterior abdominal wall is weight bearing and so is reinforced by the vertebral column and muscles that act on it; thus it is not distensible. IVC, inferior vena cava. B.Details of the disposition of the aponeurotic and fascial layers of the posterior abdominal wall. For details concerning those of the anterior abdominal wall, see Figure 2.5B. C.Dimensional view of the region demonstrated in section in B.
Figure 2.96. Muscles of posterior abdominal wall.
Figure 2.97. Muscles and nerves of posterior abdominal wall.
Most of the right psoas major has been removed to show that the lumbar plexus of nerves is formed by the anterior rami of the first four lumbar spinal nerves and that it lies in the substance of the psoas major.
If observing the anatomy of the posterior abdominal wall in only two-dimensional diagrams, such as Figure 2.97, it would be easy to suppose that it is flat. In observing a dissected cadaver or a transverse cross section such as that in Figures 2.95A & B, it is apparent that the lumbar vertebral column is a marked central prominence in the posterior wall, creating two paravertebral “gutters” on each side. The deepest (most posterior) part of these gutters is occupied by the kidneys and their surrounding fat. The abdominal aorta lies on the anterior aspect of the anteriorly protruding vertebral column. It is usually surprising to find how close the lower abdominal aorta lies to the anterior abdominal wall in lean individuals (see Fig. B2.37C). Of course, many structures lie anterior to the aorta (SMA, parts of the duodenum, pancreas and left renal vein, etc.) and so these “posterior abdominal structures” may approach the anterior abdominal wall closer than might be expected in thin persons, especially when they are in the supine position.
Fascia of Posterior Abdominal Wall
The posterior abdominal wall is covered with a continuous layer of endoabdominal fascia that lies between the parietal peritoneum and the muscles (Fig. 2.95B). The fascia lining the posterior abdominal wall is continuous with the transversalis fascia that lines the transversus abdominis muscle. It is customary to name the fascia according to the structure it covers.
The psoas fascia covering the psoas major muscle (psoas sheath) is attached medially to the lumbar vertebrae and pelvic brim. The psoas fascia (sheath) is thickened superiorly to form the medial arcuate ligament (Fig. 2.91). The psoas fascia fuses laterally with the quadratus lumborum and thoracolumbar fascias (Fig. 2.95B). Inferior to the iliac crest, the psoas fascia is continuous with the part of the iliac fascia covering the iliacus.
The thoracolumbar fascia is an extensive fascial complex attached to the vertebral column medially that, in the lumbar region, has posterior, middle, and anterior layers with muscles enclosed between them (Fig. 2.95B & C). It is thin and transparent where it covers the thoracic parts of the deep muscles, but it is thick and strong in the lumbar region. The enclosure of the vertical deep back muscles (erector spinae) by the posterior and middle layers of the thoracolumbar fascia on the posterior aspect of the trunk is comparable to the enclosure of the rectus abdominis by the rectus sheath on the anterior aspect (Fig. 2.95A). This posterior sheath is even more formidable than the rectus sheath, however, because of the thickness of its posterior layer and the central attachment to the lumbar vertebrae, as opposed to the rectus sheaths, which lack bony support where they fuse to each other at the linea alba. The lumbar part of this posterior sheath, extending between the 12th rib and the iliac crest, attaches laterally to the internal oblique and transversus abdominis muscles, as does the rectus sheath. However, in contrast to the rectus sheath, the thoracolumbar fascia is not attached to the external oblique; it is attached to the latissimus dorsi (Fig. 2.95B & C).
The anterior layer of the thoracolumbar fascia (quadratus lumborum fascia), covering the anterior surface of the quadratus lumborum—a thinner, more transparent layer than the other two layers—attaches to the anterior surfaces of the transverse processes of the lumbar vertebrae, the iliac crest, and the 12th rib (Figs. 2.95B and 2.97). The anterior layer is continuous laterally with the aponeurotic origin of the transversus abdominis muscle. It thickens superiorly to form the lateral arcuate ligament and is adherent inferiorly to the iliolumbar ligaments (Fig. 2.97).
Muscles of Posterior Abdominal Wall
The main paired muscles in the posterior abdominal wall (Fig. 2.96; Table 2.14) are the:
- Psoas major: passing inferolaterally.
- Iliacus: lying along the lateral sides of the inferior part of the psoas major.
- Quadratus lumborum: lying adjacent to the transverse processes of the lumbar vertebrae and lateral to superior parts of the psoas major.
