Humans

• Ventilation route = nasal cavity, trachea, bronchi, bronchiole, alveoli, capillaries
• The trachea is an airway supported by rings of cartilage
• Trachea can collapse slightly to allow food to pass down the oesophagus
• Walls of trachea are made from muscle with ciliated epithelial cells and goblet cells
• Goblet cells produce mucus which traps dirt and microorganisms
• Ciliated epithelial cells move mucus up the pharynx where it can be swallowed
• The bronchi is the two divisions of the trachea each leading to one lung
• Bronchi have goblet and epithelial cells
• Bronchioles are a series of branching subdivisions of the bronchi
• Bronchiole walls are made from smooth muscle lined with epithelial cells
• Muscle of the bronchioles allow them to constrict so they can control the flow of air in and out of the alveoli
• Alveoli are tiny sacs at the end of the bronchioles that contain blood capillaries
• Gas exchange occurs between the lungs and blood through diffusion across the thin membrane separating the alveoli and capillary
• Alveoli contain collagen and elastic fibres and are made from a single layer of flattened epithelial cells
• Elastic fibres allow the alveoli to stretch as they fill with air during inhalation, then they recoil during exhalation
• Alveoli have a good blood supply to maintain concentration gradient