Thorax & Respiratory System

🫀 Chest wall & mammary gland

          Mammary gland – modified sweat gland, subcutaneous. Fat determines size/shape. Suspensory (Cooper) ligaments attach skin to deep fascia; tumor infiltration causes skin dimpling (peau d'orange).
        

Vascular supply

Internal thoracic artery (subclavian branch) → medial breast.
Lateral thoracic artery (axillary branch) → lateral breast, runs with long thoracic nerve (superficial to serratus anterior).

Lymphatic drainage (key for metastasis)

75% → axillary nodes (especially pectoral group) from nipple & upper/lateral/inferior quadrants.
Medial quadrant → parasternal nodes (along internal thoracic vessels); can spread to contralateral breast.

Clinical During radical mastectomy, long thoracic nerve injury → winged scapula + weakness in arm abduction above 90°. Thoracodorsal nerve damage → weakness in extension & medial rotation of arm.

🧬 Embryology – lower respiratory system

Timeline & origin

Week 4: respiratory (laryngotracheal) diverticulum from ventral foregut endoderm.
Endoderm → epithelium; mesoderm → cartilage, muscle, CT.
Distal enlargement → lung bud → bifurcation into 2 bronchial buds → branching to main, secondary, tertiary bronchi by month 6.
Tertiary bronchi correspond to bronchopulmonary segments.

Tracheoesophageal septum

Separates trachea from esophagus.
Defective septum → tracheoesophageal fistula (most common: distal third of trachea to esophagus).
Associated with esophageal atresia, polyhydramnios, regurgitation, choking/cyanosis after feeding, abdominal distention, pneumonitis.

⚠️ Pulmonary hypoplasia causes: congenital diaphragmatic hernia (left lung affected) & bilateral renal agenesis (oligohydramnios → Potter sequence).

🫁 Pleura & pleural cavity

Serous membranes: parietal (outer) and visceral (inner). Pleural cavity = potential space with thin serous fluid for friction-free movement.

Parietal pleura – regional parts

Costal – lines ribs & intercostal spaces.
Diaphragmatic – covers thoracic surface of diaphragm.
Mediastinal – covers mediastinum, reflects at hilum to become visceral.
Cervical – extends above 1st rib (cupula).

Innervation

Parietal: intercostal nerves (costal & peripheral diaphragmatic) → local dermatomal pain.
Central diaphragmatic & mediastinal pleura: phrenic nerve (C3–C5) → referred shoulder pain.
Visceral pleura: autonomic visceral afferents.

Pleural reflections & recesses

Sternal reflection: costal → mediastinal (costal cartilages 2–4).
Costodiaphragmatic recess: space between costal & diaphragmatic pleura, lungs do not occupy except deep inspiration.
Costomediastinal recess: left side, due to cardiac notch; occupied by lingula during inspiration.

Landmark	Visceral pleura (lung)	Parietal pleura
Midclavicular	6th rib	8th rib
Midaxillary	8th rib	10th rib
Paravertebral	10th rib	12th rib

Pneumothorax Open: air enters via chest wall wound, mediastinum shifts away during inspiration, back during expiration. Tension: flap-valve mechanism, air trapped → mediastinal shift worsens, compresses opposite lung & reduces venous return – life-threatening.

Lungs – surfaces, lobes, lymph

Surfaces

Costal – smooth, convex, adjacent to ribs.
Mediastinal – concave, contains hilum and cardiac impression (deeper on left).
Diaphragmatic (base) – rests on diaphragm; right side higher due to liver.

Lobes & fissures

Right lung: 3 lobes (superior, middle, inferior) – horizontal & oblique fissures.
Left lung: 2 lobes (superior, inferior) – oblique fissure; lingula corresponds to right middle lobe.

Lymphatic drainage

Superficial plexus (under visceral pleura) & deep plexus (along bronchi).
Bronchopulmonary (hilar) nodes → receive from both plexuses.
Tracheobronchial nodes → at bifurcation → bronchiomediastinal trunks.
Left lower lobe can drain to right bronchiomediastinal trunk → right lymphatic duct (important for metastasis).

              High yield Right main bronchus is shorter, wider, more vertical → aspirated foreign bodies more often enter right lung (posterior basal segment of inferior lobe).
            

Pancoast tumor

Apical lung tumor → thoracic outlet syndrome (compression of subclavian vessels/brachial plexus).

🔬 Respiratory histology

Lung surface area ~120 m². Air-blood barrier: surfactant + type I pneumocyte + shared basal lamina + capillary endothelium (thickness ~0.1–0.5 µm).

