The gas exchange system — common mistakes

Many students confuse the roles of internal and external intercostal muscles. Remember: External for Expanding (during inspiration), Internal for pulling Inwards (during forced expiration). For quiet breathing, expiration is passive!

The key structures include the trachea, bronchi, bronchioles, and alveoli, all working together to facilitate the movement of air and the transfer of gases. Cartilage provides support, while ciliated epithelium and goblet cells clean the air.

Alveoli are highly specialised, featuring a large total surface area, extremely thin walls (short diffusion path), a rich capillary blood supply (maintaining concentration gradients), and a moist inner surface (allowing gases to dissolve). These adaptations collectively ensure rapid and efficient diffusion.

Breathing, or ventilation, involves coordinated movements of the diaphragm and intercostal muscles, which alter the volume of the thoracic cavity. This change in volume leads to corresponding changes in pressure, causing air to flow in during inspiration (active) and out during expiration (passive at rest).

Smooth muscle, mainly in the bronchioles, contracts or relaxes to regulate airflow to the alveoli. Elastic fibres, found throughout the airways and alveoli, allow for stretching during inhalation and provide elastic recoil for passive exhalation.