AP: Thorax, abdomen and pelvis

• Thorax: The Chest Wall and Surface Anatomy | Intercostal Spaces and the Diaphragm | The Mediastinum | Surface Anatomy of the Heart and Lungs | The Great Vessels and Thoracic Duct
• Abdomen: The Abdominal Wall and Regions | The Inguinal Canal and Hernias | The Peritoneum | Blood Supply of the Gut | Abdominal Organs — Position and Relations | Portal Venous System and Portosystemic Anastomoses | Retroperitoneum, Kidneys and Ureters
• Pelvis and perineum: The Pelvis and Pelvic Organs | The Perineum and Genital Anatomy
• Revision: Thorax, Abdomen and Pelvis at a Glance | Spot the Diagnosis | Exam-style Scenarios | Key Points | Guidelines and Further Reading

🫁 Thorax, abdomen and pelvis — regional and surface anatomy — page 5 of the Anatomy and Physiology chapter. Trunk anatomy is used every day — placing chest drains and central lines, finding the apex beat, localising abdominal pain, understanding hernias, reading CT scans, and knowing why a bleeding varix, a referred shoulder-tip pain or a ureteric stone behaves as it does. The organ-system pages cover physiology; this page covers where things are and what lies next to them.
Your four questions for every trunk problem: Which surface landmark marks the structure? What lies next to it — and could be damaged? What is its blood supply and lymphatic drainage? Where does its pain refer?

The Chest Wall and Surface Anatomy

• Bony thorax — 12 thoracic vertebrae, 12 pairs of ribs, sternum (manubrium, body, xiphoid).
  • True ribs (1–7) attach directly to the sternum; false ribs (8–10) join the costal margin; floating ribs (11–12).
  • First rib — short, flat; subclavian vessels and lower brachial plexus cross it; a fracture implies high-energy trauma.
• Sternal angle (angle of Louis) — junction of the manubrium and body — the key landmark at the level of T4/5:
  • 2nd costal cartilage attaches here — count ribs and intercostal spaces from it.
  • Marks the tracheal bifurcation (carina), the beginning and end of the aortic arch, the azygos vein joining the SVC, the division between superior and inferior mediastinum, the pulmonary trunk bifurcation, and where the thoracic duct crosses to the left.
• Other landmarks:
  • Jugular (suprasternal) notch — T2/3; check the trachea is central.
  • Xiphisternal joint — T9.
  • Nipple — 4th intercostal space in men (T4 dermatome).
  • Inferior angle of the scapula — 7th rib or space (arms at sides).
  • Vertebra prominens — C7 spinous process.
• Lines — midclavicular, anterior, mid- and posterior axillary lines — used to describe drain sites and the apex beat.
• Chest wall injuries — rib fractures (pain limits breathing → pneumonia — older people), flail chest (2 or more fractures in 2 or more adjacent ribs — paradoxical movement — underlying lung contusion), sternal fracture (cardiac contusion — ECG).

Intercostal Spaces and the Diaphragm

• Intercostal muscles — external (inspiration — "hands in pockets" direction), internal and innermost (forced expiration).
• Neurovascular bundle — runs in the costal groove under each rib in the order (top to bottom) Vein, Artery, Nerve ("VAN") — between the internal and innermost layers.
• Practical consequence — insert chest drains, pleural aspiration needles and thoracentesis just above the upper border of the rib below to avoid the bundle; intercostal nerve blocks are placed at the lower border.
• Safe triangle for chest drains — lateral border of pectoralis major, anterior border of latissimus dorsi, a line through the 5th intercostal space (nipple level), apex below the axilla.
• Intercostal nerves (T1–11) — supply the chest and abdominal wall muscles and skin (dermatomes); shingles follows a dermatome; intercostal neuralgia.
• Diaphragm:
  • The main muscle of inspiration (about 70% of quiet breathing) — dome-shaped; contraction flattens it and enlarges the thorax.
  • Phrenic nerve (C3, 4, 5) — "C3, 4 and 5 keep the diaphragm alive" — motor and central sensory supply → shoulder-tip pain from diaphragmatic irritation (subphrenic abscess, ruptured spleen — Kehr's sign, ectopic pregnancy, laparoscopy gas).
  • Openings — "I 8 10 Eggs At 12" — IVC at T8 (with right phrenic nerve), Oesophagus at T10 (with vagal trunks), Aorta at T12 (with thoracic duct and azygos vein — behind the diaphragm, between the crura).
  • Right dome higher than the left (liver); domes rise to about the 5th rib in expiration.
• Diaphragmatic problems — hiatus hernia (sliding — commonest; rolling/para-oesophageal), congenital diaphragmatic hernia (Bochdalek — posterolateral left — neonates), traumatic rupture (usually left), phrenic nerve palsy (raised hemidiaphragm — tumour, surgery, interscalene block, MND).