Trachea & bronchi – epithelium

Pseudostratified ciliated columnar with goblet cells (mucous), basal cells (stem cells), brush cells, neuroendocrine (Kulchitsky) cells.
Cilia beat to move mucous toward pharynx (mucociliary escalator).
Goblet cells & submucosal glands produce mucous; traps dust/pathogens.

⚠️ Cystic fibrosis: thick mucous due to defective chloride transport → impaired mucociliary clearance → recurrent infections.

Bronchioles

No cartilage, no glands. Epithelium: ciliated simple cuboidal/columnar.
Clara cells (bronchiolar secretory): nonciliated, secrete surfactant-like fluid, detoxify, stem cells; abundant in terminal bronchioles (~80% of cells).
Terminal bronchioles → last conducting airways; then respiratory bronchioles (with alveoli in walls) → alveolar ducts & sacs.

          COPD Emphysema (loss of elastic fibers) vs asthma (reversible airway narrowing). Emphysema – irreversible; asthma – reversible.
        

Trachea, bronchi & bronchioles – structural comparison

Feature	Trachea	Bronchi	Bronchioles
Epithelium	Pseudostratified ciliated columnar, goblet cells	Pseudostratified → simple columnar	Simple cuboidal/columnar, ciliated + Clara cells
Cartilage	16–20 C-shaped rings	Irregular plates	None
Glands	Seromucous	Fewer seromucous	None
Smooth muscle	Between open ends of C-rings	Prominent	Highest proportion (elastic fibers)

Epithelial cell types

Columnar – 200–300 cilia, microvilli.
Goblet – secrete mucin.
Basal – stem cells.
Neuroendocrine (Kulchitsky) – APUD, DNES; at airway branch points; can give rise to bronchial carcinoid.

          Squamous metaplasia Chronic irritation (smoking) → ciliated columnar → squamous (reversible if stimulus removed). Kartagener syndrome – dynein defect → immotile cilia → recurrent respiratory infections, male infertility.
        

Alveolar ducts, sacs & alveoli

Alveoli constitute 80–85% lung volume. ~300 million alveoli, each ~200 µm diameter.

Type I pneumocytes

Cover 90–95% of alveolar surface, but only 40% of cells.
Very thin (squamous) – main site of gas exchange.
Post-mitotic (cannot divide).

Type II pneumocytes

60% of cells, cover 5–10% of surface.
Produce surfactant (phospholipid/protein mixture) that reduces surface tension, prevents alveolar collapse.
Stem cells for type I & type II.
Contain lamellar bodies (myelin figures).

          Alveolar wall – rich capillary network (pulmonary arteries). Contains fibroblasts, macrophages, myofibroblasts, mast cells. Collagen (type I in bronchi/bronchioles) & elastic fibers (recoil). Pores of Kohn – collateral ventilation (10–15 µm).
        

Alveolar air → Surfactant → Type I cell → Basal lamina (shared) → Capillary endothelium → Blood

Respiratory distress syndrome Surfactant deficiency (prematurity, maternal diabetes, asphyxia). Leads to atelectasis, hyaline membrane disease (eosinophilic fluid). Corticosteroids accelerate surfactant synthesis; insulin antagonizes this effect.

⚕️ Clinical pearls & high-yield summary

Intercostal spaces

VAN (vein–artery–nerve) in costal groove; nerve inferior.
Thoracentesis: lower part of space to avoid neurovascular bundle.
Intercostal nerve block: upper part of space.

Diaphragm & pleura

Central diaphragmatic pleura innervated by phrenic nerve (C3–C5) → referred shoulder pain.
Costal pleura → intercostal nerves → local chest pain.

Embryology recall

Tracheoesophageal fistula: most common type (90%) – distal esophagus to trachea.
Congenital diaphragmatic hernia → left lung hypoplasia.
Bilateral renal agenesis → oligohydramnios → Potter sequence → pulmonary hypoplasia.

Lung auscultation

Upper lobes: anterior chest above 4th rib (right) / left similar.
Right middle lobe: anterior chest below 4th rib, medial.
Inferior lobes: posterior chest.

          High-yield facts
          Pancoast tumor – apical lung cancer → Horner syndrome (if sympathetic chain involved) + thoracic outlet.
Mesothelioma – malignant pleural tumor linked to asbestos.
Kulchitsky cells – neuroendocrine → carcinoid tumors (bronchial carcinoid).
Clara cells – stem cells, detoxification, chloride transport; defective in cystic fibrosis.
Emphysema – loss of elastic fibers → air trapping, irreversible.
Asthma – reversible bronchoconstriction.

        

🫀 Chest wall & mammary gland −