Table 2.14. Muscles of Posterior Abdominal Wall
|Muscle||Superior Attachment||Inferior Attachment||Innervation||Main Action|
|Psoas major*||Transverse processes of lumbar vertebrae; sides of bodies of T12–L5 vertebrae and intervening intervertebral discs||By a strong tendon to lesser trochanter of femur||Anterior rami of lumbar nerves L1, L2, L3||Acting inferiorly with iliacus, it flexes thigh; acting superiorly it flexes vertebral column laterally; it is used to balance the trunk; when sitting it acts inferiorly with iliacus to flex trunk|
|Iliacus*||Superior two thirds of iliac fossa, ala of sacrum, and anterior sacro-iliac ligaments||Lesser trochanter of femur and shaft inferior to it, and to psoas major tendon||Femoral nerve (L2–L4)||Flexes thigh and stabilizes hip joint; acts with psoas major|
|Quadratus lumborum||Medial half of inferior border of 12th ribs and tips of lumbar transverse processes||Iliolumbar ligament and internal lip of iliac crest||Anterior branches of T12 and L1–L4 nerves||Extends and laterally flexes vertebral column; fixes 12th rib during inspiration|
The attachments, nerve supply, and main actions of these muscles are summarized in Table 2.14.
The long, thick, fusiform psoas major lies lateral to the lumbar vertebrae (Figs. 2.96A and 2.97). Psoas is a Greek word meaning “muscle of the loin.” (Butchers refer to the psoas of animals as the tenderloin.) The psoas major passes inferolaterally, deep to the inguinal ligament to reach the lesser trochanter of the femur. The lumbar plexus of nerves is embedded in the posterior part of the psoas major, anterior to the lumbar transverse processes.
The iliacus is a large triangular muscle that lies along the lateral side of the inferior part of the psoas major. Most of its fibers join the tendon of the psoas major. Together the psoas and iliacus form the iliopsoas, the chief flexor of the thigh. It is also a stabilizer of the hip joint and helps maintain the erect posture at this joint. The psoas and iliacus share in hip flexion; however, only the psoas can produce movement (flexion or lateral bending) of the lumbar vertebral column.
The quadrilateral quadratus lumborum forms a thick muscular sheet in the posterior abdominal wall (Figs. 2.94A & B, 2.96, and 2.97). It lies adjacent to the lumbar transverse processes and is broader inferiorly. Close to the 12th rib, the lateral arcuate ligament crosses the quadratus lumborum. The subcostal nerve passes posterior to this ligament and runs inferolaterally on the quadratus lumborum. Branches of the lumbar plexus run inferiorly on the anterior surface of this muscle.
Nerves of Posterior Abdominal Wall
Components of both the somatic and autonomic (visceral) nervous systems are associated with the posterior abdominal wall.
The subcostal nerves (anterior rami of T12) arise in the thorax, pass posterior to the lateral arcuate ligaments into the abdomen, and run inferolaterally on the anterior surface of the quadratus lumborum (Fig. 2.97). They pass through the transversus abdominis and internal oblique muscles to supply the external oblique and skin of the anterolateral abdominal wall.
The lumbar spinal nerves (L1–L5) pass from the spinal cord through the IV foramina inferior to the corresponding vertebrae, where they divide into posterior and anterior rami. Each ramus contains sensory and motor fibers. The posterior rami pass posteriorly to supply the muscles of the back and overlying skin, whereas the anterior rami pass laterally and inferiorly, to supply the skin and muscles of the inferiormost trunk and lower limb. The initial portions of the anterior rami of the L1, L2, and occasionally L3 spinal nerves give rise to white communicating branches (L. rami communicantes), which convey presynaptic sympathetic fibers to the lumbar sympathetic trunks.
The abdominal part of the sympathetic trunks (lumbar sympathetic trunks), consisting of four lumbar paravertebral sympathetic ganglia and the interganglionic branches that connect them, are continuous with the thoracic part of the trunks deep to the medial arcuate ligaments of the diaphragm. The lumbar trunks descend on the anterolateral aspects of the bodies of the lumbar vertebrae in a groove formed by the adjacent psoas major. Inferiorly, they cross the sacral promontory and continue inferiorly into the pelvis as the sacral part of the trunks.
For the innervation of the abdominal wall and lower limbs, synapses between the presynaptic and postsynaptic fibers occur in the sympathetic trunks. Postsynaptic sympathetic fibers travel from the lateral aspect of the trunks via gray communicating branches to the anterior rami. They become the thoraco-abdominal and subcostal nerves, and the lumbar plexus (somatic nerves) that stimulate vasomotion, sudomotion, and pilomotion in the lowermost trunk and lower limb. Lumbar splanchnic nerves arising from the medial aspect of the lumbar sympathetic trunks convey presynaptic sympathetic fibers for the innervation of pelvic viscera.
The lumbar plexus of nerves is formed anterior to the lumbar transverse processes, within the proximal attachment of the psoas major. This nerve network is composed of the anterior rami of L1 through L4 nerves. The following nerves are branches of the lumbar plexus; the three largest are listed first:
- The femoral nerve (L2–L4) emerges from the lateral border of the psoas major and innervates the iliacus and passes deep to the inguinal ligament/iliopubic tract to the anterior thigh, supplying the flexors of the hip and extensors of the knee.
