The fetus does not breathe in the womb, and the baby after birth must rely on its own breathing to inhale oxygen and discharge carbon dioxide. There is no air in the lungs of the fetus. The lungs are a mass of solid tissue. After the baby is born, because the body is no longer curled up into a ball, the original bent chest suddenly expands, the chest expands immediately, and the lung lobes also open, and then the first breath of air is inhaled. When air enters the alveoli from the trachea, the inspiratory muscles relax immediately, the expiratory muscles contract immediately, and the thorax shrinks to its original size, forcing the air out of the lungs.
When the exhaled gas passes through the larynx, the muscles of the larynx contract, and the two vocal cords in the laryngeal cavity are tightened and close together. When the gas strikes the vocal cords, the vocal cords vibrate and emit a cry similar to crying. When the baby was just born, the amount of carbon dioxide in the blood was more, which stimulated and excited the respiratory center, so they all took a big breath. Therefore, every baby will cry like this for a while after birth. When the respiratory activity has established a normal rhythm, it will no longer cry like this.