Abdomen and pelvis anatomy

Gut divisions and referred pain

• Visceral pain is felt in the midline at the level of the embryonic gut segment, because visceral afferents travel with the sympathetic supply of that segment.

• Somatic referral: diaphragmatic irritation (gallbladder, blood, pus) to the shoulder tip via the phrenic nerve (C3–C5); splenic rupture to the left shoulder (Kehr sign).
• Ureteric colic spreads from loin to groin. A stone in the lower ureter refers pain to the groin and front of the upper thigh (and scrotum or labium) through the genitofemoral nerve (L1–L2), which shares spinal segments with the ureter.

Developmental anomalies

Stomach, duodenum and peritoneal spaces

• The stomach has rich anastomoses along both curvatures (left and right gastric on the lesser curve, left and right gastro-omental and short gastrics on the greater), so it survives ligation of one or two arteries. The fundus, supplied by the short gastrics, is the part most at risk in a gastric conduit.
• Truncal vagotomy removes parasympathetic drive to the antrum and pylorus, so the pylorus no longer relaxes and the stomach empties poorly (early satiety, vomiting of undigested food). A drainage procedure (pyloroplasty or gastrojejunostomy) is added for this reason. Post-vagotomy diarrhoea is another recognised effect.
• First part of duodenum: the gastroduodenal artery runs directly behind it. A posterior ulcer erodes the GDA and bleeds massively; an anterior ulcer faces the peritoneal sac and perforates, giving free gas and peritonitis. A posterior gastric ulcer can erode the left gastric or splenic artery.
• Kocher manoeuvre: incising peritoneum lateral to D2 and reflecting duodenum and pancreatic head medially; the IVC lies directly behind.
• Ligament of Treitz (suspensory muscle) holds the duodenojejunal flexure, normally left of L2, and marks the end of the duodenum.
• Root of the small bowel mesentery: about 15 cm long, from the DJ flexure to the right sacroiliac joint, crossing D3, aorta, IVC, right ureter and right gonadal vessels.
• Subphrenic spaces: the falciform ligament separates right and left anterior subphrenic (suprahepatic) spaces; the coronary ligament only limits them posteriorly. The hepatorenal pouch (Morison) is the lowest point of the peritoneal cavity in a supine patient.

Liver and biliary landmarks

• Eight Couinaud segments; the Cantlie line (gallbladder fossa to IVC) divides functional right and left livers; the caudate lobe (segment I) drains directly into the IVC, so it enlarges in Budd–Chiari syndrome.
• Portal vein supplies about 75% of blood flow and about half the oxygen; the hepatic artery the rest. In the free edge of the lesser omentum the bile duct is anterolateral, the hepatic artery anteromedial and the portal vein posterior.
• Acinar zones: zone 1 periportal (best oxygenated); zone 3 centrilobular, which has the least oxygen and the most smooth endoplasmic reticulum and cytochrome P450, so it is injured first by paracetamol, alcohol, ischaemia and heart failure (nutmeg liver).
• Percutaneous liver biopsy: right mid-axillary line, 8th–9th intercostal space in expiration, just above a rib; use the transjugular route with ascites or coagulopathy.
• Calot triangle (cystic duct, common hepatic duct, inferior surface of liver) is dissected to find the cystic artery (and Lund node). Rouvière sulcus, a cleft on the undersurface of the right lobe, marks the level of the bile duct; staying above it keeps dissection safe. The critical view of safety shows only two structures entering the gallbladder.
• Ascending cholangitis: fever with rigors, jaundice and right upper quadrant pain (Charcot triad); treat with antibiotics and early ERCP drainage.

Portal system and portal hypertension

• Portal vein forms behind the neck of the pancreas from the superior mesenteric and splenic veins; it has no valves.

• Causes by level: prehepatic (portal vein thrombosis, neonatal umbilical sepsis); presinusoidal (schistosomiasis); sinusoidal (cirrhosis, commonest); posthepatic (Budd–Chiari, constrictive pericarditis, right heart failure).
• Splenic vein thrombosis (pancreatitis) gives isolated gastric varices with a normal liver; splenectomy cures it.
• Paracentesis is done lateral to the rectus sheath in the left lower quadrant to avoid the inferior epigastric artery; give albumin (8 g per litre) after large-volume drainage. SAAG of 11 g/L or more means portal hypertension.
• Hepatorenal syndrome: rising creatinine, very low urine sodium, normal kidneys, no response to stopping diuretics and giving albumin; treat with terlipressin plus albumin after excluding spontaneous bacterial peritonitis.

