🧪 Surfactant & Alveolar Wall
🧪 Surfactant function — lowers surface tension at the air‑water interface, prevents alveolar collapse (atelectasis) during expiration. Produced by type II pneumocytes, composed of phospholipids (mainly dipalmitoylphosphatidylcholine) and surfactant proteins (SP-A, B, C, D).
- Fetal production: begins late in gestation; corticosteroids accelerate synthesis. High fetal insulin (e.g., diabetic mothers) inhibits this effect → ↑ risk of neonatal respiratory distress syndrome.
- Recycling: most surfactant is re-uptaken by type II cells; a portion is cleared by alveolar macrophages.
- Alveolar wall components: rich capillary network (from pulmonary arteries), fibroblasts, myofibroblasts, smooth muscle, mast cells. Elastic and collagen fibers (type I & II) contribute to lung recoil and compliance.
💡 Clinical pearl: Corticosteroids given antenatally accelerate surfactant maturation; infants of diabetic mothers have higher RDS risk due to insulin antagonism.
🫁 Alveolar Macrophages
- Origin: monocytes that migrate from pulmonary vessels; can also proliferate locally via mitosis.
- Location: alveolar septa and alveolar spaces; ∼1–3 per alveolus, 15–40 µm diameter.
- Functions: phagocytosis of debris, pathogens, dust (hence “dust cells”), and in heart failure, phagocytose extravasated erythrocytes (“heart failure cells”).
- Clearance: migrate toward bronchioles, enter mucus, are swept to pharynx and swallowed, or enter lymphatics.
🧬 Additional: Macrophages also participate in immune surveillance and surfactant catabolism.
❤️ Heart tube & fetal circulation
Cardiogenic development: begins week 3 from splanchnic mesoderm. Neural crest cells migrate into the heart and are critical for septation and outflow tract formation. Two primordial tubes fuse into a single tube, which undergoes dextral looping (rightward bend).
Primitive dilatations → adult structures
- Truncus arteriosus → aorta & pulmonary trunk, semilunar valves
- Bulbus cordis → smooth parts of RV (conus) and LV (aortic vestibule)
- Primitive ventricle → trabeculated parts of both ventricles
- Primitive atrium → trabeculated atria (pectinate muscles)
- Sinus venosus → smooth part of right atrium (sinus venarum) & coronary sinus
Fetal shunts (bypass)
- Ductus venosus — bypasses liver; connects umbilical vein to IVC.
- Foramen ovale — right → left atrial shunt (bypasses lungs).
- Ductus arteriosus — pulmonary trunk → aorta (bypasses lungs).
pressure Fetal: R > L (right‑to‑left shunting). Postnatal: L > R → shunts close.
Postnatal closure & remnants
| Structure | Remnant |
|---|---|
| Umbilical arteries | Medial umbilical ligaments |
| Umbilical vein | Ligamentum teres |
| Ductus venosus | Ligamentum venosum |
| Foramen ovale | Fossa ovalis |
| Ductus arteriosus | Ligamentum arteriosum |
🧩 Septation of the heart
Atrial septation
- Septum primum grows from roof toward endocardial cushions; foramen primum narrows then closes, while foramen secundum forms in septum primum to maintain right‑to‑left shunt.
- Septum secundum grows to the right, overlaps foramen secundum, leaving the foramen ovale (functional shunt).
- Closure at birth: increased left atrial pressure (from pulmonary flow) and decreased right atrial pressure (umbilical vein closure) press septum primum against septum secundum.
Ventricular septation
- Muscular IV septum grows from floor of ventricle, leaving the interventricular foramen.
- Membranous IV septum closes the foramen, formed by fusion of right/left conotruncal ridges and endocardial cushions (neural crest contribution).
⏳ Timeline: septation begins week 4, mostly complete by week 8.
🔄 Common congenital shunts
Atrial septal defect (ASD)
- Secundum (most common) — excessive resorption of septum primum or small septum secundum; central defect.
- Primum — failure of septum primum to fuse with endocardial cushions; associated with AV valve and membranous VSD defects.
- shunt left‑to‑right (non‑cyanotic).
Ventricular septal defect (VSD)
- Most common congenital heart defect (males > females).
- Membranous VSD — failure of membranous septum development (neural crest defect).
- Left‑to‑right shunt → pulmonary hypertension → Eisenmenger (late cyanosis).
Patent ductus arteriosus (PDA)
- Failure of ductus closure after birth; common in prematurity & maternal rubella.
- Left‑to‑right shunt (aorta → pulmonary trunk), machine‑like murmur.
- Prostaglandin E maintains patency; indomethacin (PGE inhibitor) promotes closure.
📌 Non‑cyanotic (L→R): ASD, VSD, PDA. Cyanotic (R→L): Tetralogy of Fallot, transposition, persistent truncus arteriosus.
🌊 Truncus arteriosus & great vessels
Septation: neural crest cells migrate into conotruncal ridges, which fuse in a spiral fashion to form the aorticopulmonary septum, dividing truncus into aorta and pulmonary trunk.
Cyanotic defects from AP septum maldevelopment
- Tetralogy of Fallot: anterosuperior displacement of septum → pulmonary stenosis, VSD, overriding aorta, RV hypertrophy. Boot‑shaped heart.
- Transposition: non‑spiral septum → aorta from RV, pulmonary trunk from LV. Two parallel circuits; requires mixing (PDA, VSD, ASD).
- Persistent truncus arteriosus: partial septation → single vessel from both ventricles, always with VSD.
📐 Mediastinum — divisions & contents
Superior
- Thymus, brachiocephalic veins, SVC, aortic arch & 3 branches, trachea, esophagus, vagus & phrenic nerves, thoracic duct.
- Left recurrent laryngeal loops under aortic arch; vulnerable to arch pathology.
Anterior
- Fat, areolar tissue, thymus (inferior). Thymoma may arise here.
Middle
- Pericardium, heart, great vessels, phrenic nerves (lateral to pericardium).
Posterior
- Descending aorta, esophagus (with plexuses), thoracic duct, azygos veins.
- Esophageal constrictions: at pharynx, aortic arch, left bronchus, diaphragmatic hiatus.
🩺 Coarctation of the aorta
- Preductal (infantile): narrowing proximal to ductus arteriosus; ductus often remains patent.
- Postductal (adult): narrowing distal to ductus; ductus closes. Collateral circulation via intercostal arteries → costal notching.
- Clinical: hypertension in upper extremities, weak pulses and hypotension in lower body.
Radiographic sign: notching of inferior rib borders from enlarged intercostals.
🧫 Pericardium
- Fibrous layer: outer tough sac.
- Serous layer: parietal (lines fibrous) and visceral (epicardium).
- Phrenic nerves lie lateral to pericardium in middle mediastinum.