Respiratory tract sequence: External nares → Nasal cavity → Pharynx → Larynx (vocal cords) → Trachea (C-rings) → Primary bronchi → Secondary bronchi → Tertiary bronchi → Bronchioles → Alveoli.

Nasal cavity functions: warms, moistens, and filters (WWF) incoming air.

Trachea: C-shaped cartilaginous rings prevent collapse; open side faces posteriorly toward oesophagus.

Lung lobes: Right = 3; Left = 2 (cardiac notch reduces left lung size).

Alveoli: functional units of gas exchange; ~700 million; ~70 $m^{2}$ surface area.

Inspiration is ACTIVE (diaphragm + external intercostals); quiet expiration is PASSIVE (elastic recoil).

Forced expiration is ACTIVE (internal intercostals + abdominals).

Tidal Volume = 500 mL; IRV = 2500–3000 mL; ERV = 1000–1100 mL; RV = 1100–1200 mL.

VC = TV + IRV + ERV (excludes RV) ≈ 3500–4600 mL.

TLC = VC + RV ≈ 5000–6000 mL.

FRC = ERV + RV ≈ 2100–2300 mL (air after normal expiration).

RV cannot be measured by spirometry.

Gas exchange = simple diffusion across 3-layered membrane (alveolar epithelium → basement membrane → capillary endothelium).

O2 gradient at alveoli: 104 → 40 mmHg (64 mmHg); CO2 gradient: 45 → 40 mmHg (5 mmHg).

O2 transport: 97% oxyhaemoglobin + 3% dissolved in plasma.

O2–Hb curve = sigmoid (cooperative binding, 4 O2 per Hb molecule).

Bohr effect: CO2↑, pH↓ (H+↑), temperature↑, 2,3-BPG↑ → right shift → O2 released.

CO2 transport: 70% HCO3– (via carbonic anhydrase in RBCs) + 23% carbaminoHb + 7% dissolved.

Chloride shift (Hamburger's phenomenon): HCO3– exits RBC → Cl– enters RBC.

CO2 binds amino groups of globin (NOT haem iron) to form carbaminohaemoglobin.

Breathing control: medullary rhythmicity centre (primary); pneumotaxic (pons) limits inspiration; peripheral chemoreceptors (carotid + aortic bodies) detect pO2, pCO2, H+.

Disorders: Asthma (reversible airway narrowing); Emphysema (alveolar destruction, barrel chest); Silicosis (silica, fibrosis); Asbestosis (asbestos, fibrosis + cancer).