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AN47.1-14 | Abdominal cavity — Part 2

Part 4: Portosystemic Anastomoses and Portal Hypertension (AN47.10, AN47.11)

The portosystemic anastomoses are natural communications between the portal venous system and the systemic venous system. In normal anatomy, they carry minimal flow. In portal hypertension, they become engorged as high-pressure portal blood seeks an alternative route to the systemic circulation.

Five sites of portosystemic anastomosis:

Site	Portal tributary	Systemic tributary	Clinical presentation
1. Lower oesophagus	Left gastric vein (portal)	Oesophageal veins → azygos → SVC	Oesophageal varices → torrential haematemesis
2. Umbilicus	Para-umbilical veins (in falciform ligament → portal)	Superior/inferior epigastric veins (systemic)	Caput medusae (dilated periumbilical veins radiating from umbilicus)
3. Anorectal junction	Superior rectal vein (portal, via IMV)	Middle + inferior rectal veins (systemic, via internal iliac and pudendal)	Rectal varices (not the same as haemorrhoids, which can occur without portal hypertension)
4. Retroperitoneum	Colic veins (portal)	Retroperitoneal veins (systemic)	Retroperitoneal varices
5. Diaphragm	Liver surface veins	Phrenic veins (systemic)	Rarely clinically prominent

Anatomical basis of haematemesis in portal hypertension (AN47.11):
Portal vein pressure rises → left gastric vein cannot drain into the portal vein → blood backs up through the left gastric vein → oesophageal venous plexus (submucosal) → azygos vein → SVC. The submucosa of the lower 2–3 cm of the oesophagus bulges into the lumen as oesophageal varices (Grade I–IV). These can rupture spontaneously → massive upper GI haemorrhage.

Caput medusae (AN47.11):
Para-umbilical veins in the falciform ligament connect the left branch of the portal vein to the epigastric veins. In portal hypertension, these veins dilate and flow reverses → blood drains away from the umbilicus to both the superior (SVC) and inferior (IVC) epigastric territories → radiating pattern of dilated veins from the umbilicus (resembling Medusa's snake-hair — hence "caput medusae").

Note on direction of flow: In caput medusae, flow is away from the umbilicus (outward) in all directions. This distinguishes it from superior vena cava obstruction, where flow in abdominal wall veins is downward (blood bypasses the SVC block via the IVC).

Part 5: Nerve Plexuses of the Posterior Abdominal Wall (AN47.12)

The posterior abdominal wall houses the major autonomic plexuses that supply the abdominal and pelvic viscera.

Coeliac plexus (solar plexus):
- Location: Surrounds the coeliac trunk at T12–L1, on the anterior surface of the aorta
- Components: Greater splanchnic (T5–T9) + lesser splanchnic (T10–T11) + least splanchnic (T12) sympathetic preganglionic fibres + parasympathetic fibres from the vagus nerve (right vagal trunk → coeliac branches)
- Function: Sympathetic (inhibits peristalsis, constricts splanchnic vessels) and parasympathetic (promotes digestion) supply to foregut organs
- Clinical: Coeliac plexus block (injection of local anaesthetic or neurolytic agents under CT guidance) relieves intractable pain in pancreatic cancer — one of the few situations where anatomy directly provides palliative care

Aortic (intermesenteric) plexus:
- Lies on the aorta between the coeliac and inferior mesenteric arteries
- Contains sympathetic fibres from the lumbar splanchnic nerves (L1–L2)
- Supplies the midgut and connects coeliac to hypogastric plexuses

Superior hypogastric plexus (presacral nerve):
- Lies anterior to L5 vertebra and the aortic bifurcation
- Formed by aortic plexus + lumbar splanchnic nerves
- Divides into left and right hypogastric nerves descending into the pelvis
- Clinical: At risk during left-sided colorectal surgery (sigmoid colectomy, anterior resection) — injury causes bladder dysfunction and retrograde ejaculation in males

Inferior hypogastric (pelvic) plexus:
- Paired, lies on the lateral wall of the rectum and bladder
- Receives hypogastric nerves (sympathetic) + pelvic splanchnic nerves (S2,3,4 — parasympathetic)
- Supplies bladder, rectum, uterus, vagina, prostate, seminal vesicles, penis/clitoris
- At risk in radical hysterectomy and total mesorectal excision — damage causes urinary incontinence and sexual dysfunction

Part 6: The Diaphragm — Attachments, Openings, Nerve Supply, Action, and Hernia (AN47.13, AN47.14)

The Diaphragm is the dome-shaped musculotendinous partition between the thoracic and abdominal cavities. It is the principal muscle of inspiration.

Attachments:
- Sternal part: Two slips from the posterior surface of the xiphisternum
- Costal part: Inner surfaces of the lower 6 costal cartilages (7–12), interdigitating with the transversus abdominis
- Lumbar part: Two crura (right and left) from the bodies of the lumbar vertebrae (right = L1–L3; left = L1–L2) + the medial and lateral arcuate ligaments (formed by thickening of the thoracolumbar fascia)
- Central tendon: The non-contractile central aponeurosis where all muscle fibres converge; fused with the pericardium above

Three main openings (and their contents):

Opening	Level	Contents	Mnemonic
Caval foramen	T8	IVC + right phrenic nerve	I ate (T8)
Oesophageal hiatus	T10	Oesophagus + right and left vagal trunks + branches of left gastric artery	10 eggs
Aortic hiatus	T12	Aorta + thoracic duct + azygos vein	at T12

The oesophageal hiatus is a muscular ring (made of the right crus fibres) — it can function as a sphincter. The aortic hiatus is not a true opening but a gap between the crura; the aorta is not compressed by diaphragmatic contraction.

Nerve supply:
- Motor + sensory to central tendon: Phrenic nerve (C3, C4, C5) bilaterally — "C3, C4, C5 keep the diaphragm alive"
- Sensory to peripheral (costal) diaphragm: Lower intercostal nerves (T7–T12)
- Referred pain: Central diaphragmatic irritation (blood, pus under the diaphragm) is referred via the phrenic nerve to the C4 dermatome = right shoulder tip pain. Classic in: Kehr's sign (splenic rupture), right subphrenic abscess, haemoperitoneum, ectopic pregnancy rupture

Action: Inspiration — the diaphragm contracts and descends, increasing the vertical diameter of the thorax → negative intrathoracic pressure → air flows in. The right hemidiaphragm is higher than the left (due to the liver).

Diaphragmatic hernias (AN47.14):

Type	Description	Clinical features
Bochdalek hernia	Congenital, posterolateral (pleuroperitoneal canal), usually left-sided (90%)	Neonatal respiratory distress; bowel in left hemithorax on CXR
Morgagni hernia	Congenital, anterior (between the sternal and costal parts), usually right-sided	Usually asymptomatic; found incidentally on CXR as a right paracardiac opacity
Hiatus hernia — Sliding	Gastro-oesophageal junction slides up into the thorax with the stomach	GORD, heartburn; 95% of hiatus hernias in India
Hiatus hernia — Rolling (paraesophageal)	Gastric fundus rolls up alongside the oesophagus through the hiatus while GEJ remains below	Risk of gastric volvulus (strangulation) — surgical emergency