Capillary network geometry during postnatal growth in rat hearts.

To determine the effects of postnatal growth on the coronary microcirculation, the geometry of capillary nets was studied in male rats at 21, 28, 60, and 240 days of age. Tissue sections were exposed to a histochemical technique that distinguished arteriolar (AC) and venular (VC) capillary regions by color. The tissue area (capillary domain) surrounding individual capillaries progressively increased with growth of the heart (P less than 0.01). In all groups, AC domain area was larger than VC domain area (P less than 0.01). The heterogeneity of capillary spacing, which is another important determinant of oxygen supply, demonstrated significant increases with age for AC regions (P less than 0.01). With increasing left ventricular mass, there was a lateral expansion of the arteriovenous capillary set, which was displayed by an increase in the number of and distance between adjacent capillaries that traversed the distance from arteriole to venule (P less than 0.01). Our data indicate that differences in the geometry of arteriolar and VC regions are present by 21 days of age. During postnatal growth, the results from both cross and longitudinal sections suggest that the geometrical conditions for oxygen supply are notably reduced.