Gas exchange system consists of:1) The trachea which is supported by incomplete rings of cartilage.
2) The trachea which divides into two bronchi the repeatedly spit into smaller tubes; the bronchioles.
3) At the ends of the bronchioles are alveoli, which is where gas exchange occurs.
How alveoli are adapted for efficient gas exchange:- Their shape and large number provides a large surface area.
- There is a fluid which lines the alveoli which allows gases to dissolve in it and diffuse across it.
- There are only 2 layers that seperate the blood and the air(in the alveoli) which provides a small diffusion pathway. (The 2 layers consist of 1) An alveolar wall; single layer of flattened epithelial cells. 2) A capillary wall; single layer of endothelial cells).
- Blood capillaries surround each alveolus providing a large surface area for absorbing oxygen and releasing carbon dioxide.
- A circulatory system which maintains a high diffusion gradient by transporting deoxygenated blood to the lungs and removing oxygenated blood from the lungs. (Think of it like taking oxygen to the cells, bringing it back to be replenished to take it back again)
- A ventilation mechanism which again maintains a high diffusion gradient by continually providing air with high oxygen concentration and removing air with high carbon dioxide concentration.
The role of the alveoli for gaseous exchange:1) Oxygen enters the alveoli into the alveolar air space.
2) This oxygen diffuses into the fluid which is lining the epithelium of the alveolus.
3) Oxygen then difusses across the alveolar wall (flattened epithelial cells lining the alveolus) and then across the capillary wall (flattened endothelial cells lining the blood capillary) and into the blood.
4) Oxygen combines with haemoglobin to create oxyhaemoglobin and the oxygenated blood is transported away from the lungs.
If you need to talk about how carbon dioxide enters the alveolus, just reverse it.
So deoxygenated blood (CO2) diffuses into capillary wall -> into alveolar wall -> dissolves into fluid lining into the alveolar air space.
All of this maintains a high diffusion gradient so gaseous exchange is efficient.
(Sorry if its too much but I hope it helped
)