[The development of intramyocardial haemorrhages or fluidaccumulation in the extracellular space].

Isolated rat hearts according to Langendorff and rabbit hearts after orthostatic collapse were studied under the light and electron microscope. The light-micrographs were also quantitatively analysed. Changes in the vessels are noticeable, especially those in the sinusoid's. Vessel ruptures occur, also isolated endothelial ruptures with intact basement membranes and perfusion fluid enters the extracellular space. Numerous 0.06-01 mu large vesicles appear in the cytoplasma of the endothelial cells. A large number of vesiculation processes can be demonstrated along the cell membrane. The extracellular space is strongly-dilated. Changes in the colloid osmotic pressure and a rise in perfusion pressure have no clearly demonstrable influence on the extent of extravasation under the experimental conditions. The mechanism possible inducing such vessel changes is discussed.