Extracellular volume and blood volume in chronically catheterized fetal sheep

1. To determine the extracellular volume (ECV) in fetal sheep and its distribution between the plasma and interstitial spaces, ECV, blood volume (BV) and haematocrit were measured in ten chronically catheterized fetal sheep aged 121-133 days. Relationships with age, weight and other fetal variables, including glomerular filtration rate (GFR), were studied. 2. ECV was measured as the mean of the volumes of distribution of [3H]inulin and [14C]mannitol extrapolated to time zero. The time zero volume of distribution was 1506 +/- 79 ml (means +/- S.E.M.) for inulin and 1590 +/- 80 ml for mannitol. The ECV was 1548 +/- 79 ml (632 +/- 18 ml (kg fetal wt)-1). BV, measured using 51Cr-labelled red cells, was 351 +/- 27 ml (141 +/- 6 ml kg-1). Haematocrit, plasma volume and interstitial volume were 34 +/- 1%, 229 +/- 17 ml (92 +/- 3 ml kg-1) and 1319 +/- 63 ml (540 +/- 17 ml kg-1), respectively. 3. Interstitial volume per kilogram fell with increasing fetal weight (P = 0.026). BV per kilogram did not change with weight or age. 4. The plasma: interstitial volume ratio was 0.17 +/- 0.01. This ratio increased as fetal weight and age increased (P = 0.026 and P = 0.044), that is, the proportion of ECV that was contained outside the vascular compartment was lower in heavier or older fetuses. 5. Since GFR was 3.4 +/- 0.4 ml min-1, the entire fetal ECV was filtered by the fetal kidney only 3.1 +/- 0.3 times per day.