The lower respiratory tract is relative to the upper respiratory tract and usually includes the trachea, the main bronchi and the bronchi at all levels in the lungs, collectively referred to as the lower respiratory tract. Generally the human body enters the trachea through the nasal cavity, mouth and throat through respiratory movements, and then the trachea carries air to the left and right bronchi, which then passes through the bronchi at all levels and enters the alveoli and lung tissue. This completes the metabolic exchange of carbon dioxide and oxygen, constantly taking away the carbon dioxide produced in the body and then constantly delivering more oxygen to the organism. This is the process of forming a normal metabolism, which ensures the normal life operation signs of the cells, tissues and organs in the body.