Thorax & Abdomen Viscera and Vessels
1. What this chapter covers, and how NEET PG actually tests it
The thorax and abdomen contain most of the body's organs, and NEET PG cannot test them descriptively.
What it tests is the vascular and embryological logic that connects a structure to its clinical behaviour.
Four such logics account for most questions in this region.
Which coronary artery supplies which wall, and therefore which ECG leads change. Why the right main bronchus receives nearly everything that is aspirated. Why gut pain is felt in the midline at a level that has nothing to do with where the organ sits. And where portal blood escapes when the liver obstructs it.
Each of these is a rule that generates many answers, rather than a fact that answers one question.
This chapter covers the heart and its blood supply, the lungs, pleura and mediastinum, the gut tube and its three arteries, and the abdominal wall with the portal system.
| In scope here | Deliberately out of scope |
|---|---|
| Coronary territories, cardiac septation defects, fetal shunt remnants | Detailed echocardiographic technique |
| Bronchial anatomy, pleural recesses, mediastinal contents, diaphragm | Respiratory physiology and lung volumes (see Physiology) |
| Foregut, midgut and hindgut supply with referred pain rules | Detailed gastrointestinal pathology (see Pathology and Surgery) |
| Inguinal canal, hernias, portosystemic anastomoses | Operative hernia repair technique |
2. The heart
2.1 Coronary territories, and the leads that show them
The coronary arteries have a fixed distribution, and it maps directly onto the ECG.
| Artery | Supplies | Leads |
|---|---|---|
| Left anterior descending | Anterior wall, anterior two-thirds of the interventricular septum, apex | V1 to V4 |
| Left circumflex | Lateral wall of the left ventricle | I, aVL, V5, V6 |
| Right coronary | Right ventricle, inferior wall of the left ventricle | II, III, aVF |
The left anterior descending is the most frequently occluded artery, and its territory includes the septum, which is why anteroseptal infarction is both common and dangerous.
Dominance is defined by which artery gives off the posterior descending artery. The right coronary does so in roughly 85% of people, making right dominance the usual pattern.
That definition is worth holding precisely, because questions test the definition rather than the frequency.
2.2 Why inferior infarcts cause bradycardia
The sinoatrial node is supplied by the right coronary artery in about 60% of people, and the atrioventricular node by the right coronary in the great majority of right-dominant hearts.
So an inferior infarct, which is a right coronary event, commonly produces bradycardia and atrioventricular block.
This is a genuinely useful bedside inference rather than a memorised association, and it explains why heart block after an inferior infarct is often transient and responsive to atropine.
Right coronary occlusion may also infarct the right ventricle, producing hypotension with clear lung fields — a state made worse rather than better by nitrates, since the failing right ventricle depends on preload.
2.3 Septation defects and the fetal shunts
The interatrial and interventricular septa each form by the fusion of separate components, and a failure of fusion leaves a defect.
Ostium secundum defect is the commonest atrial septal defect, arising from excessive resorption of septum primum or deficient septum secundum.
Ostium primum defect sits lower, involves the endocardial cushions, and is the type associated with Down syndrome and atrioventricular valve abnormalities.
The commonest ventricular septal defect is in the membranous part, where the muscular septum must meet endocardial cushion tissue — the last region to close and therefore the most likely to fail.
Fetal circulation depends on three shunts, and each leaves a named adult remnant.
| Fetal structure | Adult remnant |
|---|---|
| Ductus venosus | Ligamentum venosum |
| Umbilical vein | Ligamentum teres of the liver |
| Foramen ovale | Fossa ovalis |
| Ductus arteriosus | Ligamentum arteriosum |
A patent ductus arteriosus produces a continuous machinery murmur, because the pressure gradient between aorta and pulmonary artery persists through both systole and diastole.
Note that the ligamentum arteriosum is also where the left recurrent laryngeal nerve hooks, tying this table back to the thyroid anatomy of the head and neck chapter.
2.4 The pericardium and tamponade
The fibrous pericardium is tough and inelastic, which is the entire basis of cardiac tamponade.
Because it cannot stretch acutely, even a small rapidly accumulating effusion raises intrapericardial pressure steeply and prevents diastolic filling.