- The obturator nerve (L2–L4) emerges from the medial border of the psoas major and passes into the lesser pelvis, passing inferior to the superior pubic ramus (through the obturator foramen) to the medial thigh, supplying the adductor muscles.
- The lumbosacral trunk (L4, L5) passes over the ala (wing) of the sacrum and descends into the pelvis to participate in the formation of the sacral plexus with the anterior rami of S1–S4 nerves.
- The ilio-inguinal and iliohypogastric nerves (L1) arise from the anterior ramus of L1, entering the abdomen posterior to the medial arcuate ligament and passing inferolaterally, anterior to the quadratus lumborum. They run superior and parallel to the iliac crest, piercing the transversus abdominis near the ASIS. They then pass through the internal and external obliques to supply the abdominal muscles and skin of the inguinal and pubic regions. The division of the L1 anterior ramus may occur as far distally as the ASIS, so that often only one nerve (L1) crosses the posterior abdominal wall instead of two.
- The genitofemoral nerve (L1, L2) pierces the psoas major and runs inferiorly on its anterior surface, deep to the psoas fascia; it divides lateral to the common and external iliac arteries into femoral and genital branches.
- The lateral cutaneous nerve of the thigh, or lateral femoral cutaneous nerve (L2, L3), runs inferolaterally on the iliacus and enters the thigh deep to the inguinal ligament/iliopubic tract, just medial to the ASIS; it supplies skin on the anterolateral surface of the thigh.
- An accessory obturator nerve (L3, L4) is present almost 10% of the time. It parallels the medial border of the psoas, anterior to the obturator nerve, crossing superior to the superior pubic ramus in close proximity to the femoral vein
Although the larger branches (femoral, obturator, and lumbosacral trunk) are consistent in their placement, variation should be anticipated in the disposition of the smaller branches of the lumbar plexus.
Vessels of Posterior Abdominal Wall
The major neurovascular bundle of the inferior trunk, including the abdominal aorta, the inferior vena cava, and the aortic peri-arterial nerve plexus, courses in the midline of the posterior abdominal wall, anterior to the bodies of the lumbar vertebrae (see Figs. 2.70B and 2.89).
Most arteries supplying the posterior abdominal wall arise from the abdominal aorta (Fig. 2.98A; Table 2.15). The subcostal arteries arise from the thoracic aorta and distribute inferior to the 12th rib. The abdominal aorta is approximately 13 cm in length. It begins at the aortic hiatus in the diaphragm at the level of the T12 vertebra and ends at the level of the L4 vertebra by dividing into the right and left common iliac arteries. The abdominal aorta may be represented on the anterior abdominal wall by a band (approximately 2 cm wide) extending from a median point, approximately 2.5 cm superior to the transpyloric plane to a point slightly (2–3 cm) inferior to and to the left of the umbilicus at the level of the supracristal plane (plane of the highest points of the iliac crests) (Fig. 2.98B). In children and lean adults, the lower abdominal aorta is sufficiently close to the anterior abdominal wall that its pulsations may be detected or apparent when the wall is relaxed (see the blue box “Pulsations of Aorta and Abdominal Aortic Aneurysm”).
Figure 2.98.Arteries of posterior abdominal wall—branches of the aorta.
A.Branches of abdominal aorta. B.Branches of upper abdominal aorta. C.Vascular planes in which branches of abdominal aorta are distributed.
Table 2.15.Branches of Abdominal Aorta
|Vascular Plane||Class||Distribution||Abdominal Branches (Arteries)||Vertebral Level|
|1. Anterior midline||Unpaired visceral||Digestive tract||Celiac||T12|
|2. Lateral||Paired visceral||Urogenital and endocrine organs||Suprarenal||L1|
|Gonadal (testicular or ovarian)||L2|
|3. Posterolateral||Paired parietal (segmental)||Diaphragm; body wall||Subcostal||L2|
The common iliac arteries diverge and run inferolaterally, following the medial border of the psoas muscles to the pelvic brim. Here each common iliac artery divides into the internal and external iliac arteries. The internal iliac artery enters the pelvis. (Its course and branches are described in Chapter 3.) The external iliac artery follows the iliopsoas muscle. Just before leaving the abdomen, the external iliac artery gives rise to the inferior epigastric and deep circumflex iliac arteries, which supply the anterolateral abdominal wall.
Relations of Abdominal Aorta
From superior to inferior, the important anterior relations of the abdominal aorta are the:
- Celiac plexus and ganglion (see Figs. 2.55B and 2.71).
- Body of the pancreas and splenic vein (see Fig. 2.71)
- Left renal vein (see Figs. 2.83 and 2.92B).
- Horizontal part of the duodenum.
