Boundaries and contents
The abdominal cavity is bounded by the diaphragm above, the pelvic inlet below and the abdominal wall around it, and it is continuous with the pelvic cavity. The anterolateral wall is made of three flat muscles, external oblique, internal oblique and transversus abdominis, with rectus abdominis in front. The posterior wall is the lumbar vertebrae with psoas major, quadratus lumborum and iliacus.
Inside, the organs are either suspended in peritoneum or lie behind it. The peritoneal cavity is a potential space between the parietal and visceral layers of peritoneum. Organs on the posterior wall behind the parietal peritoneum are retroperitoneal.
- Primarily retroperitoneal (never had a mesentery): kidneys, suprarenal glands, ureters, aorta and inferior vena cava.
- Secondarily retroperitoneal (lost their mesentery during development): the duodenum apart from its first part, the pancreas apart from its tail, and the ascending and descending colon.
Embryological divisions of the gut
The blood supply, nerve supply and site of referred pain of every gut organ follow from which part of the primitive gut tube it came from.
| Division | Extent | Artery (level) | Pain felt in |
|---|---|---|---|
| Foregut | Abdominal oesophagus to the major duodenal papilla, with liver, biliary tree and most of the pancreas | Coeliac trunk (T12) | Epigastrium |
| Midgut | Major duodenal papilla to about two-thirds along the transverse colon | Superior mesenteric (L1) | Umbilical region |
| Hindgut | Distal third of transverse colon to the pectinate line of the anal canal | Inferior mesenteric (L3) | Suprapubic region |
The spleen is supplied by the coeliac trunk because it develops in the dorsal mesogastrium, although it is mesodermal rather than gut-derived.
Abdominal aorta and inferior vena cava
The abdominal aorta enters behind the median arcuate ligament at T12 and divides into the common iliac arteries at L4, slightly left of the midline. Its branches fall into three groups.
| Group | Branches and levels |
|---|---|
| Anterior, unpaired (gut) | Coeliac trunk T12, superior mesenteric L1, inferior mesenteric L3 |
| Lateral, paired (organs) | Inferior phrenic, middle suprarenal, renal (L1–L2), gonadal (L2) |
| Posterior (wall) | Four pairs of lumbar arteries, median sacral artery |
The inferior mesenteric artery gives the left colic and sigmoid arteries and continues as the superior rectal artery, which crosses the left common iliac vessels into the sigmoid mesocolon. Each common iliac artery divides in front of the sacro-iliac joint into the internal iliac, which supplies the pelvis, gluteal region and perineum, and the external iliac, which becomes the femoral artery as it passes under the inguinal ligament.
The inferior vena cava forms at L5 from the two common iliac veins, behind the right common iliac artery. It runs up to the right of the aorta and leaves the abdomen through the central tendon of the diaphragm at T8. The left common iliac vein is longer and passes behind the right common iliac artery to reach it.
Relations on the posterior abdominal wall
The kidneys, ureters, gonadal vessels and lumbar plexus all lie on the muscles of the posterior abdominal wall.
- Psoas major arises from the bodies, discs and transverse processes of T12 to L5 and joins iliacus to insert on the lesser trochanter as iliopsoas. It flexes the hip and is supplied by anterior rami of L1 to L3. The lumbar plexus forms within it.
- Quadratus lumborum runs from the iliolumbar ligament and iliac crest to the twelfth rib and lumbar transverse processes, lateral to psoas.
- Iliacus fills the iliac fossa.
Nerves emerge from psoas in a fixed pattern. The femoral nerve (L2–L4) leaves its lateral border and runs in the groove between psoas and iliacus to pass under the inguinal ligament. The lateral cutaneous nerve of the thigh (L2, L3) also leaves the lateral border but crosses iliacus towards the anterior superior iliac spine. The genitofemoral nerve pierces the front of psoas, and the obturator nerve leaves its medial border.
The ureter runs down on psoas, crossed in front by the gonadal vessels, and enters the pelvis over the bifurcation of the common iliac artery.
Clinical relevance
The organisation of the cavity explains where abdominal pain is felt and where blood or pus tracks.
- Referred visceral pain: early appendicitis is felt at the umbilicus because the appendix is midgut, before parietal peritoneal irritation localises it.
- Abdominal aortic aneurysm: most are infrarenal. In the UK, repair is generally considered at a diameter of 5.5 cm or more.
- Retroperitoneal haemorrhage from pancreatitis or a leaking aneurysm can track to the flanks as Grey Turner's sign.
- Psoas abscess can track within the psoas sheath from the spine to the groin, and an inflamed retrocaecal appendix on psoas gives pain on hip extension (the psoas sign).
- Meralgia paraesthetica is compression of the lateral cutaneous nerve of the thigh near the anterior superior iliac spine, causing burning numbness over the lateral thigh.
A lumpy, nodular kidney in an older specimen is usually due to simple cortical cysts or old scarring, but in a patient the same outline needs imaging to exclude a tumour.
On the specimen
This station is the posterior abdominal wall with most of the gut removed, so the task is to identify vessels, retroperitoneal organs and the nerves on psoas.
- Aorta and inferior vena cava: the aorta is thick-walled, round and slightly left; the inferior vena cava is flattened, thin-walled and on the right.
- Inferior mesenteric artery: the small anterior branch arising a few centimetres above the bifurcation; follow it down and left into the superior rectal artery crossing the left common iliac vessels.
- Gonadal artery or ureter: the gonadal vessel is thin and crosses the ureter from medial to lateral in front of it. The ureter is thicker, pale and lies on psoas, reaching the pelvic brim over the iliac bifurcation.
- Femoral nerve or lateral cutaneous nerve of the thigh: the femoral nerve is the broad flat nerve in the groove between psoas and iliacus; the lateral cutaneous nerve is thinner and runs across iliacus to the anterior superior iliac spine.
- Abdominal wall layers: at the cut lateral edge, external oblique is outermost, internal oblique in the middle and transversus abdominis deepest.
The left lobe of the liver, gallbladder, pancreas, spleen and portal vein are usually left in place above for orientation. The left kidney on this specimen is nodular, which is a common finding in older donors.