The microcirculation in hypertension.

Studies of the peripheral microcirculation in major organ systems during hypertension indicate that in anaesthetized spontaneously hypertensive rats (SHR) a combination of both vasoconstriction and temporary and permanent vessel closure occurs. The vasoconstriction is often limited to the largest and smallest arterioles and rarefaction is best expressed for arterioles with inner diameters less than 25 microns in the lower body musculature. Direct measurements of microvascular pressures in hypertensive rats indicate that while all arterioles in major organ systems are exposed to a pressure much higher than normal, the smallest arterioles dissipate a much higher than normal fraction of the mean arterial pressure. However, the transition vessels between the smallest arteries and the larger arterioles account for the largest fraction of total resistance (50-60%) and dominate the precapillary vascular resistance in the cerebral, intestinal and skeletal muscle vasculatures. In the established stages of hypertension, the relative influence of the transition resistance vessels is somewhat less than during the developmental phase of hypertension and also less than in adult normal animals. This latter observation has been made in a number of vasculatures and may indicate that resistance changes in the true microvessels and in the transition vessels do not occur simultaneously during the development of hypertension.