Gaseous exchange

Trachea:

• C-shaped rings of cartilage (keep airways open prevents trachea from collapsing due to large pressure changes) 
• Smooth muscle and elastic fibres
• Pseudostratified ciliated columnar epithelium (waft mucus in a synchronised way back up the trachea)
• Goblet cells (secrete sticky mucin droplets)
• Blood vessel
• Mucous secreting glands

Carina- marks where the trachea divides into 2 primary bronchi

Bronchi:

• contains cartilage plates
• Goblet cells 
• Smooth muscle 
• Simple ciliated columnar epithelium
• Mucous glands

Bronchiole:

• Smooth muscle (involved in bronchodilation and bronchoconstriction)
• Simple ciliated cuboidal epithelium 
• Loose tissue with elastic fibres
• Goblet cell
• Blood vessel

Respiratory bronchiole:

• No goblet cells

Factors affecting gaseous exchange:

1. Surface area available for diffusion (You want a large surface area to volume ratio)
2. Diffusion distance (You want it to be as short as possible)
3. Concentration gradient (steeper conc. gradient=faster diffusion) (You need a mechanism to maintain a steep conc. gradient i.e capillary network)
4. Permeability to gases

Alveoli (where gas exchange occurs):

• Large S.A to volume ratio
• 1 cell thick layer of simple squamous epithelium (short diffusion distance)
• Has 2 machanisms to maintain a concentration gradient. A circulatory (i.e capillary network) and ventilation (i.e Inspiration and Expiration).
• Moist meaning gases can dissolve

Pathway of gas:

Oxygen:

1. Squamous epithelium 
2. Connective tissue
3. Capillary endothelium
4. Blood Plasma
5. Membrane of RBC