The Mediastinum

• Mediastinum — the central compartment between the pleural sacs, from the thoracic inlet to the diaphragm.
• Divisions — superior (above the plane of the sternal angle — T4/5) and inferior (anterior, middle, posterior).
• Contents:

  Region	Contents	Masses
  Superior	Aortic arch and branches, SVC and brachiocephalic veins, trachea, oesophagus, thymus, thoracic duct, vagus and phrenic nerves, left recurrent laryngeal nerve	Retrosternal goitre, thymoma, lymphoma
  Anterior	Thymus (remnant), fat, lymph nodes, internal thoracic vessels	"4 Ts" — Thymoma, Teratoma (germ cell), Thyroid, "Terrible" lymphoma
  Middle	Heart and pericardium, ascending aorta, pulmonary trunk, main bronchi, phrenic nerves	Lymphadenopathy (sarcoid, TB, lymphoma, cancer), pericardial and bronchogenic cysts
  Posterior	Descending aorta, oesophagus, azygos and hemiazygos veins, thoracic duct, sympathetic chains, vagus nerves	Neurogenic tumours (schwannoma, neuroblastoma in children), oesophageal lesions, aneurysms

• Nerves in the thorax:
  • Phrenic nerves — pass anterior to the lung roots (with the pericardiacophrenic vessels, on the pericardium).
  • Vagus nerves — pass posterior to the lung roots; form the oesophageal plexus.
  • Left recurrent laryngeal nerve — hooks under the aortic arch (ligamentum arteriosum) → hoarseness from lung cancer (left hilum), aortic aneurysm, mitral stenosis (Ortner's), mediastinal nodes; the right hooks under the right subclavian artery (higher — less often affected).
• Pleura — visceral (no pain fibres) and parietal (sensitive — pleuritic pain; intercostal nerves, central diaphragmatic part via the phrenic nerve); the pleural cavity is a potential space with a small amount of fluid.
• Pleural reflections — "2, 4, 6, 8, 10, 12" — pleura from the apex (2–3 cm above the medial clavicle) to the sternal angle (2nd costal cartilage), runs down to the 4th (left diverges for the heart — cardiac notch) and 6th costal cartilages, crosses the 8th rib in the midclavicular line, 10th in the midaxillary line, and reaches the 12th rib posteriorly; the lungs lie about 2 ribs higher (6, 8, 10).
• Lung apex — rises above the clavicle — pneumothorax risk with subclavian and internal jugular lines and supraclavicular blocks.

Surface Anatomy of the Heart and Lungs

• Heart borders (on chest X-ray):
  • Right border — right atrium.
  • Left border — left ventricle (and left atrial appendage, pulmonary trunk and aortic knuckle above).
  • Inferior border — right ventricle (mainly).
  • Anterior surface — mostly right ventricle (most often injured by stab wounds; behind the sternum).
  • Posterior surface — left atrium — lies against the oesophagus (transoesophageal echo; enlarged LA in mitral stenosis can cause dysphagia).
• Apex beat — normally 5th intercostal space, midclavicular line; displaced with LV dilatation, mediastinal shift, chest deformity.
• Auscultation areas — aortic (2nd right intercostal space, parasternal), pulmonary (2nd left), tricuspid (4th–5th left, lower sternal edge), mitral (apex) — "All Physicians Take Money".
• Lung fissures:
  • Oblique fissure (both lungs) — from about T3 spinous process posteriorly, around to the 6th costal cartilage anteriorly — follows the line of the medial border of the scapula with the arm raised.
  • Horizontal fissure (right only) — along the 4th costal cartilage to meet the oblique fissure in the midaxillary line.
• Lobes — right lung — 3 lobes (upper, middle, lower); left — 2 lobes (upper with the lingula, lower).
  • Examine the lower lobes from behind, the right middle lobe and lingula from the front (and in the axilla).
  • Right middle lobe consolidation obscures the right heart border; lingular consolidation obscures the left heart border; lower lobe disease obscures the hemidiaphragm (silhouette sign).
• Bronchial anatomy — the right main bronchus is shorter, wider and more vertical → aspiration (supine — posterior segment of the right upper lobe and apical segment of the right lower lobe; upright — right lower lobe) and inhaled foreign bodies go right; endobronchial intubation is usually right-sided.
• Lung roots (hila) — pulmonary artery, two pulmonary veins, bronchus, bronchial vessels, lymph nodes, nerves; the left hilum is usually slightly higher than the right.

