The respiratory tract consists of the conducting zone (external nares to terminal bronchioles, no gas exchange) and the respiratory zone (respiratory bronchioles to alveoli, where gas exchange occurs).

Inspiration is active (diaphragm and external intercostals contract → thoracic volume increases → pressure falls → air enters), while quiet expiration is passive (elastic recoil of lung tissue drives air out).

The four basic lung volumes are TV (500 mL), IRV (2500–3000 mL), ERV (1000–1100 mL), and RV (1100–1200 mL); RV cannot be measured by standard spirometry.

Vital Capacity = TV + IRV + ERV (excludes RV) ≈ 3500–4600 mL; Total Lung Capacity = VC + RV ≈ 5000–6000 mL.

Gas exchange at the alveoli occurs by simple diffusion across a 3-layered membrane (alveolar epithelium + basement membrane + capillary endothelium) driven by partial pressure gradients.

O2 diffuses from alveoli (pO2 = 104 mmHg) into blood (pO2 = 40 mmHg); CO2 diffuses from blood (pCO2 = 45 mmHg) into alveoli (pCO2 = 40 mmHg).

Oxygen is transported 97% as oxyhaemoglobin (sigmoid dissociation curve due to cooperative binding) and 3% dissolved in plasma.

The Bohr effect states that increased CO2, decreased pH, elevated temperature, and increased 2,3-BPG shift the O2–Hb dissociation curve to the right, reducing O2 affinity and promoting O2 release to active tissues.

CO2 is transported as 70% HCO3– (via carbonic anhydrase in RBCs, with chloride shift), 23% carbaminohaemoglobin, and 7% dissolved; the chloride shift (Hamburger's phenomenon) maintains RBC electrical neutrality.

Breathing is controlled by the medullary rhythmicity centre (initiates rhythm), the pneumotaxic centre in the pons (limits inspiration), and peripheral chemoreceptors in the aortic and carotid bodies (detect pO2, pCO2, and H+).