Blood Capillaries of the Heart and Other Organs

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Abstract
The article is a review of work recently carried out on blood capillaries by the author in collaboration with Drs. M. G. Farquhar, G. Majno, and S. L. Wissig. It reviews the morphology of these vessels at the electron-microscope level and confirms the existence of at least 3 distinct types of blood capillaries in small laboratory mammals. It shows that the capillary wall consists of 3 concentric layers (endothelium, basement membrane, and adventitia), and indicates that the basement membrane forms a continuous layer in all capillaries so far studied. Experiments in which colloidal gold particles were used as a tracer have shown that, in capillaries with a continuous endothelium (muscle capillaries), the particles are transported across it by "pinocytic" vesicles. At the end of this step they must still transverse the basement membrane. Experiments on glomerular capillaries, which typically have a discontinuous s endothelium, were carried out on normal and nephrotic rats using ferritin as a tracer. By its accumulation on the luminal side of the basement membrane, the ferritin has identified this layer as the main filtration barrier. A similar function of the basement membrane was demonstrated in muscle venules and venous capillaries by experiments in which the endothelium was rendered discontinuous by local treatment with histamine and serotonin.

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