1. Capillary blood pressure was measured in man using a dynamic servo-nulling system and direct micropuncture. This enabled assessments of the normal variations in pressure which influence fluid filtration and reabsorption.
2. Seventy-eight capillaries in 19 subjects were punctured in one of three positions around the capillary loop with the hand at the level of the sternal angle. Mean pressure around the loop fell from 37.7 ± 3.7 mmHg (arteriolar limb, mean ± sem, n = 12) to 19.4 ± 1.0 mmHg (apex, n = 25) to 14.6 ± 0.5 mmHg (venular limb, n = 41) at skin temperatures of 18.7–33.1°C. These values agree closely with Landis' original studies in 1930 [E. Landis (1930) Heart, 15, 209–228].
3. The mean filtration/reabsorption state of any particular capillary limb was not static because of cardiac, vasomotor and respiratory fluctuations in capillary pressure. From a total of 38 capillaries in which recordings were analysed for 30 s, the fluctuations in pressure were such that 27 capillaries probably had periods of both filtration and reabsorption.
4. Computerized superimposition and coherent averaging of trains of capillary pulses enabled an accurate description of the pulse waveform to be made in three capillaries. This was remarkably similar to waveforms from the radial artery, albeit at reduced amplitude (average 3.6 ± 3.4 mmHg, mean ± sd overall). The time for the pulse to travel between the radial artery and the finger capillary was approximately 10 ms, which implies a propagation velocity of several metres per second.
5. Fourteen long recordings of capillary pressure (mean 154 s) were subjected to computerized Fourier analysis and cross-correlation with other parameters. Low frequency fluctuations between 0.02 and 0.2 Hz (0.086 ± 0.05 Hz, mean ± sd) correlated closely with vasomotion in simultaneously recorded (laser-Doppler) skin blood flow (0.080 ± 0.5 Hz), although they were not synchronous. Higher frequencies between 0.2 and 0.6 Hz (0.28 ± 0.03 Hz) correlated well with respiration (0.29 ± 0.03 Hz), were synchronous and probably relate to respiratory fluctuations in venous pressure. The dynamic system has thus enabled us to describe the considerable variations in normal capillary pressure and to relate this variability to physiological influences.