The normal respiratory movement of the human body consists of three important processes: external respiration, internal respiration and the transport of gases in the blood. External respiration is the ventilation of the lungs and pulmonary ventilation, while internal respiration is the exchange of gases between tissue cells and blood, and the metabolism of carbon dioxide and oxygen. During inspiration, the diaphragm will contract, the intercostal muscles will contract, the thorax will expand, increasing the volume of the thoracic cavity, the lungs will also expand, the air pressure in the lungs will be relatively lower, the air will be inhaled into the lungs, and the process is reversed during exhalation. A normal person breathes for 6.4 seconds when quiet, and the amount of gas breathed and exhaled each time is about 500ml.