Alimentary Glands & Respiratory System

The liver is the body's metabolic hub — its dual blood supply (75% portal, 25% hepatic artery) reflects its twin roles as a processor of absorbed nutrients and a detoxification centre for everything the portal circulation delivers. Its modern surgical anatomy is the Couinaud system: eight independent segments each with their own portal pedicle and hepatic venous drainage, allowing surgeons to remove individual segments without devascularising the rest. The zone (acinar) model explains injury patterns: Zone 3 (centrilobular) is most vulnerable to hypoxia and drug toxicity because it receives blood last and contains the highest concentration of CYP2E1, the enzyme that converts paracetamol to the hepatotoxin NAPQI. Portal hypertension — the end result of chronic liver disease — opens predictable portosystemic collaterals at every anatomical site where portal and systemic veins run side by side, and knowing those sites lets you explain oesophageal varices, caput medusae, and haemorrhoidal engorgement from first principles.

6.1.1 — Anatomy & Lobes

Fig. 4.102 — Surfaces of the liver and associated recesses — diaphragmatic and visceral surfaces, bare area and the porta hepatis.

Gray's Anatomy for Students, 4e

The largest gland in the body (~1.5 kg). Occupies the right hypochondrium and epigastrium, protected by ribs 7–11 on the right. The falciform ligament divides it into right lobe (larger) and left lobe (smaller) — this is the anatomical division.

Lobe / Structure	Location	Notes
Right lobe	Right hypochondrium	Largest; inferior surface = gallbladder fossa + right colic flexure + right kidney + suprarenal
Left lobe	Epigastrium	Smaller; overlies stomach
Caudate lobe	Posterior between IVC (right) and ligamentum venosum (left)	Receives blood from both right + left portal branches and both hepatic arteries; drains directly into IVC → preserved in Budd-Chiari syndrome
Quadrate lobe	Inferior surface between gallbladder fossa (right) and round ligament (left)	Functionally part of the left lobe (Couinaud segment IV)
Porta hepatis	H-shaped groove on inferior surface	Entry/exit of portal vein, hepatic artery, bile ducts. Left limb = ligamentum teres (obliterated umbilical vein); right limb = gallbladder fossa; crossbar = porta hepatis proper

The biliary tree's clinical significance is almost entirely about obstruction — where it occurs and what syndrome it produces. Gallstones cause 95% of biliary pathology, and their anatomical location determines the presentation: cystic duct obstruction gives biliary colic; a stone in the common bile duct gives cholangitis (Charcot's triad) or jaundice; ampullary obstruction gives both jaundice and pancreatitis. Calot's triangle is the most surgically consequential three-centimetre patch in abdominal anatomy — every iatrogenic bile duct injury during laparoscopic cholecystectomy traces back to failure to achieve the "critical view of safety" within it. Courvoisier's law links anatomy to clinical diagnosis: chronic gallstone disease fibroses and contracts the gallbladder wall so it cannot distend, whereas a sudden external compression from pancreatic carcinoma obstructs a previously normal, distensible gallbladder — hence painless jaundice plus palpable gallbladder = malignancy until proven otherwise.

6.2.1 — Extrahepatic Bile Ducts

Fig. 4.111 — Bile drainage. A. Duct system (right & left hepatic → common hepatic + cystic → bile duct → major papilla). B. Percutaneous transhepatic cholangiogram.

Gray's Anatomy for Students, 4e

Structure	Formation/Notes
Right + Left hepatic ducts	Emerge from porta hepatis; join immediately to form CHD
Common hepatic duct (CHD)	~3 cm; descends in free edge of lesser omentum, anterior to portal vein
Cystic duct	From gallbladder neck; spiral valve of Heister (prevents collapse); joins CHD to form CBD
Common bile duct (CBD)	~8 cm total; 4 parts: supraduodenal (in lesser omentum) → retroduodenal → pancreatic (in pancreatic head) → intraduodenal (joins pancreatic duct). Enters 2nd part of duodenum at ampulla of Vater

◆ Portal Triad (in free edge of lesser omentum)

The hepatoduodenal ligament (free right edge of lesser omentum) contains:
From left to right (anterior view): Bile duct (right) · Hepatic artery (left) · Portal vein (posterior)
Mnemonic: "ABP" — Anterior: bile duct + artery; Posterior: portal vein. Pringle's manoeuvre = digital compression of hepatoduodenal ligament between thumb (in epiploic foramen) and index finger to control liver bleeding.

