Histometry of the placental structures involved in the respiratory interchange.

Employing a stereometric technique, we determined the following parameters in six normal term placentae: respiratory area (total surface area of vasculo-syncytial membranes); total villous surface area; proportion of respiratory area (percentage fraction of villous surface that corresponds to the respiratory area). The value of each of these parameters was studied in each placental sample, as well as their distribution in the different placental zones, considered according to their distance from the placental rim. All parameters were distributed heterogeneously throughout the peripheral, intermediate and central zones of the placentae studied. The same parameters were also measured along the full thickness of the placenta, considering subchorial, middle and basal levels. All parameters were homogeneously distributed along the thickness of the placenta. We discuss the functional implications of our findings. Villous surface area values ranged from 8.6 to 13.3 m2 (mean = 10.9 m2). Respiratory areas ranged from 0.97 to 2.43 m2 (mean = 1.57 m2). Between 1:10 and 1:5 of the villous surface area corresponded to the respiratory area.