A decrease in blood oxygen capacity (BOC) is often seen in the juvenile form of iron deficiency anemia, which mainly affects adolescent girls (14-16 years old) due to hormonal changes during puberty and significantly hampers physical activity performance. The research focuses on studying the physiological mechanisms by which the cardiorespiratory system adapts to BOC limitations during the stepwise transfer of oxygen to tissues during physical exertion. We examined 12 adolescent girls, 14-15 years old, suffering from secondary anemia due to juvenile bleeding (Hb concentration – 109.0 ± 7.6 g/l) and 14 healthy girls of the same age (Hb – 122.0 ± 1.4 g/l). The participants performed a work of 50 W for 5 minutes with a step of 45 cm and an intensity of 35-50% of maximum O 2 consumption. The study aimed to evaluate the role of BOC in regulating O 2 mass transfer in the body and the development of tissue hypoxia during muscle activity. It was found that the O 2 demand for work in girls with anemia was 30% higher than in healthy girls, they had a higher O 2 debt, and a prolonged period of recovery of O 2 consumption. This was ensured by an increased rate of its mass transfer by blood from the lungs to the working tissues due to a significantly higher cardiac output, and mostly due to a higher heart rate. Additionally, they exhibited a greater increase in lung venti- lation, indicating reduced efficiency of external respiration. Also, a greater shift in blood buffer alkalis, severe metabolic acidosis with a decrease in pH to 7.34-7.33 in some girls and an increase in PCO 2 to 37.10 ± 2.54 mm Hg were observed, along with venous hypoxemia. All this indicated the development of secondary tissue hypoxia during exercise in these patients.