Describe the structures involved in gas exchange in the lungs and explain how gas exchange occurs

Gas exchange is where your body take in oxygen and gets rid of the carbon dioxide waste that produced.

There is a lot of different structures that are involved in gas exchange. This includes larynx, trachea, bronchus, bronchiole and alveolus.

The process of gas exchange mostly happens between the alveolus (air sac) and the blood capillaries. It occurs because of the concentration gradient between the air sac and the blood capillary. When you breathe in, the air contains a lot of oxygen, so the alveolus has a high concentration of oxygen that drives the diffusion of oxygen from the alveolus to the blood capillaries. The blood capillaries then transport the oxygen to other body parts. 

This applies same for the carbon dioxide, the carbon dioxide in the blood capillary is high, therefore it'll diffuse into the alveolus down the concentration gradient, then, we breathe out. 

So gas exchange basically occurs because of the concentration gradient of oxygen/carbon dioxide between the alveolus in lungs and the blood capillaries surrounded.

Gravitational force i believe