The respiratory system consists of respiratory tract and lung. Respiratory tract is the passage of gas into and out of the lung, which is composed of nose, pharynx, larynx, trachea, bronchus and their branches. The nose is the gateway of respiratory tract and the first barrier for external gas to enter the human body. The nasal cavity can warm and moisten the inhaled air. In addition, nasal mucus and nasal hair can adhere to the dust and bacteria inhaled in the air and 'keep them out of the door'.
There is also a unique olfactory cell in the nasal mucosa, which can play the olfactory function. When it smells the smell of irritant or harmful gas, it will immediately make a judgment and report to the brain. Under the command of the brain, people can quickly cover their nose to reduce the harm of harmful substances. As an important organ of the digestive tract, the mouth has no special function of the nose at all. When it breathes harmful gases, it can't distinguish them. The harmful substances will enter into the body in a long way. Therefore, breathing with mouth is not good for health.