The Fine Structure of Capillaries and Small Arteries by Dan

The passage of materials across capillary walls is explained by passive physicochemical mechanisms in the classical physiological literature (1). More recently the participation of enzymes and structural alterations as a means of active transport has been extensively debated (2). Morphologic studies with the electron microscope have shown that the structure of capillary walls is different and more complex than previously assumed and have provided material for a reconsideration of the various transport mechanisms. In the kidney glomerulus fluid is released from the capillary bed through pores in the capillary endothelia across the basement membrane into the Bowman space (3-7). The basement membrane, as an ultrafilter (4), retains the blood proteins but is penetrated by glucose and other dissolved materials. In most other capillaries, fluid and dissolved substances have to pass endothelial cytoplasm to leave the blood stream. Thereby active transmission and selective mechanisms other than ultrafiltration across the basement membrane, can be effective. I t has been concluded from electrophoretic studies that the passage of proteins through the human placenta is selective in that the large gamma globulin passes more readily to the fetus than do the smaller albumin molecules (8). The passage of globulins is known clinically also since antibodies of the maternal blood are found in the newborn child. Transport through the placenta involves, however, several fetal cell layers. A simpler example of selective protein transport is the prevalent shift of serum albumin into muscle tissue after the release of a tourniquet (9, 10). Electron microscopic observations gave a hint to the understanding of capillary activity in transport (11, 12). The mechanism appears to be one which has been called pinocytosis (13). The extent of pinocytic activity in capillary endothelia appears to depend on the requirements of the surrounding cells, and manifestations of this activity vary in capillaries supplying different tissues. A special problem arises in the supply of smooth muscle cells in arteries without vasa vasorum. There the membrana elastica interna would seem to restrict diffusion as well as ultrafiltration to a higher degree than does a basement membrane. 457