Question

Why do infants breathe faster than adults?

Answer 1

During early development, the lungs completely fill the thorax, so that they are not stretched to fit the relaxed rib cage. As the child develops, the rib cage grows faster than the lungs, and the linings and increased negative intrapleural pressure provide a means for the lungs to be stretched out to fill that space.
The result of this stretching is increased capacity and reserve in adults, but not in infants. Because the thorax and lungs are the same size, infants must breathe two to three times as often as an adult for adequate respiration. Lungs of adults are stretched out and are never completely compressed, so there is always a reserve of air within them that is not at the moment undergoing gas exchange.