Morphological adaptation of the cardiovascular system in fetal rats during late gestation.

: The aim of this study was to evaluate morphological changes of the cardiovascular system in fetal rats during late gestation. We used the rapid whole-body freezing technique for rats of day 17 through 21 of gestation. The right and left ventricular volumes increased markedly and significantly during this period by about 11- and 24-fold, respectively. Although the right ventricular volume was 108% larger with statistical significance than the left ventricular volume on day 17, they were almost equal after day 19. The length of the primum septum of the atrium significantly increased by 92% within 4 days, but the opening distance of foramen ovale significantly decreased by 14%. The ratio of the inner diameter (the sum of right and left pulmonary arteries to ductus arteriosus) significantly increased from 0.72+/-0.03 on day 17 to 1.17+/-0.07 on day 21. There was also a significant increase in the ratio of the inner diameters of the ascending to descending aorta. These observations suggest that the reduction of the opening distance of foramen ovale reflect the growth of pulmonary arteries.