Pigeons flying for 1 year were compared with their nonflying littermates. The physically active animals proved to have higher weights of the heart and breast muscle. In both tissues, a significantly higher water content was found in animals with restricted activity.The heart and breast muscle of the pigeon are characterized by high capillary counts (over 4000 capillaries/mm2 in the heart muscle and over 2000 capillaries/mm2 in the breast muscle). The capillary density in the heart muscle was the same in both groups, while the capillary density in the breast muscle was significantly lower in the physically active animals.
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