1. Pressure was measured within 28 capillaries of the nailfolds of nine patients with essential hypertension and in 33 capillaries of nine age- and sex-matched normotensive control subjects, using direct micropuncture, a dynamic servo-nulling system and computerized analysis.

2. Average pressure at the apex of the capillary was found to be elevated in the patients with hypertension (21.1 ± 4.9 mmHg compared with 13.0 ± 2.0 mmHg in the control subjects; mean ± SD, P < 0.01). If the two groups were combined, there was an overall correlation between average capillary pressure and mean blood pressure (r = 0.68, P < 0.01, n = 18), but within each group separately there was no significant relation between these parameters.

3. There were also abnormalities in the waveforms of pulsations in capillary pressure in the group with hypertension, with an increased attenuation of high-frequency harmonics. Pulses appeared to be conducted more rapidly along the vascular tree in the patients with hypertension.

4. The elevation of capillary pressure in essential hypertension demonstrated in this study is in agreement with indirect evidence of capillary hyperfiltration provided by other studies which showed a reduced plasma volume and increased transcapillary escape rate of plasma proteins.

5. The finding of elevated capillary pressure demands the inclusion of the postcapillary segment (and possibly vascular density) in the resistance equation in essential hypertension.

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