The result is Beck's triad of hypotension, raised jugular venous pressure and muffled heart sounds, with pulsus paradoxus.
A slowly accumulating effusion may reach a much larger volume without tamponade, because the pericardium has time to stretch.
3. Lungs, pleura and the mediastinum
3.1 Why aspiration goes to the right
The right main bronchus is wider, shorter and more vertical than the left.
So an aspirated foreign body, or vomit, preferentially enters the right lung — a fact that is asked in both anatomy and emergency medicine contexts.
Where it lands then depends on posture at the time, which is the detail that separates a good answer from a partial one.
If the patient was supine, material passes to the posterior segment of the right upper lobe or the superior segment of the right lower lobe.
If the patient was upright, it passes to the basal segments of the right lower lobe.
This posture dependence explains why aspiration pneumonia in a bedbound patient occupies different segments from that in an ambulant one.
3.2 Pleural recesses and where to put a needle
The pleural cavity extends below the lung, and the costodiaphragmatic recess is the deepest of these potential spaces.
Fluid collects there first, which is why it is the target for thoracocentesis.
The needle is inserted along the upper border of the rib below, because the intercostal neurovascular bundle runs in the costal groove on the lower border of the rib above.
The bundle's order within the groove, from above downward, is vein, artery, nerve. Aiming for the upper border of the lower rib keeps the needle away from all three.
3.3 The mediastinum and the thoracic duct
The mediastinum is divided into superior and inferior, the latter into anterior, middle and posterior compartments.
The middle mediastinum holds the heart and pericardium. The posterior mediastinum holds the descending aorta, oesophagus, thoracic duct and azygos system.
The thoracic duct drains all lymph except that from the right upper quadrant of the body, entering the venous system at the junction of the left subclavian and internal jugular veins.
Its injury during thoracic or neck surgery therefore produces a chylothorax, and the milky effusion's chemistry distinguishes it from other exudates.
An anterior mediastinal mass has a memorable differential in the four Ts: thymoma, teratoma, thyroid (retrosternal), and terrible lymphoma.
3.4 The diaphragm
Three major structures pass through the diaphragm at three levels, and the numbers are worth learning as a set.
| Level | Structure |
|---|---|
| T8 | Inferior vena cava, with the right phrenic nerve |
| T10 | Oesophagus, with the vagal trunks |
| T12 | Aorta, with the thoracic duct and azygos vein |
A useful hook is that each structure's name has as many letters as its vertebral level: "vena cava" has eight, "oesophagus" has ten, and "aortic hiatus" has twelve.
The diaphragm's motor supply is entirely the phrenic nerve, from C3, C4 and C5 — the source of the saying that they keep the diaphragm alive.
That segmental origin explains diaphragmatic referred pain. Irritation of the diaphragmatic peritoneum, from a subphrenic abscess or blood after splenic rupture, is felt in the shoulder tip, which shares the C4 dermatome.
4. The gut tube and its three arteries
4.1 Three divisions, three arteries, three pain zones
The primitive gut divides into foregut, midgut and hindgut, each with its own artery and its own referred pain level.
| Division | Extent | Artery | Referred pain |
|---|---|---|---|
| Foregut | Oesophagus to the second part of the duodenum | Coeliac trunk | Epigastrium |
| Midgut | Second duodenum to two-thirds along the transverse colon | Superior mesenteric | Periumbilical |
| Hindgut | Distal transverse colon to upper rectum | Inferior mesenteric | Suprapubic |
Visceral pain is poorly localised and referred to the midline at the level of the organ's embryonic origin, not its final position.
That is why early appendicitis hurts around the umbilicus even though the appendix sits in the right iliac fossa — the appendix is a midgut derivative.
4.2 Why appendicitis pain moves
The shift of appendicitis pain is one of the clearest examples of two different pain systems in sequence.
Initially the inflamed appendix generates visceral pain, carried by autonomic afferents, referred to the periumbilical region.
As inflammation progresses and reaches the parietal peritoneum of the abdominal wall, somatic afferents are recruited.
Somatic pain is precisely localised, so the pain migrates to McBurney's point, at the junction of the lateral and middle thirds of a line from the anterior superior iliac spine to the umbilicus.
The migration itself is diagnostic, because it reports the transition from visceral to parietal involvement rather than a change in the disease's location.
