Gilbert W M
Department of Reproductive Medicine, University of California, San Diego.
Am J Obstet Gynecol. 1992 Dec;167(6):1742-7. doi: 10.1016/0002-9378(92)91769-7.
Previously we found that water infused into the ovine amniotic cavity was rapidly absorbed into the fetal circulation through the vascularized fetal membranes and fetal surface of the placenta (i.e., the intramembranous pathway). The purposes of this study were to (1) estimate the conductance of the intramembranous pathway from the allantoic cavity and (2) determine if the conductance is adequate to offset the inflow of urine, which may be up to 500 ml/day in the near-term ovine fetus.
Seven chronically catheterized fetal sheep averaging 132 +/- 2 (+/- SE) days' gestation underwent an infusion of warmed distilled water into the allantoic cavity at 6 ml/min. The infusions were continued until steady states were obtained in allantoic and amniotic fluid and in fetal and maternal blood osmolalities. During the steady state the conductance of the intramembranous pathway was estimated as the ratio of osmotic gradient to infusion rate.
The allantoic and amniotic fluid and the fetal and maternal blood osmolalities decreased by 188 +/- 14, 36 +/- 8, 13 +/- 2, and 3 +/- 1 mOsm/kg, respectively, at steady state. From the fetal-allantoic osmolality gradients the conductance of the intramembranous pathway was 1.72 +/- 0.14 or 0.53 +/- 0.08 microliter/min/mm Hg/kg fetal weight. Assuming a similar conductance during the preinfusion period, the next volume movement would equal 0.67 ml/min (965 ml/day).
The conductance of the intramembranous pathway in combination with the normal osmotic gradient is sufficient to remove the large volume of fetal urine that may enter the allantoic cavity each day.
此前我们发现，注入羊膜腔的水会通过血管化的胎膜和胎盘的胎儿表面（即膜内途径）迅速被吸收进入胎儿循环。本研究的目的是：（1）估计从尿囊腔到膜内途径的传导率；（2）确定该传导率是否足以抵消尿液的流入量，在接近足月的绵羊胎儿中，尿液流入量每天可能高达500毫升。
7只平均妊娠132±2（±标准误）天且长期插管的胎羊，以6毫升/分钟的速度向尿囊腔输注温热的蒸馏水。输注持续进行，直到尿囊液、羊水以及胎儿和母体血液渗透压达到稳态。在稳态期间，膜内途径的传导率估计为渗透梯度与输注速率的比值。
在稳态时，尿囊液、羊水以及胎儿和母体血液渗透压分别下降了188±14、36±8、13±2和3±1毫摩尔渗透压/千克。根据胎儿 - 尿囊渗透压梯度，膜内途径的传导率为1.72±0.14或0.53±0.08微升/分钟/毫米汞柱/千克胎儿体重。假设在输注前阶段传导率相似，下一个体积移动量将等于0.67毫升/分钟（965毫升/天）。
膜内途径的传导率与正常渗透梯度相结合，足以清除每天可能进入尿囊腔的大量胎儿尿液。