The Great Vessels and Thoracic Duct

• Aorta — ascending (coronary arteries arise from the aortic sinuses) → arch (at the sternal angle) → descending thoracic → through the diaphragm at T12 → abdominal aorta.
• Arch branches (right to left) — brachiocephalic trunk (→ right subclavian and right common carotid), left common carotid, left subclavian.
• Veins — internal jugular + subclavian → brachiocephalic vein (behind the sternoclavicular joint); both brachiocephalic veins → SVC (the left crosses in front of the arch branches) → right atrium; the azygos vein drains the posterior chest wall into the SVC.
• Superior vena cava obstruction — lung cancer, lymphoma — facial swelling and plethora, distended neck and chest veins, breathlessness, headache worse bending forward — steroids, stenting, treat the cause.
• Central venous access:
  • Internal jugular — at the apex of the triangle between the two heads of sternocleidomastoid, lateral to the carotid — ultrasound-guided (preferred).
  • Subclavian — below the clavicle — higher pneumothorax risk, lower infection risk.
  • Femoral — medial to the femoral artery.
  • Catheter tip at the cavoatrial junction (about at the level of the carina on X-ray).
• Aortic dissection — Stanford type A (involves the ascending aorta — surgery) vs type B (descending only — usually medical); may occlude branches (stroke, MI, limb, gut, kidneys).
• Traumatic aortic injury — deceleration — tears at the isthmus (just distal to the left subclavian, at the ligamentum arteriosum) — widened mediastinum on X-ray.
• Thoracic duct — drains lymph from the whole body except the right upper quadrant (right head, neck, arm and chest — right lymphatic duct); starts at the cisterna chyli (L1–2), passes through the aortic hiatus, ascends behind the oesophagus, crosses to the left at T4/5 and drains into the junction of the left internal jugular and subclavian veins → injury causes chylothorax (milky effusion — triglycerides high); Virchow's node (left supraclavicular — Troisier's sign — gastric cancer).

The Abdominal Wall and Regions

• Nine regions — defined by two vertical midclavicular lines and two horizontal planes (subcostal and intertubercular):

  Right hypochondrium (liver, gallbladder)	Epigastrium (stomach, pancreas, duodenum, aorta)	Left hypochondrium (spleen, stomach)
  Right flank (lumbar) Right kidney, ascending colon	Umbilical Small bowel, aorta, transverse colon	Left flank (lumbar) Left kidney, descending colon
  Right iliac fossa Appendix, caecum, right ovary	Suprapubic (hypogastrium) Bladder, uterus	Left iliac fossa Sigmoid colon, left ovary

• Important planes:
  • Transpyloric plane (L1) — halfway between the jugular notch and pubic symphysis — pylorus, neck of pancreas, duodenojejunal flexure, hila of kidneys, fundus of gallbladder (9th costal cartilage), end of the spinal cord, origin of the SMA, renal hila.
  • Subcostal plane (L3); umbilicus (L3/4); supracristal plane (L4) — aortic bifurcation and LP level; intertubercular plane (L5).
• Layers of the anterolateral wall — skin → superficial fascia (Camper's fatty and Scarpa's membranous layers) → external oblique → internal oblique → transversus abdominis → transversalis fascia → extraperitoneal fat → parietal peritoneum.
• Rectus abdominis — in the rectus sheath; the linea alba in the midline (laparotomy — little bleeding); the arcuate line (below which the posterior sheath is absent); linea semilunaris (lateral border — Spigelian hernia).
• Vessels — inferior epigastric artery (from the external iliac — runs up behind the rectus; injured with laparoscopic ports or paracentesis — use the left iliac fossa lateral to the rectus for ascitic drains) and the superior epigastric (internal thoracic).
• Nerves — T7–T12 and L1 (thoracoabdominal, iliohypogastric, ilioinguinal); dermatomes — T10 umbilicus; TAP block (between internal oblique and transversus abdominis).
• Incisions — midline laparotomy, Kocher's (right subcostal — gallbladder), Lanz and gridiron (appendix — McBurney's point: one-third of the way from the ASIS to the umbilicus), Pfannenstiel (caesarean), Rutherford Morison, laparoscopic ports.

