Simionescu M
Ciba Found Symp. 1980;78:11-36. doi: 10.1002/9780470720615.ch2.
As a result of its special position in the circulatory system the lung can screen and monitor the composition of the blood which comes from and is returned to all the tissues. This function, together with the exchange of gases, takes place at the level of the alveolar-capillary unit. The cellular components of the unit are: the epithelium, lining the air spaces, and the endothelium, facing the blood compartment. The epithelium is composed of two types of cells: type I--broad, squamous, highly branched cells occupying approximately 97% of the total alveolar surface; these cells seem to be involved mostly in exchange of gases; and type II--cuboidal cells containing characteristic osmiophilic lamellar bodies in their cytoplasm. The epithelial cells are connected to one another by tight junctions. A thin layer of a surface-active material--the surfactant--covers the epithelium toward the air space, where its role is to lower the surface tension. Surfactant is a mixture of lipids (approximately 75%) and proteins; dipalmitoyl-phosphatidylcholine is the major lipid component and is synthesized and secreted by type II epithelial cells. The endothelium is of continuous type and has a large population of plasmalemmal vesicles; the cells are linked together by tight junctions. Morphometric studies indicate that in humans the capillary surface area is approximately 120 m2 and the alveolar surface area is approximately 140 m2. Some of the metabolic functions ascribed to the lung have been localized to cellular components. Phospholipids needed for the constantly renewed surfactant are synthesized in type II epithelial cells. Angiotensin-converting enzyme is associated with the endothelial cell membrane and vesicles opening to the blood front. There are indications that pulmonary cells also intervene in the metabolism of circulating vasoactive substances which during their passage through the lung can be activated (angiotensin I), inactivated (bradykinin) or removed from the circulation (5-hydroxytryptamine). The lung is a metabolically active organ where the anabolism and catabolism of pharmacologically active substances and the synthesis of lipids take place and a proper balance for blood homeostasis is maintained.
由于肺在循环系统中的特殊位置，它能够筛选和监测来自所有组织并回流至这些组织的血液成分。这一功能与气体交换一起，在肺泡 - 毛细血管单元水平上进行。该单元的细胞成分包括：衬于气腔的上皮细胞和面向血液腔的内皮细胞。上皮细胞由两种类型的细胞组成：I型——宽大、扁平、高度分支的细胞，占据肺泡总表面积的约97%；这些细胞似乎主要参与气体交换；以及II型——立方体细胞，其细胞质中含有特征性嗜锇板层小体。上皮细胞通过紧密连接相互连接。一层薄的表面活性物质——表面活性剂——朝向气腔覆盖上皮细胞，其作用是降低表面张力。表面活性剂是脂质（约75%）和蛋白质的混合物；二棕榈酰磷脂酰胆碱是主要的脂质成分，由II型上皮细胞合成并分泌。内皮细胞为连续型，有大量质膜小泡；细胞通过紧密连接相连。形态计量学研究表明，在人类中，毛细血管表面积约为120平方米，肺泡表面积约为140平方米。一些归因于肺的代谢功能已定位到细胞成分。不断更新表面活性剂所需的磷脂在II型上皮细胞中合成。血管紧张素转换酶与内皮细胞膜以及通向血液面的小泡相关。有迹象表明，肺细胞也参与循环血管活性物质的代谢，这些物质在通过肺时可被激活（血管紧张素I）、失活（缓激肽）或从循环中清除（5 - 羟色胺）。肺是一个代谢活跃的器官，在此发生药理活性物质的合成代谢和分解代谢以及脂质合成，并维持血液稳态的适当平衡。
