Erythropoiesis is the process of producing red blood cells (RBCs) in the bone marrow. It is regulated by the hormone erythropoietin (EPO), which is secreted by the kidneys in response to low oxygen levels in the blood. There are various factors that can trigger erythropoiesis, but the most common one is increased tissue demand for oxygen.

Increased tissue demand for O2:
When the body's tissues require more oxygen than usual, the oxygen-carrying capacity of the blood needs to be increased. This can occur due to various reasons such as during exercise, pregnancy, or in individuals living in high-altitude areas. In response, the kidneys secrete erythropoietin, which stimulates the bone marrow to produce more RBCs. The newly produced RBCs contain hemoglobin, a protein that binds with oxygen and transports it to the tissues, thereby increasing the oxygen-carrying capacity of the blood.

Decreased tissue demand for O2:
Conversely, when the body's tissues require less oxygen than usual, such as during rest or in certain medical conditions, the kidneys secrete less erythropoietin, which results in decreased erythropoiesis.

An increased number of RBCs:
In some medical conditions, such as polycythemia vera, the bone marrow produces too many RBCs, leading to an increased number of circulating RBCs. This can cause thickening of the blood, leading to various complications.

Moving from a high altitude to a lower altitude:
When an individual moves from a high-altitude area to a lower-altitude area, the oxygen concentration in the air increases. This results in an increase in the oxygen-carrying capacity of the blood, which can cause a temporary decrease in erythropoietin production and erythropoiesis. However, this effect is usually short-lived, and the body adjusts to the new oxygen levels within a few days.

In conclusion, erythropoiesis is primarily triggered by an increased tissue demand for oxygen, which stimulates the production of RBCs in the bone marrow. Other factors such as decreased tissue demand for oxygen, an increased number of RBCs, and changes in altitude can also affect erythropoiesis.

Erythropoiesis is triggered by increased tissue demand for oxygen as we know that erythropoiesis means the formation of erythrocytes or red blood cells and mature red blood cells help in transporting oxygen to tissues or cells demand of oxygen is high...
At high altitude due to low atmospheric pressure many people tend faintbor even get breathless but in a span of few days they get acclimatised to the pressure deficiency as the body stimulates erythropoiesis as a result more erythrocytes are present and the body gets its required demands to perform the necessary metabolic functions !!!