Alveoli:
- Large SA:V ratio which allows more gases to be exchanged at a time.
- Thin membrane (1 cell thick) which reduces diffusion distance.
- Good blood supply and ventilation; so carbon dioxide is brought to the alveoli and oxygen is taken away quickly, maintaining a constant high diffusion gradient.
Red Blood Cells:
- Biconcave disc shape to provide a large SA for gas exchange.
- Only slightly larger than capillary diameter and flexible, so cells pass in a single file line through capillaries, reducing the speed of blood flow and allowing more time for gas exchange.
- Has haemoglobin- which has a higher affinity for oxygen, the greater the partial pressure of oxygen. So takes in oxygen to form oxyhaemoglobin where oxygen concentration is high and releases oxygen where oxygen concentration is low. (Works the same way for Carbon Dioxide)
- No nucleus so there is more room for haemoglobin.
All of these increase the rate of diffusion and exchange of gases.