Lowered tissue-fluid oncotic pressure protects the blood volume in the nephrotic syndrome.

We have studied the role of adjustments of tissue-fluid colloid osmotic pressure (COP) in the maintenance of the blood volume in 10 patients with the nephrotic syndrome before and after diuretic treatment until dry weight. A mean weight reduction of 13.5 +/- 6.4 kg was attended by a fall in blood volume in 3 patients and no change in 6, but the final blood volume was within the normal range: 84.3 +/- 3.7 ml/kg (normal value: 87.6 +/- 8.8 ml/kg). Albumin content and COP of tissue-fluid, obtained with subcutaneous wicks, were low before edema removal and rose slightly after it, parallel to changes in the plasma. Thus, the transcapillary gradient in COP did not change: 6.5 +/- 1.5 mm Hg before and 6.2 +/- 1.7 mm Hg after diuretic treatment. Considering the low COP, 8.6 +/- 1.6 mm Hg in edematous and 11.7 +/- 3.7 mm Hg in dry conditions, this gradient was only slightly below the value of about 10 mm Hg normally found with this technique. We conclude that a lowered tissue-fluid COP is important for the preservation of blood volume in dry patients with the nephrotic syndrome. In addition, this adaptation can explain why the blood volume is often normal and not expanded despite the sometimes huge overhydration in these patients.