Abstract
1. 1. Djungarian dwarf hamsters ( Phodopus sungorus) show an annual cycle in weight-specific metabolic rate with a high level during winter. 2. 2. These seasonal changes in oxygen demand are met by hematological adjustments, primarily based on an increased number of erythrocytes, but a decreased erythrocyte volume during winter. 3. 3. Subsequently, the diffusion area for blood gas exchange is increased during this time of high metabolic capabilities. 4. 4. Blood oxygen capacity (hemoglobin, 2,3-diphosphoglycerate (2,3-DPG) does not change with the season. 5. 5. However, seasonal changes in heart weight suggest changes in cardiac output, causing an increased blood flow per unit tissue weight during winter. 6. 6. This increase in circulatory efficiency, as well as changes in erythrocyte surface, are primarily controlled by photoperiod, since it occurred in hamsters living indoors at thermoneutrality but subjected to seasonal changes in photoperiod to the same extent as in hamsters living outdoors.
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