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Brachial and carotid artery diameters, blood velocities and blood flows were determined by a pulsed-Doppler flowmeter in 22 normotensive and 74 mild to moderate essential hypertensive subjects, aged 46 ± 8 years. The product age x MAP/1000 (where MAP is mean arterial blood pressure), called 'arterial age' (expressed in years x mmHg/1000), was used to assess the progress of age and blood pressure. The moving-mean method was first used to explore the chronic hemodynamic changes in the two circulations with a progressive increase in arterial age. The method indicated that at an arterial age of about 4-4.5units a noticeable decrease in both brachial and carotid blood velocities occurred, and at an arterial age of about 5 units an important increase in brachial artery diameter occurred. In order to test these findings, the total sample (n=96) was divided into three groups of equal size with low, median and high arterial ages. Brachial blood velocity decreased significantly in groups 2 (P<0.05) and 3 (P<0.05) compared with group 1, and carotid blood velocity decreased significantly from group 1 to group 2 (P<0.02) and from group 2 to group 3 (P<0.05). Brachial artery diameter did not change from group 1 to group 2, but increased significantly from group 2 to group 3 (P<0.05). Over the whole arterial age range, the carotid artery diameter remained identical. As a result, brachial blood flow remained unchanged in the three groups, and carotid blood flow decreased significantly from group 1 to group 2 (P<0.05) and from group 2 to group 3 (P<0.05). A positive relationship was found between brachial and carotid blood velocities (r=0.78, P<0.0001) and between brachial and carotid-blood flows (r=0.51, P<0.005) in group 3. No similar relationship was observed in groups 1 and 2. The results suggest that: (1) blood velocity in the brachial and carotid arteries progressively decreases under the combined effects of age and hypertension, and this decrease begins to occur at a moderate arterial age; and (2) at a high arterial age, an adaptation mechanism occurs in the brachial side to conserve blood flow, despite a persistent decrease in blood velocity, but not in the carotid side.