The primary function of lungs in mammals is to facilitate gas exchange. They enable oxygen from the air to enter the bloodstream and carbon dioxide to be expelled from the body. This process occurs in the alveoli, tiny air sacs within the lungs, where oxygen diffuses into the blood while carbon dioxide diffuses out. Lungs are essential for maintaining respiration, which is vital for cellular metabolism and overall survival, enabling mammals to meet their oxygen demands during varying levels of physical activity.