Properties of a good respiratory surface

Thin, moist, large SA:V ratio, vascularised, permeable

Insects

Spiracles → trachea (chitin) → tracheoles (no chitin); independent of blood

Bony fish gills

Counter-current flow — up to 80–90% O₂ extraction

Cartilaginous fish gills

Parallel flow — only ~50% O₂ extraction

Alveoli

~7 million; one-cell-thick walls; surfactant reduces surface tension

Inspiration

Active; external intercostals + diaphragm contract

Expiration

Passive; elastic recoil + internal intercostal contraction

Nervous control

Medulla (inspiratory/expiratory centres); vagus, phrenic, intercostal nerves

Chemical control

Rising blood CO₂ → chemoreceptors (aorta/carotid) → ↑ breathing rate

Vital capacity

TV + IRV + ERV

Plant gaseous exchange

Via stomata; guard cell turgidity controls pore size