- Coils of small intestine.
The abdominal aorta descends anterior to the bodies of the T12–L4 vertebrae (Fig. 2.98A). The left lumbar veins pass posterior to the aorta to reach the IVC (Fig. 2.99). On the right, the aorta is related to the azygos vein, cisterna chyi, thoracic duct, right crus of the diaphragm, and right celiac ganglion. On the left, the aorta is related to the left crus of the diaphragm and the left celiac ganglion.
Figure 2.99.Inferior vena cava and its tributaries.
The asymmetry in the renal and common iliac veins reflects the placement of the IVC to the right of the midline.
Branches of the Abdominal Aorta
The branches of the descending (thoracic and abdominal) aorta may be described as arising and coursing in three “vascular planes” and can be classified as being visceral or parietal and paired or unpaired (Fig. 2.98A & C; Table 2.15). Paired parietal branches of the aorta serve the diaphragm and posterior abdominal wall.
The median sacral artery, an unpaired parietal branch, may be said to occupy a fourth (posterior) plane because it arises from the posterior aspect of the aorta just proximal to its bifurcation. Although markedly smaller, it could also be considered a midline “continuation” of the aorta, in which case its lateral branches, the small lumbar arteries and lateral sacral branches, would also be included as part of the paired parietal branches.
Veins of Posterior Abdominal Wall
The veins of the posterior abdominal wall are tributaries of the IVC, except for the left testicular or ovarian vein, which enters the left renal vein instead of entering the IVC (Fig. 2.99). The IVC, the largest vein in the body, has no valves except for a variable, non-functional one at its orifice in the right atrium of the heart. The IVC returns poorly oxygenated blood from the lower limbs, most of the back, the abdominal walls, and the abdominopelvic viscera. Blood from the abdominal viscera passes through the portal venous system and the liver before entering the IVC via the hepatic veins.
The inferior vena cava (IVC) begins anterior to the L5 vertebra by the union of the common iliac veins. The union occurs approximately 2.5 cm to the right of the median plane, inferior to the aortic bifurcation and posterior to the proximal part of the right common iliac artery (see Fig. 2.76). The IVC ascends on the right side of the bodies of the L3–L5 vertebrae and on the right psoas major to the right of the aorta. The IVC leaves the abdomen by passing through the caval opening in the diaphragm and enters the thorax at the T8 vertebral level. Because it is formed one vertebral level inferior to the aortic bifurcation, and traverses the diaphragm four vertebral levels superior to the aortic hiatus, the overall length of the IVC is 7 cm greater than that of the abdominal aorta, although most of the additional length is intrahepatic. The IVC collects poorly oxygenated blood from the lower limbs and non-portal blood from the abdomen and pelvis. Almost all the blood from the gastrointestinal tract is collected by the hepatic portal system and passes through the hepatic veins to the IVC.
The tributaries of the IVC correspond to the paired visceral and parietal branches of the abdominal aorta. The veins that correspond to the unpaired visceral branches of the aorta are instead tributaries of the hepatic portal vein. The blood they carry does ultimately enter the IVC via the hepatic veins, after traversing the liver.
The branches corresponding to the paired visceral branches of the abdominal aorta include the right suprarenal vein, the right and left renal veins, and the right gonadal (testicular or ovarian) vein. The left suprarenal and gonadal veins drain indirectly into the IVC because they are tributaries of the left renal vein.
Paired parietal branches of the IVC include the inferior phrenic veins, the 3rd (L3) and 4th (L4) lumbar veins, and the common iliac veins. The ascending lumbar and azygos veins connect the IVC and SVC, either directly or indirectly providing collateral pathways (see the blue box “Collateral Routes for Abdominopelvic Venous Blood”).
Lymphatic Vessels and Lymph Nodes of Posterior Abdominal Wall
Lymphatic vessels and lymph nodes lie along the aorta, IVC, and iliac vessels (Fig. 2.100A). The common iliac lymph nodes receive lymph from the external and internal iliac lymph nodes. Lymph from the common iliac lymph nodes passes to the right and left lumbar lymph nodes. Lymph from the alimentary tract, liver, spleen, and pancreas passes along the celiac and superior and inferior mesenteric arteries to the pre-aortic lymph nodes (celiac and superior and inferior mesenteric nodes) scattered around the origins of these arteries from the aorta. Efferent vessels from these nodes form the intestinal lymphatic trunks, which may be single or multiple, and participate in the confluence of lymphatic trunks that gives rise to the thoracic duct (Fig. 2.100B).
Figure 2.100.Lymphatic vessels and lymph nodes of posterior abdominal wall and lymphatic trunks of abdomen.
A.The parietal lymph nodes are shown. B.The abdominal lymphatic trunks are shown. All lymphatic drainage from the lower half of the body converges in the abdomen to enter the beginning of the thoracic duct.
