Q2 LONG ANSWER 10M ★ High-yield

Describe the histology of the trachea and the bronchial tree. Describe the alveolus and the blood–air barrier.

Anatomy · 1st Professional — Pre-Clinical

Portion	Parts	Function
Conducting	Nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, terminal bronchioles	Conducts, warms, humidifies and cleans the air; the anatomical dead space
Respiratory	Respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli	Gas exchange

Epithelium — pseudostratified ciliated columnar with goblet cells, the "respiratory epithelium"
A distinct thick basement membrane
Lamina propria with elastic tissue; submucosa with seromucous glands
15–20 C-shaped rings of hyaline cartilage, deficient posteriorly where the trachealis (smooth muscle) bridges the gap
Adventitia outside

Structure	Epithelium	Cartilage	Smooth muscle	Glands	Goblet cells
Trachea	Pseudostratified ciliated columnar	C-shaped rings	Trachealis only	Present	Present
Bronchus	Pseudostratified ciliated columnar	Irregular plates	Complete spiral layer	Present	Present
Bronchiole	Simple ciliated columnar → cuboidal	Absent — the defining feature	Relatively prominent	Absent	Few, then absent
Terminal bronchiole	Simple cuboidal with CLARA (club) cells	Absent	Present	Absent	Absent
Respiratory bronchiole	Cuboidal, interrupted by alveoli	Absent	Scanty	Absent	Absent

Cell	Share of cells	Share of surface	Function
Type I pneumocyte	About 40%	About 95%	Extremely thin squamous cell — gas exchange; cannot divide
Type II pneumocyte	About 60%	About 5%	Cuboidal, with lamellar bodies; secretes surfactant; the stem cell that replaces type I after injury
Alveolar macrophage (dust cell)	Variable	—	Phagocytosis; in heart failure they contain haemosiderin and are called "heart failure cells"

Alveolar pores of Kohn connect adjacent alveoli and allow collateral ventilation
Elastic fibres in the alveolar wall provide the recoil of expiration

Secreted by type II pneumocytes from about the 24th week, adequate by 34–36 weeks
Chiefly dipalmitoyl phosphatidylcholine (lecithin) with surfactant proteins
Reduces surface tension, so it prevents alveolar collapse, reduces the work of breathing, and keeps small alveoli from emptying into large ones (Laplace)
Also keeps the alveoli dry, by opposing the transudation of fluid

Smoking paralyses and then destroys the cilia, which is why the escalator fails and a smoker must cough to clear secretions — worst in the morning after a night without coughing
Coal dust and silica are in the 1–5 micrometre range, which is precisely why they reach and damage the alveoli in pneumoconiosis

Region	Epithelium	Distinguishing feature
Vestibule of the nose	Stratified squamous keratinised	Vibrissae and sebaceous glands
Respiratory mucosa of the nose	Pseudostratified ciliated columnar	Very rich venous plexus (swell bodies) over the conchae, for warming
Olfactory mucosa	Pseudostratified with bipolar olfactory neurones, sustentacular and basal cells	Yellow-brown; serous BOWMAN glands; the only neurones exposed to the exterior and capable of replacement
Epiglottis — lingual surface	Stratified squamous	Elastic cartilage core
Epiglottis — laryngeal surface	Respiratory epithelium	Transition visible in one section
True vocal cord	Stratified squamous non-keratinised	Avascular; dense vocal ligament; NO glands
False vocal cord	Respiratory epithelium	Seromucous glands present
Pleura	Mesothelium — simple squamous	Thin layer of connective tissue with elastic fibres

The true cord is covered by stratified squamous epithelium and has no glands, which is why it appears pearly white at laryngoscopy and why carcinoma there is squamous
Olfactory neurones are replaced throughout life from basal cells, unique among neurones, and this is why smell can recover after a viral infection

Respiratory distress syndrome of the newborn — surfactant deficiency in prematurity; treated with exogenous surfactant, and prevented by antenatal steroids that accelerate type II cell maturation. The lecithin:sphingomyelin ratio in amniotic fluid predicts maturity
Emphysema — destruction of the alveolar walls and elastic tissue, so the surface area falls and the airways collapse on expiration; 1-antitrypsin deficiency and smoking are the causes
Asthma — bronchiolar smooth muscle hypertrophy, mucus plugging, thickened basement membrane and eosinophilic infiltration
Chronic bronchitis — hypertrophy of the submucosal glands, measured as the Reid index, with goblet cell hyperplasia
Ciliary dyskinesia — a defect of the dynein arms; mucus is not cleared, causing bronchiectasis and sinusitis
Type I pneumocytes cannot divide, so all repair depends on type II cells — and their proliferation after injury is what produces the thickened alveolar wall of fibrosis
Pneumoconiosis — silica and coal dust in the 1–5 micrometre range reach the alveoli, are taken up by macrophages, and provoke progressive fibrosis
Bronchial carcinoma — squamous cell carcinoma arises centrally from metaplastic bronchial epithelium, adenocarcinoma peripherally, and small cell carcinoma from neuroendocrine cells

All questions in Systemic Histology