A 20-year-old young man, who started to train systematically in athleti¬cs, has the following resting-state blood values: erythrocytes - 5,5 • 1012/l, reti¬culocytes - 12%, hemoglobin - 160 g/l, color index - 1,03. Such blood values indicate erythropoiesis stimulation due to the following occurring in the process of his training: