Renkin E M
Department of Human Physiology, University of California, School of Medicine, Davis 95616.
Am Rev Respir Dis. 1992 Nov;146(5 Pt 2):S28-31. doi: 10.1164/ajrccm/146.5_Pt_2.S28.
The endothelium of lung alveolar capillaries is of the continuous type, that of airway exchange vessels (capillaries and pericytic venules) includes both continuous and fenestrated types. Water and small lipophilic solutes penetrate via the endothelial cells (cell membrane pathway) as well as through intercellular junctions. Hydrophilic solutes are limited to junctional pathways and cytoplasmic vesicles. Permeation of hydrophilic solutes is progressively restricted with increasing molecular size, as by a sieve, with many openings 8 nm and a few 40 to 60 nm wide. In response to local tissue injury or to certain chemical mediators, larger junctional pathways may be opened, greatly increasing permeability to large molecules. Both alveolar capillaries and airway exchange vessels exhibit this response, but the effective stimuli may differ (e.g., alveolar capillaries are insensitive to histamine and bradykinin). Hydrophilic solutes are transported by diffusion, convection, and vesicular exchange (transcytosis). For small ions and molecules (radii < 2 nm), diffusion is the dominant transport mode; contributions of convection and transcytosis are negligibly small. Because diffusion decreases with increasing molecular size, all three mechanisms may contribute substantially to transport of large molecules (radii > 2 nm). Fenestrated endothelia have higher hydraulic conductivities and are more permeable to small ions and molecules than are continuous endothelia. However, their permeabilities to plasma proteins are about the same. Lung alveolar capillary endothelium has lower hydraulic conductivity and lower solute permeabilities than do other continuous endothelia (heart, skeletal muscle). Airway exchange vessel endothelium has about the same permeability to serum albumin as alveolar capillary endothelium.
肺泡毛细血管的内皮属于连续型，气道交换血管（毛细血管和周细胞小静脉）的内皮则包括连续型和有孔型。水和小分子亲脂性溶质可通过内皮细胞（细胞膜途径）以及细胞间连接渗透。亲水性溶质则限于连接途径和细胞质小泡。随着分子大小增加，亲水性溶质的渗透逐渐受到限制，就像通过一个有许多8纳米开口以及一些40至60纳米宽开口的筛子。对局部组织损伤或某些化学介质的反应中，可能会打开更大的连接途径，大大增加对大分子的通透性。肺泡毛细血管和气道交换血管都有这种反应，但有效刺激可能不同（例如，肺泡毛细血管对组胺和缓激肽不敏感）。亲水性溶质通过扩散、对流和小泡交换（转胞吞作用）运输。对于小离子和分子（半径<2纳米），扩散是主要的运输方式；对流和转胞吞作用的贡献可忽略不计。由于扩散随分子大小增加而降低，所有三种机制可能对大分子（半径>2纳米）的运输都有很大贡献。有孔内皮比连续内皮具有更高的水力传导率，对小离子和分子的通透性也更高。然而，它们对血浆蛋白的通透性大致相同。肺泡毛细血管内皮的水力传导率和溶质通透性低于其他连续内皮（心脏、骨骼肌）。气道交换血管内皮对血清白蛋白的通透性与肺泡毛细血管内皮大致相同。
