Why is Co2 lost in the lung?

CO2 is not lost in the lungs, it is exhaled.

During the process of respiration, oxygen is inhaled and carbon dioxide is exhaled. This happens because the lungs are responsible for gas exchange, which means that they allow oxygen from the air to enter the bloodstream and carbon dioxide from the bloodstream to exit the body. The carbon dioxide is carried to the lungs by the blood, and then it is released into the air when we exhale.

The rate of carbon dioxide exhalation is regulated by the respiratory centre in the brain, which responds to changes in the concentration of carbon dioxide in the blood. If the carbon dioxide concentration in the blood increases, the respiratory rate will increase in order to remove more carbon dioxide from the body.