Fetal Defecation

Dilatation of the rectosigmoid in newborn infants with ARM supports the hypothesis that fetal colonic peristalsis and defecation is a normal physiological process [56]. What is contentious is whether colonic emptying occurs during the third trimester. The determination of fetal intestinal disaccharidases in the amniotic fluid suggests that they are released by defecation [92]. Their disappearance after the 22nd week suggests that rectal emptying is absent during the third trimester.

This evidence must be correlated with morphological data regarding the anatomical development of the anal sphincteric mechanism [10]. Digestive enzyme assays indicate that anal membrane perforation occurs at about 12 weeks of gestation; their absence in amniotic fluid subsequent to 22 weeks of gestation apparently cannot be explained by development of anal continence, as morphological evidence shows that the functional components of the sphincteric mechanism are not adequate by the 30th week of gestation. The use of ultrasound to observe anal activity revealed that the greatest frequency occurs between weeks 28 and 34 of gestation; however, defining fetal defecation as the expulsion of contents through the anus showed that emptying of rectal content occurs until term. The discrepancy between these studies remains speculative.

Hypoxic defecation in the mature fetus is unchallenged when meconium staining of liquor occurs. If, as it is suggested, defecation in utero is a physiological fetal function, what happens to the evacuated material? Filtration mechanisms clearing unwanted content have been demonstrated in animal experiments, which are rendered inoperative during hypoxic stress [17,18]. These findings point to the presence of a similar physiological cycle in the human. It is obvious that in the fetus with an ARM and fistula, emptying of the rectum is possible and determined by the size and site of this channel/tract. Meconium reaches the colon by the 18th week of gestation and builds up thereafter in the colon. How much of it is evacuated before term remains a matter of conjecture [97].

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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