What is the mechanism of inspiration in human?

Inspiration and expiration are the terms used to describe breathing in and out. The way in which we breathe depends entirely upon a pressure gradient that determines the direction of air movement in or out of our lungs. The main principle to remember is : increasing volume = decreasing pressure - as the density of particles in that specified space is more widely distributed. The opposite is true for a decreased volume which results in an increased pressure. To breathe in, our diaphragm (usually in a dome shape at rest) and the intercostal muscles between the ribs contract. This results in the diaphragm flattening itself and the intercostal muscles moving the ribcage upwards and outwards. These 2 movements help to increase the volume of the chest cavity, hence, decreasing the pressure inside the lungs. As a result, the air pressure in the external environment is now higher than that inside the lungs and air moves down its pressure gradient from the environment into the lungs. Allowing us to breathe in ( inspire).