When ascending to a certain altitude, the amount of oxygen that reaches the lungs decreases due to the effect of lower partial pressure, and this phenomenon is called hypoxia. The body responds by producing a glycoprotein hormone – erythropoietin (EPO) in order to increase the level of hemoglobin. After staying in such conditions for a certain period of time, the body acclimatizes to the altitude level (adapts to the environment) and increases the level of hemoglobin in the blood to a sufficient level, so that the blood can transport the necessary amount of oxygen to the tissues. After that, when the body returns to sea level or lower altitudes where the oxygen level is higher, much more oxygen is delivered to the tissues because the blood of a person retains a high content of hemoglobin for some time

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