The Inguinal Canal and Hernias

• Inguinal canal — about 4 cm long, running obliquely above the medial half of the inguinal ligament from the deep ring (midpoint of the inguinal ligament, lateral to the inferior epigastric vessels) to the superficial ring (above and medial to the pubic tubercle).
• Walls — "MALT: 2Ms, 2As, 2Ls, 2Ts":
  • Roof — 2 Muscles (internal oblique and transversus abdominis).
  • Anterior wall — 2 Aponeuroses (external oblique, reinforced laterally by internal oblique).
  • Floor — 2 Ligaments (inguinal and lacunar).
  • Posterior wall — 2 Ts (transversalis fascia and conjoint tendon).
• Contents — spermatic cord (men) or round ligament of the uterus (women), and the ilioinguinal nerve (passes through the superficial ring, not the deep ring).
• Spermatic cord — vas deferens, testicular artery, pampiniform plexus, artery to vas, cremasteric artery, genital branch of the genitofemoral nerve, sympathetic fibres, lymphatics, processus vaginalis remnant — covered by three fascial layers.
• Hernias:

  Hernia	Anatomy	Clinical features
  Indirect inguinal	Through the deep ring, lateral to the inferior epigastric vessels, along the canal (patent processus vaginalis)	Commonest hernia (both sexes); young men and children; can reach the scrotum; controlled by pressure over the deep ring
  Direct inguinal	Through the posterior wall (Hesselbach's triangle), medial to the inferior epigastric vessels	Older men; weak abdominal wall; rarely strangulates
  Femoral	Through the femoral canal, below and lateral to the pubic tubercle	Women; high risk of strangulation — repair urgently
  Umbilical / paraumbilical	Through or beside the umbilicus	Congenital (infants — most close by age 4–5) or acquired (obesity, ascites)
  Incisional	Through a surgical scar	After laparotomy — obesity, infection
  Spigelian, obturator, epigastric	Linea semilunaris; obturator canal; linea alba above the umbilicus	Uncommon; obturator — elderly thin women, bowel obstruction

• Hesselbach's triangle — inferior epigastric vessels (lateral), lateral border of rectus (medial), inguinal ligament (inferior).
• Complications — incarceration (irreducible), obstruction, strangulation (compromised blood supply — tender, red, painful — emergency surgery); Richter's hernia (part of the bowel wall — strangulation without obstruction).
• Scrotal swelling — can you get above it? (No — inguinoscrotal hernia); transilluminates (hydrocele); separate from the testis (epididymal cyst).

The Peritoneum

• Peritoneum — a serous membrane — parietal (lines the wall — somatic nerves — sharp, localised pain) and visceral (covers organs — autonomic — dull, poorly localised pain).
• Intraperitoneal vs retroperitoneal organs:
  • Intraperitoneal (on a mesentery) — stomach, first part of the duodenum, jejunum, ileum, transverse and sigmoid colon, liver, spleen, gallbladder, appendix, caecum (variably).
  • Retroperitoneal — "SAD PUCKER" — Suprarenal (adrenal) glands, Aorta and IVC, Duodenum (2nd–4th parts), Pancreas (except the tail), Ureters, Colon (ascending and descending), Kidneys, Esophagus (lower), Rectum (middle and lower).
• Mesenteries and omenta:
  • Greater omentum — hangs from the greater curvature of the stomach — "abdominal policeman" (walls off inflammation — appendix mass); site of cancer deposits (omental cake — ovarian cancer).
  • Lesser omentum — liver to the lesser curvature — its free edge (the hepatoduodenal ligament) contains the portal triad: portal vein (posterior), hepatic artery (left), common bile duct (right) — the Pringle manoeuvre compresses it to control liver bleeding.
  • Epiploic foramen (of Winslow) — behind the free edge — entrance to the lesser sac (behind the stomach — pancreatic pseudocysts, posterior gastric ulcer perforation).
• Peritoneal spaces:
  • Hepatorenal pouch (Morison's pouch) — the lowest point of the peritoneal cavity when lying supine — fluid collects here (FAST scan).
  • Rectouterine pouch (of Douglas) in women / rectovesical pouch in men — lowest point when upright — pelvic abscesses, ectopic blood, metastases (Blumer's shelf).
  • Subphrenic spaces — abscesses after surgery (shoulder-tip pain, swinging fever).
  • Paracolic gutters — fluid tracks from the upper abdomen to the pelvis (perforated duodenal ulcer can mimic appendicitis).
• Peritonitis — guarding, rigidity (board-like in perforation), rebound tenderness, lying still; pain from inflamed visceral peritoneum moves to parietal peritoneum as inflammation spreads (appendicitis — periumbilical then right iliac fossa).

