Reactivity of small blood vessels in hypertension: relation with structural changes. State of the art lecture.

Small blood vessels, particularly small arteries of 150-300 microns in lumen diameter and larger arterioles of 50-150 microns in lumen diameter, are the most important location of the arterial bed that undergoes changes resulting in the increased peripheral resistance that characterizes elevated blood pressure. This article reviews these morphological and functional alterations of small blood vessels. Study of mesenteric small arteries with a lumen diameter of 220-260 microns revealed consistently a reduced external and lumen diameter in renal and deoxycorticosterone acetate-salt hypertensive rats early in the evolution of hypertension. The media of the vessel wall was significantly thickened in the hypertensive rats, and the media/lumen ratio was increased. Although the tension that developed in response to different vasoconstrictors was not elevated or was even decreased in hypertensive rats, exaggerated transmural pressures resulted in the isolated blood vessels as a consequence of the law of Laplace because of the narrowed lumen. Similar findings were obtained in small resistance arteries from subcutaneous gluteal biopsies of hypertensive humans. In arterioles, on the other hand, decreases in the density of blood vessels (rarefaction) and in vasomotion amplitude may play a more important role than reductions in lumen diameter. As a consequence of the design of small resistance blood vessels and as a result of functional and structural alterations, which may be primary to or a consequence of high blood pressure, the pressor effect of vasoconstrictors is amplified and interacts with other factors to contribute to the maintenance of elevated blood pressure even if the intrinsic response of vascular smooth muscle to these agents is not exaggerated.