The right and left lumbar (caval and aortic) lymph nodes lie on both sides of the IVC and aorta. These nodes receive lymph directly from the posterior abdominal wall, kidneys, ureters, testes or ovaries, uterus, and uterine tubes. They also receive lymph from the descending colon, pelvis, and lower limbs through the inferior mesenteric and common iliac lymph nodes. Efferent lymphatic vessels from the large lumbar lymph nodes form the right and left lumbar lymphatic trunks.
The inferior end of the thoracic duct lies anterior to the bodies of the L1 and L2 vertebrae between the right crus of the diaphragm and the aorta. The thoracic duct begins with the convergence of the main lymphatic ducts of the abdomen, which in only a small proportion of individuals takes the form of the commonly depicted, thin-walled sac or dilation, the cisterna chyli (chyle cistern) (Fig. 2.100B). Cisterna chyli vary greatly in size and shape. More often there is merely a simple or plexiform convergence at this level of the right and left lumbar lymphatic trunks, the intestinal lymph trunk(s), and a pair of descending thoracic lymphatic trunks, which carry lymph from the lower six intercostal spaces on each side. Consequently, essentially all the lymphatic drainage from the lower half of the body (deep lymphatic drainage inferior to the level of the diaphragm and all superficial drainage inferior to the level of the umbilicus) converges in the abdomen to enter the beginning of the thoracic duct.
The thoracic duct ascends through the aortic hiatus in the diaphragm into the posterior mediastinum, where it collects more parietal and visceral drainage, particularly from the left upper quadrant of the body. The duct ultimately ends by entering the venous system at the junction of the left subclavian and internal jugular veins (the left venous angle).
Hiccups (hiccoughs) are involuntary, spasmodic contractions of the diaphragm, causing sudden inhalations that are rapidly interrupted by spasmodic closure of the glottis (aperture of the larynx) that checks the inflow of air and produces the characteristic sound. Hiccups result from irritation of afferent or efferent nerve endings, or of medullary centers in the brainstem that control the muscles of respiration, particularly the diaphragm. Hiccups have many causes, such as indigestion, diaphragm irritation, alcoholism, cerebral lesions, and thoracic and abdominal lesions, all which disturb the phrenic nerves.
Section of a Phrenic Nerve
Section of a phrenic nerve in the neck results in complete paralysis and eventual atrophy of the muscular part of the corresponding half of the diaphragm, except in persons who have an accessory phrenic nerve (see Chapter 8). Paralysis of a hemidiaphragm can be recognized radiographically by its permanent elevation and paradoxical movement. See the blue box “Paralysis of Diaphragm,”
Referred Pain from Diaphragm
Pain from the diaphragm radiates to two different areas because of the difference in the sensory nerve supply of the diaphragm (Table 2.12). Pain resulting from irritation of the diaphragmatic pleura or the diaphragmatic peritoneum is referred to the shoulder region, the area of skin supplied by the C3–C5 segments of the spinal cord (see the blue box “Visceral Referred Pain,”). These segments also contribute anterior rami to the phrenic nerves. Irritation of peripheral regions of the diaphragm, innervated by the inferior intercostal nerves, is more localized, being referred to the skin over the costal margins of the anterolateral abdominal wall.
Rupture of Diaphragm and Herniation of Viscera
Rupture of the diaphragm and herniation of viscera can result from a sudden large increase in either the intrathoracic or intra-abdominal pressure. The common cause of this injury is severe trauma to the thorax or abdomen during a motor vehicle accident. Most diaphragmatic ruptures are on the left side (95%) because the substantial mass of the liver, intimately associated with the diaphragm on the right side, provides a physical barrier.
A non-muscular area of variable size called the lumbocostal triangle usually occurs between the costal and lumbar parts of the diaphragm (see Figs. 2.91 and 2.97). This part of the diaphragm is normally formed only by fusion of the superior and inferior fascias of the diaphragm. When a traumatic diaphragmatic hernia occurs, the stomach, small intestine and mesentery, transverse colon, and spleen may herniate through this area into the thorax.
Hiatal (hiatus) hernia, a protrusion of part of the stomach into the thorax through the esophageal hiatus, was discussed earlier in this chapter. The structures that pass through the esophageal hiatus (vagal trunks, left inferior phrenic vessels, esophageal branches of the left gastric vessels) may be injured in surgical procedures on the esophageal hiatus (e.g., repair of a hiatus hernia)
Congenital Diaphragmatic Hernia
In congenital diaphragmatic hernia (CDH), part of the stomach and intestine herniate through a large posterolateral defect (foramen of Bochdalek) in the region of the lumbocostal trigone of the diaphragm (Fig. B2.34). Herniation almost always occurs on the left owing to the presence of the liver on the right. This type of hernia results from the complex development of the diaphragm. Posterolateral defect of the diaphragm is the only relatively common congenital anomaly of the diaphragm, occurring approximately once in 2200 newborn infants (Moore, Persaud, and Torchia, 2012). With abdominal viscera in the limited space of the prenatal pulmonary cavity, one lung (usually the left lung) does not have room to develop normally or to inflate after birth. Because of the consequent pulmonary hypoplasia, the mortality rate in these infants is high (approximately 76%).
