The balance of fluid across capillaries is given by the Starling equation. Because the
plasma protein concentration (one of its components) is decreased in pregnancy, we decided to explore the question as to whether
hypoproteinemia with intact
protein mass (produced by blood volume expansion) or
hypoproteinemia with decreased total
protein mass (produced by removal of circulating
proteins) alters the oncotic pressure differences across capillaries. We calculated the oncotic pressure difference obtained in seven nonpregnant ewes during periods of normoproteinemia and
hypoproteinemia; the influence of fluid infusion under both conditions was also observed. There was an increase in the oncotic pressure difference across the capillary wall during
hypoproteinemia produced by a decrease in the total
protein mass (p less than 0.01); however, the response to
hypoproteinemia produced by fluid infusion was similar (p greater than 0.1). The venous pressure (used as an index of interstitial fluid pressure changes) did not differ in either
hypoproteinemia or fluid infusion. Capillary permeability was decreased during
hypoproteinemia, as evidenced by a higher lymph/
protein ratio of labeled
albumin during the control period (p less than 0.05). In conclusion, during a state of decreased
plasma protein concentration similar to that of pregnancy, the difference in the oncotic pressure and hydrostatic pressure forces acting to prevent transfer of fluid to the interstitium is increased. Therefore other factors that influence fluid transfer across the capillaries must be investigated to explain the
edema of pregnancy.