Aorta, IVC and portal vein

Portal vein

• Formed behind the neck of the pancreas (about L1–L2) by the superior mesenteric vein and splenic vein; the inferior mesenteric vein usually joins the splenic vein. Other tributaries: left and right gastric veins, cystic and paraumbilical veins.
• Behind the pancreas: IVC behind the head, aorta behind the uncinate process, portal vein and SMV behind the neck; the SMA passes in front of the uncinate process; the splenic artery runs along the upper border.
• Runs up in the right border of the lesser omentum, behind the bile duct (right) and hepatic artery (left), to divide at the porta.
• On transverse ultrasound below the pancreas the SMV lies to the right of the SMA. An SMV to the left of the SMA, with bilious vomiting in a newborn and a right-sided DJ flexure with a corkscrew duodenum, means midgut malrotation with volvulus risk.
• Normal Doppler: flow towards the liver (hepatopetal), gently phasic; main portal vein diameter above about 13 mm suggests portal hypertension. Reversed flow is hepatofugal.
• Portal hypertension: hepatic venous pressure gradient (wedged minus hepatic vein pressure) above 5 mmHg; varices form above about 10 mmHg. TIPS joins a hepatic vein to a portal vein through the liver.
• Hepatic artery buffer response: when portal flow stops (acute thrombosis), the hepatic artery enlarges with high diastolic flow and low resistance, so liver function holds.
• Normal hepatic vein Doppler is triphasic because right atrial pressure changes are transmitted back; it flattens in cirrhosis.

Portosystemic collaterals

• Gamna–Gandy bodies: in portal hypertension with splenomegaly, small haemorrhages in the spleen organise into iron and calcium deposits: many tiny foci dark on all MRI sequences that bloom on gradient-echo images.
• Cavernous transformation: after portal vein thrombosis (umbilical catheter, neonatal sepsis) the vein is replaced by a tangle of small hepatopetal collaterals; varices bleed but liver function is normal.
• Portal venous gas reaches the periphery of the liver (carried with blood flow), typical of bowel infarction; pneumobilia stays central. Infected thrombus with gas in the SMV and portal vein after appendicitis is pylephlebitis. A filling defect expanding the SMV with thick oedematous bowel (oral contraceptive use) is mesenteric venous thrombosis.

Abdominal aorta

• Enters through the aortic hiatus at T12, behind the median arcuate ligament; bifurcates at L4 (umbilicus level). Normal diameter about 2 cm; an aneurysm is 3 cm or more.

• Paired: inferior phrenic, middle suprarenal, renal, gonadal, and four pairs of lumbar arteries from the posterior aspect of the aorta; the unpaired median sacral artery leaves the back of the aorta just above the bifurcation. The coeliac trunk gives the left gastric, splenic and common hepatic arteries; the common hepatic gives the gastroduodenal artery, which runs behind D1 and is eroded by a bleeding posterior duodenal ulcer.
• Autonomic: the greater splanchnic nerve (T5–T9) pierces the crus and synapses mainly in the coeliac ganglion around the coeliac trunk; the coeliac plexus lies between the coeliac trunk and the SMA origin.
• Median arcuate ligament syndrome: coeliac trunk compressed in expiration, velocity falls on inspiration.
• Ruptured AAA: elderly hypertensive smoker with a pulsatile expansile mass, sudden back pain and hypotension; the infrarenal aneurysm is commonest: this segment has fewer elastic lamellae and little vasa vasorum supply. Rupture bleeds mainly into the left retroperitoneum.
• Inflammatory aneurysm: thick enhancing cuff in front and at the sides, posterior wall spared, ureters pulled medially.
• Aortoenteric fistula years after graft repair: the proximal graft erodes into the third or fourth part of the duodenum (which crosses in front of the aorta); a small herald haematemesis, loss of the fat plane and ectopic gas round the graft. A normal endoscopy that stops at D2 does not exclude it.
• Type II endoleak after stent grafting: sac filled by retrograde flow in the IMA or lumbar arteries.
• Aortic dissection: the false lumen is usually larger, crescentic and opacifies more slowly.
• Aortoiliac occlusion (Leriche): collaterals through internal thoracic, superior and inferior epigastric arteries in the abdominal wall, and lumbar to iliolumbar and gluteal branches. Conversely, after both internal thoracic arteries are harvested for coronary grafts, the upper rectus is supplied from below by the deep inferior epigastric artery; the superficial epigastric supplies only skin.
• IMA ligation is tolerated because the marginal artery carries SMA blood to the descending and sigmoid colon; if it is deficient, the splenic flexure and sigmoid become ischaemic.
• Acute SMA embolism (atrial fibrillation): pain out of proportion, raised lactate, non-enhancing bowel with pneumatosis.