Figure B2.34.Congenital diaphragmatic hernia (CDH).
A.As seen on autopsy. B.As seen radiographically in a newborn.
Posterior Abdominal Wall
Although the prevalence of tuberculosis (TB) has been greatly reduced, there is currently a resurgence of TB, especially in Africa and Asia, sometimes in pandemic proportions, owing to AIDS and drug resistance. TB of the vertebral column is quite common. An infection may spread through the blood to the vertebrae (hematogenous spread), particularly during childhood. An abscess resulting from tuberculosis in the lumbar region tends to spread from the vertebrae into the psoas fascia (sheath), where it produces a psoas abscess (Fig. B2.35). As a consequence, the psoas fascia thickens to form a strong stocking-like tube. Pus from the psoas abscess passes inferiorly along the psoas muscle within this fascial tube over the pelvic brim and deep to the inguinal ligament. The pus usually surfaces in the superior part of the thigh. Pus can also reach the psoas fascia by passing from the posterior mediastinum when the thoracic vertebrae are diseased.
Figure B2.35.Psoas abscess (arrow).
The inferior part of the iliac fascia is often tense and raises a fold that passes to the internal aspect of the iliac crest. The superior part of this fascia is loose and may form a pocket, the iliacosubfascial fossa, posterior to the above-mentioned fold. Part of the large intestine, such as the cecum and/or appendix on the right side and the sigmoid colon on the left side, may become trapped in this fossa, causing considerable pain.
Posterior Abdominal Pain
The iliopsoas muscle has extensive, clinically important relations to the kidneys, ureters, cecum, appendix, sigmoid colon, pancreas, lumbar lymph nodes, and nerves of the posterior abdominal wall. When any of these structures is diseased, movement of the iliopsoas usually causes pain. When intra-abdominal inflammation is suspected, the iliopsoas test is performed. The person is asked to lie on the unaffected side and extend the thigh on the affected side against the resistance of the examiner’s hand (Bickley, 2009). The elicitation of pain with this maneuver is a positive psoas sign. An acutely inflamed appendix, for example, will produce a positive right psoas sign (Fig. B2.36).
Figure B2.36.Anatomical basis of psoas sign.
Because the psoas lies along the vertebral column and the iliacus crosses the sacro-iliac joint, disease of the intervertebral and sacro-iliac joints may cause spasm of the iliopsoas, a protective reflex. Adenocarcinoma of the pancreas in advanced stages invades the muscles and nerves of the posterior abdominal wall, producing excruciating pain because of the close relationship of the pancreas to the posterior abdominal wall.
Partial Lumbar Sympathectomy
The treatment of some patients with arterial disease in the lower limbs may include a partial lumbar sympathectomy, the surgical removal of two or more lumbar sympathetic ganglia by division of their rami communicantes. Surgical access to the sympathetic trunks is commonly through a lateral extraperitoneal approach because the sympathetic trunks lie retroperitoneally in the extraperitoneal fatty tissue (Fig. 2.97). The surgeon splits the muscles of the anterior abdominal wall and moves the peritoneum medially and anteriorly to expose the medial edge of the psoas major, along which the sympathetic trunk lies. The left trunk is often overlapped slightly by the aorta. The right sympathetic trunk is covered by the IVC. The intimate relationship of the sympathetic trunks to the aorta and IVC also makes these large vessels vulnerable to injury during lumbar sympathectomy. Consequently, the surgeon carefully retracts them to expose the sympathetic trunks that usually lie in the groove between the psoas major laterally and the lumbar vertebral bodies medially. These trunks are often obscured by fat and lymphatic tissue. Knowing that identification of the sympathetic trunks is not easy, great care is taken not to remove inadvertently part of the genitofemoral nerve, lumbar lymphatics, or ureter.
Pulsations of Aorta and Abdominal Aortic Aneurysm
Because the aorta lies posterior to the pancreas and stomach, a tumor of these organs may transmit pulsations of the aorta that could be mistaken for an abdominal aortic aneurysm, a localized enlargement of the aorta (Fig. B2.37A & B). Deep palpation of the midabdomen can detect an aneurysm, which usually results from a congenital or acquired weakness of the arterial wall (Fig. B2.37C & D). Pulsations of a large aneurysm can be detected to the left of the midline; the pulsatile mass can be moved easily from side to side. Medical imaging can confirm the diagnosis in doubtful cases.