Fig. 4.107 — Pancreas: head (with uncinate process), neck, body and tail, with the main pancreatic duct and relation to the duodenum and spleen.

Gray's Anatomy for Students, 4e

The pancreas is a retroperitoneal gland with dual exocrine (digestive enzyme) and endocrine (insulin, glucagon) roles, and its anatomy maps directly onto its disease patterns. The head sits in the C-loop of the duodenum and shares the common bile duct — carcinoma of the pancreatic head obstructs the CBD early, causing painless progressive jaundice (Courvoisier's gallbladder) and requiring the Whipple's pancreaticoduodenectomy that removes both structures en bloc. The neck lies directly anterior to the SMV–portal vein junction — vascular involvement at this plane is the key surgical boundary for resectability. Acute pancreatitis activates digestive enzymes within the gland itself; the retroperitoneal position explains why haemorrhagic extravasation tracks along fascial planes to the periumbilical skin (Cullen's sign via the falciform ligament) and flanks (Grey Turner's sign via retroperitoneal fat).

Three pairs of salivary glands produce up to 1.5 litres of saliva daily, and their clinical anatomy is governed by two asymmetries. The parotid is the largest but the most surgically dangerous — CN VII does not supply it, but splits into five terminal branches within it, so malignancy or surgery here threatens the whole facial nerve and produces ipsilateral facial palsy. The submandibular duct (Wharton's duct) is the longest and most tortuous, running against gravity from the floor of the mouth upward to the submandibular triangle — the "uphill" course allows saliva to pool and calcium salts to deposit, explaining why 80% of salivary calculi form here. Secretomotor innervation is parasympathetic in both cases but via different routes: parotid via CN IX (glossopharyngeal → lesser petrosal → auriculotemporal), submandibular and sublingual via CN VII (chorda tympani → lingual nerve → submandibular ganglion).

The trachea runs from the cricoid cartilage (C6) to the carina at T4/T5 — the level of the sternal angle, which is simultaneously the most important thoracic surface landmark in anatomy, marking the 2nd rib, the start of the aortic arch, and the boundary between superior and inferior mediastinum. Its 16–20 C-shaped cartilage rings leave the posterior wall membranous (trachealis muscle), which is why posterior tracheal tears are the most common complication of emergency intubation. The carina's normal angle is ~70°; widening signals left atrial enlargement (Samson's sign on CXR) or subcarinal lymphadenopathy. The recurrent laryngeal nerves ascend in the tracheo-oesophageal groove bilaterally, explaining why thyroid surgery, tracheal tumours, and oesophageal carcinoma all present with hoarseness as their first neurological symptom.

Fig. 3.47 — A. Bronchial tree (trachea → main → lobar → segmental bronchi, carina). B. A bronchopulmonary segment supplied by its own segmental bronchus and pulmonary artery branch.

Gray's Anatomy for Students, 4e

The bronchopulmonary segment is the anatomical unit that underpins modern lung surgery and bronchoscopy — each segment has its own segmental bronchus, artery, and intersegmental venous drainage, making it independently resectable without devascularising adjacent segments. The right upper lobe bronchus is eparterial (the only bronchus in the body that runs above its accompanying pulmonary artery), while all other bronchi are hyparterial — an anatomical distinction that appears in every respiratory anatomy viva. The right lower lobe is the most common site of aspiration pneumonia because the right bronchus is more vertical and gravity directs aspirated material here, especially when supine (specifically into the superior segment of the right lower lobe, the "dependent" segment).

The left lung is smaller than the right — the cardiac notch and the cardiac impression on its mediastinal surface reflect the heart's left-sided displacement — and this structural asymmetry explains why the left has only two lobes and one fissure while the right has three lobes and two fissures. Hilar asymmetry on CXR is physiological: the left hilum is always higher than the right because the left pulmonary artery arches over the left main bronchus, while on the right the bronchus is above the artery in the upper lobe region; a left hilum lower than the right is pathological (left lower lobe collapse). The lung root contains the bronchus, pulmonary artery, and two pulmonary veins (superior and inferior), and their three-dimensional arrangement differs between sides in ways that matter for thoracic surgery and radiological interpretation.

