Subcutaneous Starling forces in the unanaesthetized guinea-pig fetus with indwelling catheters.

We placed capsules in fetal guinea-pigs for measuring the hydrostatic pressure of the interstitial fluid and for collecting samples of interstitial fluid. The vitelline artery and vein were cannulated to measure blood pressures. Plasma colloid osmotic pressure (mmHg) and total protein concentration (g/100 ml) were measured in samples taken from other adult and fetal guinea-pigs and were found to be related by colloid osmotic pressure = 0.16 + 2.32 X protein concentration. The colloid osmotic pressures of fetal plasma and fetal interstitial fluid, calculated from measured protein concentrations, were 10.6 +/- 0.4 mmHg and 9.1 +/- 0.6 mmHg (mean +/- SEM). The hydrostatic pressure of the interstitial fluid was 0.2+/- 0.3 mmHg below intra-uterine pressure and the mean capillary blood pressure was calculated to be 1.3 +/- 0.8 mmHg above intra-uterine pressure. Arterial and venous blood pressures in the vitelline vessels were 29.0 +/- 0.9 and 3.8 +/- 0.6 mmHg above intra-uterine pressure. The ratio of pre- to -post-capillary resistance was, therefore, greater than 10.