Acute rupture of an abdominal aortic aneurysm is associated with severe pain in the abdomen or back. If unrecognized, such an aneurysm has a mortality rate of nearly 90% because of heavy blood loss (Swartz, 2009). Surgeons can repair an aneurysm by opening it, inserting a prosthetic graft, and sewing the wall of the aneurysmal aorta over the graft to protect it. Many vascular problems formerly treated with open repair, including aneurysm repair, are now being treated by means of endovascular catheterization procedures.
When the anterior abdominal wall is relaxed, particularly in children and thin adults, the inferior part of the abdominal aorta may be compressed against the body of the L4 vertebra by firm pressure on the anterior abdominal wall, over the umbilicus (Fig. B2.37C & D). This pressure may be applied to control bleeding in the pelvis or lower limbs.
Collateral Routes for Abdominopelvic Venous Blood
Three collateral routes, formed by valveless veins of the trunk, are available for venous blood to return to the heart when the IVC is obstructed or ligated. Two of these routes (one involving the superior and inferior epigastric veins, and another involving the thoraco-epigastric vein) were discussed earlier in this chapter with the anterior abdominal wall. The third collateral route involves the epidural venous plexus inside the vertebral column (illustrated and discussed in Chapter 4—Back), which communicates with the lumbar veins of the inferior caval system, and the tributaries of the azygos system of veins, which is part of the superior caval system.
The inferior part of the IVC has a complicated developmental history because it forms from parts of three sets of embryonic veins (Moore, Persaud, and Torchia, 2012). Therefore, IVC anomalies are relatively common, and most of them, such as a persistent left IVC, occur inferior to the renal veins (Fig. B2.38). These anomalies result from the persistence of embryonic veins on the left side, which normally disappear. If a left IVC is present, it may cross to the right side at the level of the kidneys.
The Bottom Line
Diaphragm and Posterior Abdominal Wall
The diaphragm is the double-domed, musculotendinous partition separating the thoracic and abdominal cavities and is the chief muscle of inspiration.
- The muscular portion arises from the ring-like inferior thoracic aperture from which the diaphragm rises steeply, invaginating the thoracic cage and forming a common central tendon.
- The right dome (higher because of the underlying liver) rises nearly to the level of the nipple, whereas the left dome is slightly lower.
- The central portion of the diaphragm is slightly depressed by the heart within the pericardium and is fused to the mediastinal surface of the central tendon. In the neutral respiratory position, the central tendon lies at the level of the T8–T9 IV disc and the xiphisternal joint.
- When stimulated by the phrenic nerves, the domes are pulled downward (descend), compressing the abdominal viscera. When stimulation ceases and the diaphragm relaxes, the diaphragm is pushed upward (ascends) by the combined decompression of the viscera and tonus of the muscles of the anterolateral abdominal wall.
- The diaphragm is perforated by the IVC and phrenic nerves at the T8 vertebral level.
- The fibers of the right crus of the diaphragm form a sphincteric hiatus for the esophagus at the T10 vertebral level.
- The descending aorta and thoracic duct pass posterior to the diaphragm at the T12 vertebral level, in the midline between the crura, overlapped by the median arcuate ligament connecting them.
- Superior and inferior phrenic arteries and veins supply most of the diaphragm, with additional drainage occurring via the musculophrenic and azygos/hemi-azygos veins.
- In addition to exclusive motor innervation, the phrenic nerves supply most of the pleura and peritoneum covering the diaphragm.
- Peripheral parts of the diaphragm receive sensory innervation from the lower intercostal and subcostal nerves.
- The left lumbocostal triangle and the esophageal hiatus are potential sites of acquired hernias through the diaphragm. Developmental defects in the left lumbocostal region account for most congenital diaphragmatic hernias.
Fascia and muscles: Large, complex aponeurotic formations cover the central parts of the trunk both anteriorly and posteriorly, forming dense sheaths centrally that house vertical muscles and attach laterally to the flat muscles of the anterolateral abdominal wall.
- The thoracolumbar fascia is the posterior aponeurotic formation. In addition to ensheathing the erector spinae between its posterior and middle layers, it encloses the quadratus lumborum between its middle and anterior layers.
- The anterior layer, part of the endoabdominal fascia, is continuous medially with the psoas fascia (enclosing the psoas) and laterally with the transversalis fascia (lining the transversus abdominis).
- The tube-like psoas fascia provides a potential pathway for the spread of infections between the vertebral column and hip joint.
- The endoabdominal fascia covering the anterior aspects of both the quadratus lumborum and psoas is thickened over the superiormost aspects of the muscles, forming the lateral and medial arcuate ligaments, respectively.
- A highly variable layer of extraperitoneal fat intervenes between the endoabdominal fascia and peritoneum. It is especially thick in the paravertebral gutters of the lumbar region, comprising the paranephric fat (pararenal fat body).