Blood Supply of the Gut

• Coeliac trunk branches — left gastric (oesophageal branches), splenic (short gastrics, left gastro-omental, pancreatic branches), common hepatic (→ proper hepatic, gastroduodenal artery, right gastric).
• Gastroduodenal artery — runs behind the first part of the duodenum — posterior duodenal ulcers erode it → major upper GI bleeding.
• Left gastric artery — bleeding from lesser curvature gastric ulcers.
• Watershed areas — splenic flexure (Griffiths' point — SMA/IMA junction) and the rectosigmoid (Sudeck's point) — vulnerable to ischaemic colitis in low-flow states.
• Marginal artery of Drummond — connects SMA and IMA along the colon.
• Acute mesenteric ischaemia — SMA embolus (AF) or thrombosis — pain out of proportion to the signs, raised lactate (late) — CT angiography — emergency revascularisation and resection.
• Rectum and anal canal — superior rectal (IMA), middle rectal (internal iliac), inferior rectal (internal pudendal) arteries.
• Pectinate (dentate) line — marks the embryological junction:
  • Above — columnar epithelium, visceral innervation (painless — internal haemorrhoids), drains to the portal system and internal iliac/inferior mesenteric nodes — adenocarcinoma.
  • Below — squamous epithelium, somatic innervation (painful — external haemorrhoids, fissures), drains to systemic veins and superficial inguinal nodes — squamous cell carcinoma.

Abdominal Organs — Position and Relations

• Oesophagus — 25 cm; begins at C6 (cricopharyngeus); three constrictions (upper sphincter, aortic arch/left main bronchus, diaphragm — about 15, 25 and 40 cm from the incisors) — where foreign bodies and strictures lodge; lower third drains to portal and systemic veins (varices).
• Stomach — cardia, fundus, body, antrum, pylorus (transpyloric plane, L1); lesser and greater curvatures; posterior relations (the "stomach bed") — pancreas, spleen, left kidney and adrenal, splenic artery.
• Duodenum — C-shaped around the head of the pancreas: 1st (superior — duodenal cap — commonest ulcer site; intraperitoneal), 2nd (descending — major papilla (ampulla of Vater) — common bile and pancreatic ducts enter), 3rd (horizontal — crossed by the SMA — superior mesenteric artery syndrome), 4th (ascending → duodenojejunal flexure — ligament of Treitz — boundary between upper and lower GI bleeding).
• Small bowel — jejunum (proximal, thicker wall, fewer arcades, long vasa recta, plicae circulares) and ileum (more fat, more arcades, Peyer's patches; terminal ileum — B12 and bile salt absorption — Crohn's); Meckel's diverticulum (rule of 2s — 2 feet from the ileocaecal valve, 2% of people, 2 inches long, may contain gastric or pancreatic tissue).
• Large bowel — caecum, appendix (base at the confluence of the taeniae coli; tip position varies — retrocaecal commonest), ascending, transverse, descending and sigmoid colon, rectum; taeniae coli, haustra, appendices epiploicae distinguish colon from small bowel (on X-ray — haustra don't cross the full width).
• Liver — right upper quadrant under ribs 7–11; divided into 8 functional segments (Couinaud) by hepatic and portal veins; dual blood supply — portal vein (75%) and hepatic artery (25%); falciform ligament (ligamentum teres — obliterated umbilical vein).
• Gallbladder — fundus at the tip of the right 9th costal cartilage (Murphy's sign); Calot's triangle (cystic duct, common hepatic duct, inferior border of the liver) contains the cystic artery — identified in cholecystectomy ("critical view of safety").
• Biliary tree — right and left hepatic ducts → common hepatic duct + cystic duct → common bile duct → joins the pancreatic duct → ampulla of Vater (sphincter of Oddi).
• Pancreas — head (in the duodenal C-loop), neck (in front of the portal vein/SMV — L1), body, tail (in the splenic hilum); retroperitoneal — pain radiates to the back; head tumours obstruct the bile duct → painless jaundice with a palpable gallbladder (Courvoisier's law).
• Spleen — "1, 3, 5, 7, 9, 11" — about 1 × 3 × 5 inches, weighs about 7 oz, lies under the 9th–11th ribs on the left; enlarges towards the right iliac fossa; notch on the anterior border; can't get above it; injured with left lower rib fractures.