Kidney

• Retroperitoneal, T12–L3, right lower than left; hilum at L1 with renal vein, artery and pelvis from front to back. The kidney lies on the diaphragm, so it descends 2–3 cm on inspiration as the diaphragm pushes it down; it is ballotable, with colonic resonance in front (unlike the spleen).
• Coverings: capsule, perirenal fat, renal (Gerota) fascia enclosing kidney and adrenal, pararenal fat.
• Left kidney anterior relations: adrenal, spleen, stomach and jejunum (with peritoneum between), and body of pancreas with splenic vessels and the splenic flexure directly in contact, with no peritoneum.
• The left renal vein crosses in front of the aorta below the SMA and receives the left gonadal and adrenal veins; the right renal artery passes behind the IVC.
• Arterial sequence: renal, segmental (end arteries), interlobar (between pyramids), arcuate (arching along the bases of the pyramids at the corticomedullary junction), interlobular, afferent arteriole.
• Medullary rays are straight collecting ducts and tubules running up from the medulla into the cortex; renal columns are cortex extending down between pyramids.
• Juxtaglomerular apparatus: renin-secreting granular cells in the afferent arteriole, macula densa in the distal tubule sensing NaCl (low NaCl raises renin), and lacis cells. Autoregulation keeps renal blood flow steady over a mean arterial pressure of about 80–180 mmHg; vasa recta act as countercurrent exchangers.
• Filtration barrier: fenestrated endothelium, basement membrane with negatively charged heparan sulphate (charge selectivity) and podocyte slit diaphragms (nephrin).
• The straight proximal tubule and medullary thick ascending limb are most vulnerable to ischaemia (high energy use, low medullary oxygen).
• Renal biopsy: lower pole, patient prone, ultrasound guided; haematuria is the commonest complication.

Adrenal gland

• Medullary chromaffin cells are modified postganglionic neurones, so they receive preganglionic fibres directly. Right adrenal vein drains straight into the IVC; left into the left renal vein.

Ureter, bladder and aorta

• Ureter runs on psoas, crosses the pelvic brim at the common iliac bifurcation and passes under the uterine artery and under the vas. Narrowings: pelviureteric junction, pelvic brim, vesicoureteric junction (narrowest, commonest site for a stone to lodge).
• At surgery the ureter is told from vessels and nerves by peristalsis when pinched (vermiculation); injury is commonest in hysterectomy and colorectal surgery.
• Bladder control: parasympathetic S2–S4 contracts detrusor; sympathetic T11–L2 aids storage; pudendal nerve to the external sphincter. Cord lesions above the sacral centre give a spastic bladder; cauda equina or diabetic neuropathy a flaccid, overflowing one.

• Abdominal aortic aneurysm is usually infrarenal. Rupture causes back or abdominal pain, hypotension and a pulsatile expansile mass in an older hypertensive smoker.

Remember Posterior duodenal ulcers bleed (gastroduodenal artery); anterior ones perforate.

Exam trap The falciform ligament divides the right and left subphrenic spaces; the coronary ligament only bounds them.

Exam trap Calcium and insulin protect against hyperkalaemia but do not lower total body potassium; in AKI with refractory hyperkalaemia or overload, dialysis is needed.

How FCPS asks about this

• Massive bleed from a posterior first-part duodenal ulcer: gastroduodenal artery.
• Newborn bilious vomiting, DJ flexure on the right, corkscrew duodenum: malrotation with volvulus.
• Duplex kidney cause and the girl who dribbles constantly: bifid ureteric bud; ectopic upper-pole ureter below the bladder neck.
• Aldosterone source and control; nerve supply of the adrenal medulla: zona glomerulosa, angiotensin II and potassium; preganglionic sympathetic fibres.
• Commonest site for a ureteric stone and the nerve for groin and thigh pain: vesicoureteric junction; genitofemoral nerve.
• Laparoscopic cholecystectomy landmarks: Calot triangle for the cystic artery; stay above Rouvière sulcus.
• Blow to full bladder, contrast around bowel loops: intraperitoneal rupture needing surgical repair.