The microanatomy of the alveolar duct system.

Observations made on wet and dry slices of inflated human lungs taken at necropsy have been used to give a more complete account of the alveolar duct system, the basic respiratory structure. This includes consideration of the central spiral alveolar duct fibre, which is thought to consist of elastic and collagen, the spiral arrangement of alveoli and the special nature of the terminal alveolus, the interdigitation of alveoli from adjacent alveolar ducts and the arrangement of pulmonary arterioles. This suggests that the alveolar duct system is specifically designed so that changes in volume are not accompanied by changes in alveolar surface area. Of the small holes seen in the cut surface of the fixed inflated lung (Fig. 1) the vast majority are found to be alveolar ducts, separated by translucent brownish alveolar tissue. The structure of the alveolar ducts, which, with the alveoli opening off them, make up the basic respiratory unit of the lung, has been considered by many authors. The diagrams of Miller (1947) represent an older opinion, and those of Oderr (1964) a more recent view. Further observations are reported here from which a more complete account of the alveolar duct system and its relationship to pulmonary vessels and to other alveolar duct systems can be inferred. METHODS Normal and diseased lungs were obtained from unselected adult patients of various ages and of Negro extraction coming to necropsy at the University Hospital of the West Indies. Some of the lungs were inflated with 10% formalin via the main bronchus, sliced after 24 hours into four or five sagittal slices, 1-5 cm. thick, and examined with a dissecting microscope, and blocks were taken for his-tology. Rapid paper sections (Whimster, 1969) were then made. Other lungs were fixed and dried in inflation by the method of Blumenthal and Boren (1959) with or without prior injection of the arteries and veins with a barium-gelatine solution. The dried lungs were sliced on a bacon slicer at approximately 750 p, and examined unstained between transparent plastic sheets perforated at 1-cm. intervals under an ordinary binocular microscope; photomicrographs were taken as required. Adult lungs yielded up to 100 slices. 141 Selected areas of the dried slices were cut out, soaked in water in a vacuum chamber to remove bubbles, and stained. They were mounted on 3 x I in. slides by covering the 750 ,t thickness with Mara-removing bubbles in a vacuum chamber …