Portal Venous System and Portosystemic Anastomoses

• Hepatic portal vein — formed behind the neck of the pancreas by the superior mesenteric vein and the splenic vein (the inferior mesenteric vein usually joins the splenic vein); carries nutrient-rich blood from the gut, spleen and pancreas to the liver; about 8 cm long.
• Hepatic veins (3 — right, middle, left) drain into the IVC — obstruction → Budd–Chiari syndrome (pain, hepatomegaly, ascites).
• Portal hypertension (cirrhosis, portal vein thrombosis, schistosomiasis) — blood diverts through portosystemic anastomoses:

  Site	Portal side	Systemic side	Clinical result
  Lower oesophagus	Left gastric vein	Oesophageal veins → azygos	Oesophageal varices (life-threatening bleeding)
  Umbilicus	Paraumbilical veins	Superficial epigastric veins	Caput medusae
  Anorectal junction	Superior rectal vein	Middle and inferior rectal veins	Rectal varices (not ordinary haemorrhoids)
  Retroperitoneum	Colic veins	Retroperitoneal and renal veins	Retroperitoneal varices; spontaneous splenorenal shunts

• Other consequences — splenomegaly (hypersplenism — low platelets), ascites (with low albumin and sodium retention), hepatic encephalopathy (toxins bypass the liver).
• TIPSS (transjugular intrahepatic portosystemic shunt) — a stent between a hepatic vein and portal vein branch to lower portal pressure (refractory varices or ascites) — risk of encephalopathy.
• First-pass metabolism — drugs absorbed from the gut pass through the liver before reaching the systemic circulation (oral morphine, GTN) — bypassed by sublingual, rectal (partly), transdermal and IV routes.

Retroperitoneum, Kidneys and Ureters

• Abdominal aorta — enters at T12; bifurcates at L4 (level of the umbilicus/iliac crests) into the common iliac arteries.
  • Branches — coeliac (T12), SMA (L1), renal arteries (L1/2), gonadal arteries (L2), IMA (L3), lumbar arteries, median sacral.
  • AAA — usually infrarenal; screening of men at 65 in the UK (ultrasound); pulsatile, expansile mass above the umbilicus; rupture — back or flank pain, collapse.
• IVC — formed at L5 (right of the aorta); left renal vein crosses in front of the aorta (below the SMA — "nutcracker" compression); passes through the diaphragm at T8.
• Kidneys — retroperitoneal at T12–L3; right lower than the left (liver); hila at L1; move with respiration; surrounded by perirenal fat and Gerota's fascia.
  • Hilum structures (anterior to posterior) — renal vein, renal artery, ureter (pelvis) — "VAU".
  • Relations — posteriorly the 12th rib, diaphragm, psoas and quadratus lumborum; right kidney — liver, duodenum, hepatic flexure; left kidney — stomach, spleen, pancreas tail, splenic flexure.
  • Left gonadal vein drains into the left renal vein (right into the IVC) → left varicocele; a sudden left varicocele may signal a left renal tumour.
• Adrenal glands — on the upper poles; right adrenal vein drains into the IVC, left into the left renal vein.
• Ureters — about 25–30 cm; run down on psoas, cross the pelvic brim at the bifurcation of the common iliac artery (sacroiliac joint), then into the pelvis to the bladder.
  • Three narrowings where stones lodge — pelviureteric junction, pelvic brim (crossing the iliac vessels), vesicoureteric junction (narrowest).
  • On X-ray (KUB) — run along the tips of the lumbar transverse processes, over the sacroiliac joints, to the ischial spines.
  • "Water under the bridge" — the ureter passes under the uterine artery (women) and under the vas deferens (men) — at risk in hysterectomy.
  • Pain — renal colic radiates "loin to groin" (T11–L2) as the stone descends.
• Psoas major — retroperitoneal; lumbar plexus forms within it; psoas abscess; psoas sign (appendicitis — retrocaecal).