6.7.1 — Surfaces, Fissures & Lobes

Fig. 3.45 — A. Right lung. B. Major structures related to the mediastinal surface of the right lung (hilum, grooves for azygos vein, oesophagus and SVC).

Gray's Anatomy for Students, 4e

	Right Lung	Left Lung
Weight	~625 g	~565 g (smaller — cardiac notch)
Lobes	3: upper, middle, lower	2: upper (+ lingula), lower
Fissures	Oblique (major) + horizontal (minor)	Oblique (major) only
Horizontal fissure	From right anterior chest (4th costal cartilage) to oblique fissure (5th rib, midaxillary line)	Absent
Oblique fissure	From T4 (posteriorly) around to 6th rib (anteriorly)	Same orientation; from T4 to 6th rib
Lingula	—	Tongue-like projection of upper lobe (equivalent to right middle lobe)

The pleural space is normally a potential space containing just enough lubricating fluid to allow frictionless respiratory excursion — any pathological accumulation of air, blood, or fluid disrupts this and compresses the lung. The critical anatomical fact is the two-rib gap between the lower lung border and the lower pleural reflection (the costodiaphragmatic recess): the lung ends at rib 6 (MCL), rib 8 (MAL), rib 10 (posterior), while the pleura continues to rib 8, 10, and 12 respectively — this recess is where fluid collects first and from where thoracocentesis is safely performed. Parietal pleura has somatic nerve supply (intercostal + phrenic) and generates sharp localised pleuritic pain; visceral pleura has only autonomic supply and is insensate — inflammation of the lung parenchyma is therefore painless until it reaches the parietal pleura.

6.8.1 — Layers & Recesses

Fig. 3.40 — Parietal pleural reflections and recesses — note the costodiaphragmatic and costomediastinal recesses where the pleural layers meet.

Gray's Anatomy for Students, 4e

Parietal pleura: lines the thoracic wall, diaphragm, and mediastinum. Has 4 named parts: costal, diaphragmatic, mediastinal, cervical (cupola — extends 2.5 cm above medial clavicle). Innervated by somatic nerves → sharp, well-localised pleural pain. Visceral pleura: covers lung surface; innervated by autonomic → insensitive to pain; cannot be separated from lung.

Pleural Recess	Location	Clinical Significance
Costodiaphragmatic recess	Between costal and diaphragmatic pleura. Deepest at midaxillary line (rib 8 → rib 10)	Fluid first collects here on CXR (blunting of costophrenic angle — requires ~200–300 mL). Site for diagnostic pleural aspiration (thoracocentesis)
Costomediastinal recess	Between costal and mediastinal pleura anteriorly	Smaller; accomodates anterior lung expansion; relevant in anterior mediastinal procedures

The spleen lies obliquely along the 10th rib in the left hypochondrium, protected by the lower ribs — it must double in size before it becomes palpable below the left costal margin, which is why massive splenomegaly (malaria, kala-azar, myelofibrosis) is so striking when you finally feel it. As the body's largest lymphoid organ, the white pulp mounts immune responses to encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis), explaining why post-splenectomy patients face overwhelming infection from exactly these organisms and require lifelong vaccination and prophylaxis. The tail of the pancreas physically contacts the splenic hilum within the splenorenal ligament — distal pancreatectomy for tail tumours typically becomes a combined splenopancreatectomy. Kehr's sign (left shoulder pain from splenic haemorrhage) is the classic example of referred pain via the phrenic nerve (C4).

6.9.1 — Anatomy & Relations

Fig. 4.114 — Surfaces and hilum of the spleen — visceral surface impressions (gastric, renal, colic) and the hilum transmitting the splenic vessels.

Gray's Anatomy for Students, 4e

Definition

The largest lymphoid organ in the body (~150 g). Intraperitoneal in the left hypochondrium, related to the 9th–11th ribs. It is NOT normally palpable below the left costal margin — if palpable, it is at least twice its normal size.

