Part 1: Respiratory System Anatomy
The human respiratory tract runs from the external nares to the alveoli. The nasal cavity conditions incoming air by warming, moistening, and filtering it. The pharynx is a shared passage for air and food. The larynx houses the vocal cords. The trachea has C-shaped cartilaginous rings (open posteriorly) to prevent collapse. The trachea branches into primary bronchi and then progressively smaller bronchi and bronchioles. The alveoli (~700 million, ~70 total surface area) are the terminal gas exchange units. The right lung has three lobes; the left lung has two (cardiac notch). The lungs are enclosed in double-layered pleura with intrapleural fluid (slightly negative pressure keeps lungs inflated).
Part 2: Mechanism of Breathing
Breathing is driven by pressure changes. Inspiration is active: diaphragm contracts (flattens) and external intercostals lift ribs up and out → thoracic volume increases → intra-pulmonary pressure falls below atmospheric → air flows in. Quiet expiration is passive: inspiratory muscles relax → elastic recoil raises intra-pulmonary pressure → air flows out. Forced expiration is active: internal intercostals and abdominal muscles contract to forcefully expel more air.
Part 3: Respiratory Volumes and Capacities
Four basic volumes: TV (500 mL), IRV (2500–3000 mL), ERV (1000–1100 mL), RV (1100–1200 mL). Key capacities: VC = TV + IRV + ERV (3500–4600 mL; excludes RV), TLC = VC + RV (5000–6000 mL), FRC = ERV + RV (2100–2300 mL; air remaining after normal expiration), IC = TV + IRV (3000–3500 mL). RV cannot be measured by standard spirometry.
Part 4: Gas Exchange at Alveoli
Exchange by simple diffusion across the 3-layered diffusion membrane (alveolar epithelium + basement membrane + capillary endothelium). O2 gradient: 104 → 40 mmHg (64 mmHg). CO2 gradient: 45 → 40 mmHg (5 mmHg). CO2 compensates with ~20× higher solubility. Fick's Law determines exchange rate: proportional to surface area and partial pressure gradient; inversely proportional to membrane thickness.
Part 5: Gas Transport in Blood
O2 transport: 97% oxyhaemoglobin (sigmoid cooperative curve, 4 O2 per Hb), 3% dissolved. Bohr effect: CO2↑, pH↓, temperature↑, 2,3-BPG↑ → right shift → more O2 released. CO2 transport: 70% HCO3– (carbonic anhydrase in RBCs, chloride shift), 23% carbaminohaemoglobin (CO2 + globin amino groups), 7% dissolved. Haldane effect: oxygenation of Hb releases CO2 at the lungs.
Part 6: Regulation of Breathing
The respiratory rhythmicity centre (medulla oblongata) initiates the breathing pattern. The pneumotaxic centre (pons) inhibits inspiration, setting breathing frequency. Peripheral chemoreceptors (aortic and carotid bodies) detect pO2, pCO2, and H+ and adjust ventilation. CO2/H+ is the dominant ventilatory stimulus (medullary chemoreceptors are very sensitive to CO2 changes in CSF).
Part 7: Respiratory Disorders
Asthma: reversible allergic/inflammatory airway narrowing → wheezing, expiratory difficulty. Emphysema: irreversible alveolar wall destruction → loss of elastic recoil → air trapping, barrel chest, ↑RV, ↓VC. Silicosis: silica dust → progressive lung fibrosis, dyspnoea. Asbestosis: asbestos fibres → fibrosis + significantly increased lung cancer/mesothelioma risk.