Source: Respiratory system complete — Wikimedia Commons $\frac{Public Domain}{CC BY}$

Cue Column | Note-Taking Area

What is the respiratory tract? | The pathway from external nares to alveoli: External nares → Nasal cavity (warms, moistens, filters) → Pharynx → Larynx (vocal cords) → Trachea (C-shaped cartilaginous rings) → Primary bronchi → Secondary bronchi → Tertiary bronchi → Bronchioles → Alveoli.

What are the lungs? | Paired organs enclosed in double-layered pleural membranes with intrapleural fluid. Right lung = 3 lobes; Left lung = 2 lobes (cardiac notch accommodates heart).

How does inspiration work? | Active process: Diaphragm contracts (flattens) + External intercostals lift ribs up and out → thoracic volume ↑ → intra-pulmonary pressure ↓ below atmospheric → air flows in.

How does expiration work? | Quiet expiration: PASSIVE — diaphragm and external intercostals relax; elastic recoil drives air out. Forced expiration: ACTIVE — internal intercostals + abdominal muscles contract.

Key lung volumes: | TV = 500 mL; IRV = 2500–3000 mL; ERV = 1000–1100 mL; RV = 1100–1200 mL. VC = TV + IRV + ERV ≈ 3500–4600 mL. TLC = VC + RV ≈ 5000–6000 mL.

How does gas exchange occur? | Simple diffusion across a 3-layered diffusion membrane (squamous alveolar epithelium + shared basement membrane + capillary endothelium). Driven by partial pressure gradients: O2 from alveoli (104 mmHg) → blood (40 mmHg); CO2 from blood (45 mmHg) → alveoli (40 mmHg).

How is O2 transported? | 97% as oxyhaemoglobin (sigmoid curve, cooperative binding); 3% dissolved in plasma. Bohr effect: increased CO2, decreased pH, elevated temperature, increased 2,3-BPG → right shift → more O2 released to tissues.

How is CO2 transported? | 70% as HCO3– (via carbonic anhydrase in RBCs, chloride shift); 23% as carbaminohaemoglobin (binds globin amino groups); 7% dissolved in plasma.

How is breathing regulated? | Respiratory rhythmicity centre (medulla oblongata) sets basic rhythm. Pneumotaxic centre (pons) limits inspiration. Peripheral chemoreceptors (aortic and carotid bodies) detect pO2, pCO2, H+ and modulate ventilation.

Summary

Breathing involves mechanically driven airflow (pressure gradients via muscle action), gas exchange at alveoli (diffusion driven by partial pressure gradients), and blood transport of gases (primarily oxyhaemoglobin for O2; primarily HCO3– for CO2). Central (medullary) control plus peripheral chemoreceptor feedback maintains homeostatic blood gas levels.