Document 5D3b7GVgdZkvKM5YZK9NZpzoV

PULMONARY DUST DISEASES 365 terminal bronchioles are the last of a series of subdivisions of these nonrespirabronchioles. The muscular tissue in their walls is more highly developed than in any other part of the bronchial tree and when fully contracted exerts 'a incterlike action which can completely shut off the air supply to the chambers ond. The structures lying distal to the terminal bronchioles are the "leaves" of the |nchial tree. They have a respiratory function: the interchange of gases between and blood occurs through their walls. The respiratory portion consists of the ^`lUiratory bronchioles, alveolar ducts, alveolar sacs, and pulmonary alveoli. The ster formed by a related group of these structures constitutes a lung unit or mary lobule. This is the distensible or bellows part of the lung, p The respiratory bronchiole as described by Best and Taylor9 has the same Hineter as the terminal bronchiole, of which it appears as a branch or a continuafn. I've or six alveolar ducts arise from each respiratory bronchiole. After a friable number of rebranchings each alveolar duct gives rise to from three to six Natations, the alveolar sacs. The bays in the walls of these sacs constitute the 'monary alveoli, which are lined by a single layer of flattened epithelial cells Rented together. The alveolar walls contain elastic fibers and a rich network of ,,ipillaries. Frequently only a single capillary channel intervenes between the SS\lls of adjacent alveoli. The blood in the capillaries is therefore separated from flgQ/ c^air in the alveoli by only two membranes of the utmost delicacy--the alveolar ''SfS'an.d capillary walls, so the greatest freedom is afforded for the diffusion of gases oil the blood to the alveolar air and from the alveolar air to the blood. fE.The bronchioles, as they approachThe'periphery of'the lung, branch and repnch repeatedly, diminishing in length with each subdivision. The first branch- are about 1.5 mm. in length and from 0.3 to 0.4 mm. in diameter. The terminal ^'respiratory bronchioles are from 0.2 to 0.5 mm. in length but of about the same Imeter as the more central subdivisions. That is, the bronchioles, though profsively shorter, show practically no decrease in diameter as they pass toward "'periphery. The alveolar sac, however, is considerably .wider than either the espiratory bronchiole or the alveolar duct from which it arises. The pulmonary Sveoli are semiglobular and have diameters ranging from 0:075 to 0.125 mm.; the 111, number in the lungs has been estimated by Zuntz at 750 million. Willson10 flmates the total epithelial surface of the lungs at 70 sq..meters; of this,, prob||55 sq. meters, over 25 times the "surface area of'the"skih,'is-respiratory. ' From these facts it is apparent that lung tissue offers 'by far the best medium of ny,in the body for the absorption of materials that may come in contact with it. Blood vessels and lymphatics. Except in the alveolar areas the blood vessels ^lvihphatics-^follow-rath&^doseLv-the-Dath-CijLthe-hEonchial tree. "The relation- .^Hip in the .periphery of the lung is shown clearly in Figure 3.11 I * C. H. Best and N. B. Taylor, The Physiological Basis of Medical Practice. 3rd ed., (Jfc&tf.iiliams & Wilkins, Baltimore, 1943. '( "H. G. Willson, Am. J. Anat., 30, 267 (1922). 5|R' 11W. S. Miller, as reproduced by Pendergrass in A. J. Lanza, ed., Silicosis ond Asbestosis. llfford Univ. Press, New York, 1938.