Influence of body posture on transcapillary pressures in human subcutaneous tissue

Abstract

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 (II_i) was measured in fluid collected by implantation of multi-filamentous nylon wicks. Interstitial fluid hydrostatic pressure (P_i) was measured by a ‘wick-in-needle’ method. Samples for determination of plasma colloid osmotic pressure (II_p) were obtained by venipuncture. In the upright position II_i 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) II_i on thorax was practically unchanged, while II_i at the ankle rose from 10.1 (SD 2.9) mmHg to 12.2 (SD 2.8) mmHg. P_i 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 II_i and P_i are small, and can compensate for only a fraction of the changes in capillary pressure.

Key-words:

• orthostasis
• oedema-preventing mechanisms
• interstitial fluid colloid osmotic pressure
• interstitial fluid hydrostatic pressure
• Starling equilibrium