Fetal gastrointestinal composition: implications for water and electrolyte absorption.

In the late-gestation human or ovine fetus, daily swallowed volumes (100-300 ml/kg body weight) are markedly greater than adult values (40-60 ml/kg). This volume of amniotic fluid water, electrolytes and cellular debris is almost completely resorbed within the fetal gastrointestinal (GI) tract, resulting in the formation of meconium within the distal fetal colon. Despite this capacity for fetal GI water resorption, little is known regarding the sites or processes by which absorption occurs. We examined the composition of GI contents of 13 ovine fetuses (121-148 days). Fetuses were operatively delivered and killed and the Gi tract (rumen to sigmoid colon) was ligated in sections. Luminal samples were obtained and analysed for water and organic content and osmolality and electrolyte composition. The water content significantly decreased (98.7% to 70.1%) while the organic content significantly increased (55.0% to 95.3% of dry weight) from the rumen to colon. Osmolality significantly increased from the rumen to the distal small intestine (303 +/- 3 to 596 +/- 50 mOsm/kg) and returned to near isotonic values in the sigmoid colon. These results indicate significant water absorption within the fetal GI tract, with a pattern similar to that of the adult. However, fetal colonic sodium absorption is either suppresses or immature near term.