1. Histamine exerts a local dilator effect upon capillaries and upon the smallest arterioles and venules which border the capillary system. There occurs also an opening up of large numbers of capillaries of which no trace can be seen before the application of histamine.
2. When injected intravenously in amounts sufficient to produce shock, histamine causes a quickly progressive dilatation of both the visible and the occult capillaries and of their immediately adjacent arterioles and venules, all of which become engorged with blood that moves through them in a strikingly sluggish manner. The circulatory failure which characterizes histamine shock results from the dilatation of the peripheral vascular bed.