The appendix's position is variable, most often retrocaecal, then pelvic, which explains why a retrocaecal appendix may irritate the psoas and give a positive psoas sign.
4.3 Watershed areas
Where two arterial territories meet, perfusion is marginal and ischaemia strikes first.
The splenic flexure lies at the junction of superior and inferior mesenteric supply, and the rectosigmoid junction at the junction of inferior mesenteric and internal iliac supply.
These are the classic sites of ischaemic colitis, and the anatomy is the reason.
4.4 The biliary triangle
Calot's triangle is bounded by the cystic duct, the common hepatic duct, and the inferior surface of the liver.
It contains the cystic artery, and it is dissected in every cholecystectomy.
The clinically important variation is a right hepatic artery running within or close to the triangle, which can be mistaken for the cystic artery and ligated in error.
5. The abdominal wall and the portal system
5.1 The inguinal canal and the two hernias
The inguinal canal transmits the spermatic cord in men and the round ligament in women.
The deep ring is a defect in the transversalis fascia, lateral to the inferior epigastric vessels. The superficial ring is a defect in the external oblique aponeurosis.
The relationship to the inferior epigastric vessels is the single fact that classifies a groin hernia.
An indirect hernia passes through the deep ring, therefore lateral to the inferior epigastric vessels, and follows the canal. It is congenital in origin, from a patent processus vaginalis, and is commoner in the young.
A direct hernia pushes through the weak posterior wall of the canal in Hesselbach's triangle, therefore medial to the vessels. It is acquired, from wall weakness, and commoner with age.
Hesselbach's triangle is bounded by the inferior epigastric vessels laterally, the rectus sheath medially, and the inguinal ligament inferiorly.
A femoral hernia passes below the inguinal ligament through the femoral canal, medial to the femoral vein. It is commoner in women and more likely to strangulate, because the femoral ring is narrow and rigid.
5.2 Portal hypertension and where the blood goes
The portal vein is formed behind the neck of the pancreas by the union of the splenic and superior mesenteric veins.
Portal blood has no valves, so when hepatic resistance rises it reverses into any communication with the systemic circulation.
Those communications are anatomically fixed, which is why the clinical consequences of portal hypertension are so predictable.
| Site | Portal tributary meets systemic vein | Clinical result |
|---|---|---|
| Lower oesophagus | Left gastric meets oesophageal | Oesophageal varices |
| Umbilicus | Paraumbilical meets epigastric | Caput medusae |
| Anal canal | Superior rectal meets middle and inferior rectal | Anorectal varices |
| Retroperitoneum | Colic meets retroperitoneal veins | Retroperitoneal varices |
Anorectal varices are not haemorrhoids, and the distinction is examinable. Haemorrhoids are dilated normal anal cushions and are far commoner; varices at this site are a specific consequence of portal hypertension.
The watershed at the dentate line also determines everything else about the anal canal. Above it, the epithelium is columnar, sensation is visceral, drainage is portal, and lymph goes to internal iliac nodes.
Below it, the epithelium is squamous, sensation is somatic, drainage is systemic, and lymph goes to superficial inguinal nodes.
That is why a lesion below the dentate line is painful and one above it is not, and why the two spread to entirely different nodal groups.
Worked clinical vignettes
Q1. A 58-year-old man has chest pain with ST elevation in leads II, III and aVF. His heart rate is 42 per minute with second-degree atrioventricular block. Which artery is occluded, and why the bradycardia?
Pick an option to check your answer.
Show explanation
Solution. Leads II, III and aVF are the inferior leads, which is right coronary territory.
The bradycardia and block are not coincidental. The right coronary supplies the sinoatrial node in about 60% of people and the atrioventricular node in most right-dominant hearts, so an inferior infarct commonly produces conduction disturbance.
Answer: (b).
Q2. A 30-year-old presents with periumbilical pain that over six hours shifts to the right iliac fossa and becomes sharply localised. What explains the migration?
Pick an option to check your answer.
Show explanation
Solution. The appendix is a midgut derivative, so its visceral pain is referred to the periumbilical midline regardless of where the organ physically lies.
Once inflammation reaches the parietal peritoneum, somatic afferents take over, and somatic pain is precisely localised — hence the shift to McBurney's point.
