ObjectiveTo determine the effects of capillary rarefaction on cardiovascular reactivity and microcirculatory functioning in essential hypertension.DesignHypertension is associated with abnormal cardiovascular reactivity and increased vasoconstriction. Capillary rarefaction amplifies these abnormalities, which modify microcirculatory hemodynamics. Hence this study of the hemorheological pattern and the veno-arteriolar reflex in hypertensive patients and normotensive control subjects.MethodsSixty-one men with never-treated essential hypertension and capillary rarefaction (<80 capillaries per field) and 20 age-matched and sex-matched controls underwent a strenuous cycle ergometer test to monitor, during exercise and recovery, the blood pressure profile and the hemorheological pattern: blood viscosity at low shear, hematocrit and leukocyte counts, soluble P-selectin levels, and red and white blood cell filterability rates. The veno-arteriolar reflex was determined by laser-Doppler flowmetry before exercise and at recovery.ResultsHypertensive men with ⩽ 72 capillaries per field had an abnormal hemorheological profile before exercise. The physiological response to exercise was observed only in the controls and in hypertensives with ⩾ 73 capillaries per field. Abnormal responses to exercise worsened as capillaries were more rarefied. At recovery, hemorheological parameters in hypertensives with 65–72 capillaries per field returned to baseline, remaining significantly (P<0.05) different to control values. Variations in the hemorheological pattern in hypertensives with<64 capillary per field persisted at recovery. The veno-arteriolar reflex followed the same pattern.ConclusionA reduced microvascular network may contribute to abnormal cardiovascular reactivity and to exercise-induced rheological abnormalities in hypertension.