Stress Failure in the Wall of the Pulmonary Capillary

The ultrastructural changes that occur in the walls of pulmonary capillaries when the wall stresses are increased have been extensively studied. Measurements have been made in anesthetized rabbits where the pulmonary artery and left atrium were cannulated so that the capillary transmural pressure could be raised and accurately measured. The lungs were prepared for electron microscopy using intravascular fixation with buffered glutaraldehyde at the required pressure. An example of the ultrastructural changes seen in such experiments is shown in Figure 3. Note that the alveolar epithelial layer is disrupted near the top of the micrograph and, in addition, there is disruption of the capillary endothelial layer near the bottom. Interestingly, a blood platelet

Figure 3 Electron micrograph showing some effects of raising the capillary transmural pressure in rabbit lung. Note that there is disruption of the alveolar epithelial layer (top arrows) and also a disruption of the capillary endothelial layer (bottom arrows). A platelet is adhering to the exposed basement membrane below. ALV, alveolus; CAP, capillary. (From Ref. [5].)

Figure 3 Electron micrograph showing some effects of raising the capillary transmural pressure in rabbit lung. Note that there is disruption of the alveolar epithelial layer (top arrows) and also a disruption of the capillary endothelial layer (bottom arrows). A platelet is adhering to the exposed basement membrane below. ALV, alveolus; CAP, capillary. (From Ref. [5].)

appears to be adhering to the exposed basement membrane, which is not surprising because the membrane is electrically charged and highly reactive.

The pressures required to cause these ultrastructural changes have been investigated and it has been shown that the first indications of stress failure in rabbit lung occur at capillary transmural pressures of about 24 mmHg, but the number of breaks is much increased at a pressure of 39mmHg [4]. It is perhaps remarkable that pressures as low as 24 mmHg result in some changes in view of the fact that in the human lung capillary transmural pressures as high as 36 mmHg apparently occur during heavy exercise. Of course, we cannot assume that the strength of the capillary walls is the same in rabbit and human lung, and in fact we have shown that different pressures are required for stress failure in rabbit, dog, and horse lung [7].

An important feature of these ultrastructural changes is that many are rapidly reversible when the capillary transmural pressure is reduced. This has been shown in our rabbit preparation by first raising the pressure to a high level, and then reducing it to a low level for several minutes with intravascular fixation at the low pressure. The result was that about 70 percent of both the endothelial and epithelial breaks closed within a few minutes [8]. The breaks that closed were mainly those that were initially small and associated with an intact basement membrane.

The micromechanics of stress failure are not fully understood, but one possibility is that the high tensile stress in the type IV collagen layer causes this to stretch with distortion of the matrix arrangement. It is known that type IV collagen molecules have sites that allow bending to occur. Thus it is possible that the basement membrane elongates under stress, causing the overlying cell to disrupt. When the tensile stress is removed the matrix reestablishes its normal configuration and the disruptions in the cells close. However, there is no direct evidence for this hypothesis.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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