Blood vascular casts of the rat adrenal glands were observed with a scanning electron microscope. The cortical capillary plexus drains, through the corticomedullary venous radicles, into the subcortical veins continuous with the medullary collecting veins. The medullary capillary plexus drains into the corticomedullary venous radicles, subcortical veins and medullary collecting veins. No portal vessel was noted between the cortical and medullary capillaries. These findings indicate that the cortical blood rich in glucocorticoids preferentially and continuously flows into the corticomedullary venous radicles, subcortical veins and medullary collecting veins all three of which are fenestrated in type, and also suggest that the vascular route from the cortical capillaries to the medullary collecting veins functions as a substitute for the portal system, controling the biosynthesis of catecholamines in the adrenal medulla. The vascular bed of the accessory adrenal gland (extra-adrenal cortical or chromaffin body) is sometimes annexed to that of the adrenal gland. On rare occasions, the vascular beds of the extra-adrenal cortical and chromaffin bodies fuse with each other. Additional scanning of tissue samples confirmed the direct drainage of cortical capillaries into the medullary veins and also the endothelial fenestrations of these capillaries and veins.
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