Structure of Pulmonary Capillaries

Pulmonary capillaries typically have a thick and thin side as shown in Figure 1. On the thin side, the blood-gas barrier consists only of a thin epithelium, this being the protoplasmic extension of a type I alveolar epithelial cell, an interstitium or extracellular matrix, and a thin capillary endothelial layer. There is also a very thin layer of surfactant lining the alveolar space, but this is not shown in Figure 1 because it was lost during preparation of the section.

Figure 1 Electron micrograph of a pulmonary capillary. The thin side of the blood-gas barrier is in the upper part of the micrograph while the thick side is below. The micrograph emphasizes the extreme thinness of some portions of the barrier and the fact that it is completely unsupported by surrounding tissue. (From Ref. [1].)

Figure 1 Electron micrograph of a pulmonary capillary. The thin side of the blood-gas barrier is in the upper part of the micrograph while the thick side is below. The micrograph emphasizes the extreme thinness of some portions of the barrier and the fact that it is completely unsupported by surrounding tissue. (From Ref. [1].)

By contrast, the thick side of the pulmonary capillary contains additional structures in the interstitium, notably fibers of type I collagen that contribute to the structural framework of the lung, and also interstitial cells including fibroblasts and other pericytes. This anatomy means that the thin side of the blood-gas barrier is mainly responsible for diffusional gas exchange, whereas the thick side is concerned with fluid exchange and also mechanical stability. Discussion about the fragility of pulmonary capillaries mainly concerns the thin side where the stresses will be highest, other things being equal.

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Essentials of Human Physiology

Essentials of Human Physiology

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