Feature	Details
Position	Left hypochondrium; long axis along 10th rib; deep to ribs 9–11
Surfaces	Diaphragmatic (smooth, convex) + Visceral (concave): gastric impression (anteromedial), renal impression (posteromedial), colic impression (inferior pole)
Hilum	Visceral surface; splenic artery + vein enter/exit; tail of pancreas contacts the hilum
Peritoneal ligaments	Splenorenal ligament (to left kidney; contains splenic vessels + tail of pancreas) · Gastrosplenic ligament (to greater curvature; contains short gastric + left gastroepiploic vessels)
Blood supply	Splenic artery (largest branch of coeliac trunk; tortuous course along upper border of pancreas). Splenic vein drains to join SMV → portal vein behind neck of pancreas
Notch	Superior border has 1–2 notches (remnant of lobulation) — palpable in massive splenomegaly; confirms spleen vs renal mass

Jaundice is a symptom with three anatomical addresses: pre-hepatic (too much bilirubin being produced), hepatic (the liver cannot process it), or post-hepatic (the processed bilirubin cannot escape). The bilirubin pathway is the physiological skeleton that decodes all three: unconjugated bilirubin is lipid-soluble and albumin-bound so it cannot reach the urine (no urinary bilirubin in haemolytic jaundice); conjugated bilirubin is water-soluble and filtered by the glomerulus (dark urine in obstructive jaundice); stercobilin colours stools brown — pale stools mean bile is not reaching the gut. Gilbert's syndrome — a UGT1A1 promoter polymorphism causing isolated unconjugated hyperbilirubinaemia with completely normal LFTs in 5–10% of the population — is one of the most commonly missed benign conditions in medicine, and students who know the bilirubin pathway will never confuse it with hepatocellular disease.

The mediastinum is divided at the sternal angle (T4/T5) — the single most important surface landmark in thoracic anatomy — into superior and inferior compartments, with the inferior split into anterior (thymic/lymphoid), middle (cardiac/vascular), and posterior (neural/oesophageal). This compartment scheme is a clinical algorithm: the anterior "4 Ts" (thymoma, teratoma, terrible lymphoma, thyroid) account for the great majority of anterior mediastinal masses; posterior masses are almost always neurogenic. The left recurrent laryngeal nerve's intrathoracic loop under the aortic arch at the ligamentum arteriosum is the most clinically exposed cranial nerve branch in the chest — aortic aneurysm, left hilar tumour, and mediastinal lymphadenopathy all cause hoarseness by the same mechanism. The thoracic duct is the drainage route for all lymph below the diaphragm and the entire left side above it; chylothorax (milky fluid, TG >1.1 mmol/L) after oesophagectomy is the classic complication of inadvertent division.

6.11.1 — Divisions

Fig. 3.5 — Subdivisions of the mediastinum — superior mediastinum and the anterior, middle and posterior divisions of the inferior mediastinum.

Gray's Anatomy for Students, 4e

Definition

The mediastinum is the central compartment of the thoracic cavity, between the two pleural cavities. Divided by the sternal angle (angle of Louis, T4/T5 level) into superior (above) and inferior (below, further divided into anterior/middle/posterior).

Compartment	Key Contents
Superior mediastinum (above T4/T5)	Thymus (anterior) · Arch of aorta + 3 branches (brachiocephalic trunk → right common carotid + right subclavian; left common carotid; left subclavian) · Brachiocephalic veins + SVC (upper) · Trachea (ends at carina T4/5) · Oesophagus · Thoracic duct (left side) · Vagus nerves (CN X) · Left recurrent laryngeal nerve (loops under arch of aorta at ligamentum arteriosum) · Phrenic nerves (C3, C4, C5)
Anterior mediastinum (between sternum and pericardium)	Thymic remnants · Internal thoracic (mammary) vessels · Lymph nodes · Fat · Sternopericardial ligaments
Middle mediastinum	Pericardium + heart · Ascending aorta + pulmonary trunk + pulmonary veins · SVC (lower) + IVC · Phrenic nerves (run laterally through middle mediastinum) · Carina + main bronchi · Tracheal bifurcation lymph nodes
Posterior mediastinum (between pericardium and vertebral column)	Descending thoracic aorta (T5–T12) · Oesophagus + oesophageal plexus · Azygos + hemiazygos veins · Thoracic duct · Vagus nerves (reforming as oesophageal plexus) · Thoracic sympathetic chain + splanchnic nerves · Thoracic vertebral bodies