Human Respiration System:-

Human respiration relies on an involuntary muscle reflex that reflex that makes the diaphragm and external intercostals muscle contract. This increases the volume of the chest cavity, and the pressure inside it drops. Air rushes in through the nose, down the trachea, and into the lungs, where an exchange of oxygen and carbon dioxide takes place. As the muscle relax, the air is forced back out of lungs.