Lung growth and alveolar multiplication.

As the lung grows in volume from infancy to adult life, considerable amounts of tissue are added to the lung, mostly as a result of alveolar multiplication. Species differences may exist: at birth alveoli are absent in mice and rats but alveoli are generally thought to be present in humans at birth. Alveolar multiplication is brought about by the subdivision of the primitive terminal units, primary pulmonary saccules, by secondary alveolar crests, and by alveolarization of nonalveolated and partly alveolated airways. The exact method of alveolar development and the relative importance of the above modes of alveolar growth are not known. In the first few days of life in rats and mice, there is a phase of dilatation of the lung, followed by a phase of rapid cellular and tissue proliferation. Subsequently, remodeling of the lung occurs, during which stage lung tissue increases little and dilatation is more prominent; this leads to stretching of the alveolar walls. Alveolar multiplication may occur throughout life in the rat. In human subjects, alveolar multiplication is most rapid in the first few years of life. After this, it appears to slow and perhaps stop by age 8 years, although there is some suggestion that alveolar multiplication may continue until somatic growth stops. Pneumonectomy produces enlargement and increase in tissue of the contralateral lung by virtue of cellular hyperplasia. Alveolar multiplication likely does not occur. Diminution of intrathoracic volumes produces small lungs which, in the human, may also have too few alveoli if the chest wall deformity has its onset in infancy or in intrauterine life. High altitude produces large, heavy lungs which may have more alveoli than normal.