What's the mechanism behind inspiration in human?

 This entire process is based on a pressure gradient or the difference in air pressure between the air inside our lungs and the air in our external environment. Just like the normal principle of concentration gradient, air will flow from an area of higher pressure into an area of lower pressure. Therefore, in order to inspire or breathe in, a number of changes need to happen to our chest cavity to create lower air pressure inside our lungs compared to the outer environment.  During inspiration, the diaphragm (normally in a curved, dome shape when relaxed) contracts and flattens itself, which increases the volume of the chest cavity. The external intercostal muscles between our ribs also contract which moves the rib cage upwards and outwards, also increasing the volume of the chest cavity.  As the volume of the chest cavity increases, the pressure inside decreases because the air particles are not as tightly packed together as they were before. All of this causes air from an area of higher pressure – the surrounding environment – to move down the pressure gradient into an area of lower pressure, which is our lungs. As a result, we inspire or breathe in.