- The muscles of the posterior abdominal wall are the quadratus lumborum, psoas major, and iliacus.
Nerves: The lumbar sympathetic trunks deliver postsynaptic sympathetic fibers to the lumbar plexus for distribution with somatic nerves, and presynaptic parasympathetic fibers to the abdominal aortic plexus, the latter ultimately innervating pelvic viscera.
- With the exception of the subcostal nerve (T12) and lumbosacral trunk (L4–L5), the somatic nerves of the posterior abdominal wall are products of the lumbar plexus, formed by the anterior rami of L1–L4 deep to the psoas.
- Only the subcostal nerve and derivatives of the anterior ramus of L1 (iliohypogastric and ilio-inguinal nerves) have an abdominal distribution—to the muscles and skin of the inguinal and pubic regions. All other nerves pass to the muscles and skin of the lower limb.
Arteries: Except for the subcostal arteries, the arteries supplying the posterior abdominal wall arise from the abdominal aorta.
- The abdominal aorta descends from the aortic hiatus, coursing on the anterior aspects of the T12–L4 vertebra, immediately left of the midline, and bifurcates into the common iliac arteries at the level of the supracristal plane.
- Branches of the aorta arise and course in three vascular planes: anterior (unpaired visceral branches), lateral (paired visceral branches), and posterolateral (paired parietal).
- The median sacral artery may be considered a diminutive continuation of the aorta, which continues to give rise to paired parietal branches to the lower lumbar vertebrae and sacrum.
Veins: The veins of the posterior abdominal wall are mostly direct tributaries of the IVC, although some enter indirectly via the left renal vein.
- The IVC:
- is the largest vein and lacks valves;
- formed at the L5 vertebral level by the union of the common iliac veins;
- ascends to the T8 vertebral level, passing through the caval opening of the diaphragm and entering the heart almost simultaneously;
- drains poorly oxygenated blood from the body inferior to the diaphragm; and • receives the venous drainage of the abdominal viscera indirectly via the hepatic portal vein, liver, and hepatic veins.
- Except for the hepatic veins, the tributaries of the IVC mostly correspond to the lateral paired visceral and posterolateral paired parietal branches of the abdominal aorta.
- Three collateral routes (two involving the anterior abdominal wall, and one involving the vertebral canal) are available to return blood to the heart when the IVC is obstructed.
Lymph vessels and lymph nodes: Lymphatic drainage from the abdominal viscera courses retrograde along the ramifications of the three unpaired visceral branches of the abdominal aorta.
- Lymphatic drainage from the abdominal wall merges with that from the lower limbs, both pathways following the arterial supply retrograde from those parts.
- Ultimately, all lymphatic drainage from structures inferior to the diaphragm, plus that draining from the lower six intercostal spaces via the descending thoracic lymphatic trunks, enters the beginning of the thoracic duct at the T12 level, posterior to the aorta.
- The origin of the thoracic duct may take the form of a saccular cisterna chyi (chyle cistern).
Sectional Medical Imaging of Abdomen
Ultrasound, CT scans, and MRIs are used to examine the abdominal viscera (Figs. 2.101,2.102,2.103–2.104). Because MRIs provide better differentiation between soft tissues, its images are more revealing. An image in virtually any plane can be reconstructed after scanning is completed. Abdominal angiographic studies may also now be performed using MRA (magnetic resonance angiography) (Fig. 2.104C).
Figure 2.101. Ultrasound scans of abdomen.
A.A transverse scan through the celiac trunk is shown. B.A transverse scan through the pancreas is shown. C.A sagittal scan through the aorta is shown.
(Courtesy of Dr. A. M. Arenson, Assistant Professor of Medical Imaging, University of Toronto, Toronto, ON, Canada.)
Figure 2.102. Computed tomographic (CT) scans of abdomen at progressively lower levels showing viscera and blood vessels.
(Courtesy of Dr. Tom White, Department of Radiology, The Health Sciences Center, University of Tennessee, Memphis, TN.)
Figure 2.103. Transverse magnetic resonance images (MRIs) of abdomen.
A.Level of T10 vertebra and esophageal hiatus. B.Level of L1–L2 vertebra and renal vessels and hilum. C.Level of L5 vertebra and bifurcation of aorta.
(Courtesy of Dr. W. Kucharczyk, Professor of Medical Imaging, University of Toronto, and Clinical Director of Tri-Hospital Resonance Centre, Toronto, ON, Canada.)
Figure 2.104. Magnetic resonance images (MRIs) and magnetic resonance (MR) angiogram of abdomen.
A.Coronal MRI through viscera (almost all intestine) of anterior abdominal cavity. B.Sagittal MRI in right midclavicular line. C.Anteroposterior MR angiogram demonstrating great vessels of thorax and aorta and portal vein in abdomen.