Inferior vena cava

• Formed at L5 by the common iliac veins, to the right of the aorta; passes behind the liver (bare area) and through the diaphragm at T8.
• Direct tributaries: right gonadal, both renal veins, right suprarenal, inferior phrenic, hepatic veins, lumbar veins. The left gonadal and left suprarenal veins drain into the left renal vein, which crosses in front of the aorta just below the SMA (compressed there in nutcracker syndrome, giving haematuria and left varicocele).
• Variants from persistence of embryonic veins: retroaortic or circumaortic left renal vein; duplicated IVC (left limb joins the left renal vein); azygos continuation. A retrocaval ureter comes from a persistent posterior cardinal vein.
• Imaging traps: in early-phase CT, opacified renal vein blood meets unopacified infrarenal blood and causes mixing artefact; a flat slit-like IVC with intensely enhancing bowel means hypovolaemic shock.
• IVC filter: infrarenal, below the lowest renal vein. Primary leiomyosarcoma grows from the IVC wall.
• Anterior relations of the IVC: right common iliac artery, root of the mesentery, D3, head of pancreas and bile duct, portal vein in the epiploic foramen, and the liver.
• IVC obstruction collaterals: deep, ascending lumbar veins to azygos and hemiazygos, so a block below the azygos inflow dilates the azygos and hemiazygos systems; superficial, thoracoepigastric veins on the flank (flow upwards). Postpartum right ovarian vein thrombosis can extend into the IVC.

Posterior abdominal wall

• Psoas major: a tuberculous psoas abscess from spinal TB tracks within the psoas fascia beneath the inguinal ligament to the femoral triangle, giving groin swelling and hip flexion contracture; the lumbar plexus lies inside it, so a psoas haematoma on anticoagulation causes femoral and obturator nerve palsy. The medial arcuate ligament is thickened psoas fascia.
• Lumbar sympathetic trunk lies along the medial border of psoas; damage at para-aortic surgery gives a warm, dry, pink leg.
• Cisterna chyli lies at L1–L2, right of the aorta behind the right crus; injury gives chylous ascites.
• Inferior lumbar (Petit) triangle: iliac crest below, external oblique in front, latissimus dorsi behind; a site of lumbar hernia.
• Ureter: lies on psoas and crosses the iliac bifurcation; a ureter injured at hysterectomy (near the uterine artery) causes loin pain and hydronephrosis on that side.
• Lumbar puncture in adults is done at L3–L4 or L4–L5, below the end of the spinal cord (L1–L2), using the line joining the iliac crests (L4).

Remember Coeliac T12, SMA L1, renals L1–L2, IMA L3, bifurcation L4; IVC starts at L5.

Exam trap Portal venous gas lies at the liver periphery; gas in the bile ducts lies centrally near the porta.

Exam trap The left renal vein normally passes in front of the aorta, below the SMA, not behind it.

How FCPS asks about this

• Aortic branch levels: coeliac T12, SMA L1, renals L1–L2, IMA L3, bifurcation L4; IVC forms at L5.
• Commonest site of AAA rupture: infrarenal aorta, into the left retroperitoneum.
• Herald bleed after aortic graft: aortoenteric fistula with D3 or D4.
• Bleeding oesophageal varices: left gastric vein to oesophageal veins.
• Dark blooming splenic foci in cirrhosis: Gamna–Gandy bodies.
• Greater splanchnic nerve synapses in: the coeliac ganglion.
• Lumbar arteries arise from: the posterior abdominal aorta (four pairs).