Influence of body posture on transcapillary pressures in human subcutaneous tissue

Capillary pressure in the human circulation varies within a wide range depending on the height difference between the capillary and the heart. To study the influence of body posture on transcapillary pressures in subcutaneous tissue, 28 healthy volunteers were examined. Interstitial colloid osmotic pressure (IIi) was measured in fluid collected by implantation of multi-filamentous nylon wicks. Interstitial fluid hydrostatic pressure (Pi) was measured by a 'wick-in-needle' method. Samples for determination of plasma colloid osmotic pressure (IIp) were obtained by venipuncture. In the upright position IIi was 15.2 (SD 2.1) mmHg on the thorax and 10.4 (SD 2.1) mmHg at the ankle. Similar values were obtained in subjects examined after 2 h in a horizontal position. During sustained rest in bed (40 h) IIi on thorax was practically unchanged, while IIi at the ankle rose from 10.1 (SD 2.9) mmHg to 12.2 (SD 2.8) mmHg. Pi averaged -1.3 (SD 1.6) mmHg on the thorax and -0.4 (SD 2.5) mmHg at the ankle, but the difference was not statistically significant. Altogether the body-posture dependent variations in IIi and Pi are small, and can compensate for only a fraction of the changes in capillary pressure.