The migration therefore reports a change in which pain system is active, and is itself diagnostic. Answer: (b).
Q3. During elective hernia repair, the sac is found to emerge medial to the inferior epigastric vessels. What is the diagnosis and its usual mechanism?
Pick an option to check your answer.
Show explanation
Solution. The relationship to the inferior epigastric vessels is definitive. Medial means the sac has pushed through Hesselbach's triangle in the posterior wall, which is a direct hernia.
Direct hernias are acquired from wall weakness and are commoner with age, whereas indirect hernias traverse the deep ring lateral to the vessels and are congenital in origin.
(c) A femoral hernia lies below the inguinal ligament, not within the canal. Answer: (b).
7. Common exam traps
- Defining dominance by size rather than by the posterior descending artery. Dominance is determined by which artery gives off the posterior descending artery, right in roughly 85% of people.
- Forgetting the conducting system's supply in inferior infarcts. Bradycardia and block with inferior ST elevation are anatomically expected, not incidental.
- Giving nitrates freely in right ventricular infarction. The failing right ventricle is preload-dependent, so nitrates worsen hypotension.
- Placing the needle below the rib in thoracocentesis. The neurovascular bundle lies in the costal groove on the lower border of the rib above, so the needle goes along the upper border of the rib below.
- Ignoring posture in aspiration questions. Supine aspiration reaches the posterior segment of the right upper lobe or the superior segment of the right lower lobe; upright aspiration reaches the basal segments.
- Localising visceral pain to the organ's actual position. Referred pain follows the embryonic division, which is why appendicitis begins periumbilically.
- Classifying a groin hernia by where it appears externally. Only the relationship to the inferior epigastric vessels distinguishes direct from indirect.
- Calling anorectal varices haemorrhoids. They are distinct entities with different causes, and only the varices indicate portal hypertension.
Summary
- Coronary territories map onto the ECG: left anterior descending to V1-V4, circumflex to I, aVL, V5 and V6, right coronary to II, III and aVF.
- Dominance is defined by the origin of the posterior descending artery, which is the right coronary in roughly 85% of people.
- The right coronary supplies the sinoatrial node in about 60% and the atrioventricular node in most right-dominant hearts, so inferior infarcts cause bradycardia and block.
- Ostium secundum is the commonest atrial septal defect; ostium primum involves the endocardial cushions and links to Down syndrome; membranous defects are the commonest ventricular type.
- Fetal shunt remnants are ligamentum venosum, ligamentum teres, fossa ovalis and ligamentum arteriosum, the last also being where the left recurrent laryngeal nerve hooks.
- The pericardium is inelastic, so a small rapid effusion causes tamponade while a slow one may not.
- The right main bronchus is wider, shorter and more vertical, so aspiration goes right, with the segment determined by posture at the time.
- The costodiaphragmatic recess is the deepest pleural space and the thoracocentesis target; insert along the upper border of the rib below, since the bundle runs vein, artery, nerve in the groove above.
- The thoracic duct drains all lymph except the right upper quadrant, so its injury causes chylothorax; anterior mediastinal masses are the four Ts.
- Diaphragmatic openings are T8 inferior vena cava, T10 oesophagus, T12 aorta, with phrenic supply from C3 to C5 explaining shoulder-tip referred pain.
- Foregut, midgut and hindgut have the coeliac, superior mesenteric and inferior mesenteric arteries, with pain referred to epigastrium, umbilicus and suprapubic region respectively.
- Appendicitis pain migrates because visceral referred pain gives way to precisely localised somatic pain once the parietal peritoneum is involved.
- The splenic flexure and rectosigmoid junction are watershed areas and the classic sites of ischaemic colitis.
- Calot's triangle contains the cystic artery, and an aberrant right hepatic artery within it is the dangerous variation in cholecystectomy.
- Indirect hernias pass lateral to the inferior epigastric vessels and are congenital; direct hernias pass medial through Hesselbach's triangle and are acquired; femoral hernias lie below the inguinal ligament and strangulate more readily.
- Portosystemic anastomoses at the lower oesophagus, umbilicus, anal canal and retroperitoneum explain varices and caput medusae, and the dentate line divides the anal canal into painful somatic and painless visceral halves with different lymphatic drainage.