The Pelvis and Pelvic Organs

• Bony pelvis — two hip bones, sacrum and coccyx; the pelvic inlet (brim) divides the greater (false) pelvis from the lesser (true) pelvis.
• Male vs female pelvis — female — wider, rounder (gynaecoid) inlet, wider subpubic angle (over 80°), more everted ischial spines — adapted for childbirth.
• Pelvic floor — levator ani (pubococcygeus, puborectalis, iliococcygeus) and coccygeus — supports pelvic organs; puborectalis sling maintains the anorectal angle (continence); nerve supply S3–4 and pudendal nerve; damaged in childbirth → stress incontinence and prolapse.
• Bladder — behind the pubic symphysis; rises into the abdomen when full (palpable; suprapubic catheter site — 2–3 cm above the symphysis in the midline with a full bladder); trigone (between the ureteric orifices and internal urethral meatus); detrusor (parasympathetic S2–4 — voiding).
• Male:
  • Prostate — below the bladder, around the prostatic urethra; zones — peripheral (70% — most cancers; felt on DRE), transition (BPH), central; venous plexus connects with vertebral veins (bone metastases to the spine).
  • Male urethra — prostatic, membranous (narrowest; external sphincter; injured in pelvic fractures — blood at the meatus, high-riding prostate — don't catheterise urethrally), spongy (penile).
• Female:
  • Uterus — usually anteverted and anteflexed; supported by the pelvic floor and ligaments (uterosacral, cardinal (transverse cervical), round ligaments — the round ligament has little support role); broad ligament contains the uterine tubes, ovarian and uterine vessels.
  • Uterine artery (internal iliac) crosses above the ureter near the cervix.
  • Ovaries — on the lateral pelvic wall; ovarian artery from the aorta (via the suspensory ligament); lymph to para-aortic nodes; ovarian pain may refer to the medial thigh (obturator nerve).
  • Female urethra — short (about 4 cm) — UTIs more common.
• Rectum — about 12–15 cm; no taeniae; supported by the mesorectum (total mesorectal excision for rectal cancer); digital rectal examination — prostate (men) or cervix (women) anteriorly; rectouterine/rectovesical pouch above.
• Internal iliac artery — supplies the pelvic organs, perineum, gluteal region; ligated or embolised for pelvic haemorrhage (PPH, trauma).

The Perineum and Genital Anatomy

• Perineum — the diamond-shaped region below the pelvic floor, between the thighs — divided by a line between the ischial tuberosities into the urogenital triangle (anterior) and the anal triangle (posterior).
• Perineal body — fibromuscular central point — supports the pelvic floor; damaged in childbirth (perineal tears — 1st to 4th degree; 3rd and 4th involve the anal sphincter); episiotomy is mediolateral to avoid it.
• Anal canal — about 4 cm; internal sphincter (smooth muscle — involuntary — sympathetic) and external sphincter (skeletal — voluntary — pudendal nerve, S2–4); pectinate line (see above).
• Anorectal conditions:
  • Haemorrhoids — vascular cushions (at 3, 7 and 11 o'clock in the lithotomy position) — internal (painless bleeding) and external.
  • Anal fissure — tear below the pectinate line (usually posterior midline) — painful defecation.
  • Perianal abscess and fistula — anal glands; Goodsall's rule.
  • Ischioanal fossa — fat-filled space beside the anal canal (ischiorectal abscess).
• Pudendal nerve (S2–4) — passes around the ischial spine through Alcock's canal — external anal and urethral sphincters, perineal skin, penis/clitoris — pudendal block in labour.
• Testis:
  • Develops in the abdomen and descends through the inguinal canal (gubernaculum) — undescended testis (orchidopexy — cancer and fertility risk).
  • Testicular artery from the aorta (L2); venous drainage via the pampiniform plexus → testicular vein (left into the left renal vein — varicoceles commoner on the left); lymph drains to para-aortic nodes (not inguinal — scrotal skin drains to inguinal nodes).
  • Coverings from the abdominal wall; tunica vaginalis (hydrocele).
  • Testicular torsion — "bell-clapper" deformity; pain may refer to the lower abdomen (T10); cremasteric reflex (L1–2 — genitofemoral) absent.
• Penis — corpora cavernosa (erectile — parasympathetic, nitric oxide) and corpus spongiosum (urethra); priapism (sickle cell, drugs).
• Female external genitalia — vulva; Bartholin's glands (4 and 8 o'clock — cysts and abscesses); vulval lymph to the superficial inguinal nodes.