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Erythropoiesis is the process by which new red blood cells are produced. It occurs primarily in the bone marrow, which is a soft, spongy tissue found in the hollow centers of certain bones. Red blood cells, or erythrocytes, are crucial for transporting oxygen throughout the body.
This process is regulated by the hormone erythropoietin, mainly produced by the kidneys. When oxygen levels in the blood drop, the kidneys release this hormone to stimulate the bone marrow to increase the production of red blood cells.
In patients undergoing chemotherapy, the production of erythropoietin can be disrupted, leading to decreased erythropoiesis. Consequently, fewer red blood cells are produced, resulting in anemia. This weakened state manifests as fatigue, shortness of breath, and weakness.

Chemotherapy often targets rapidly dividing cells, which is why it is effective against cancer cells. However, this treatment also affects other rapidly dividing cells in the body, including those in the bone marrow.
The bone marrow's function as a primary site for blood cell production becomes compromised under chemotherapy. Its capability to generate new red blood cells, white blood cells, and platelets diminishes.
As a result, patients might experience not only anemia but also a decreased immune response and increased bleeding risks due to fewer white blood cells and platelets being available. The bone marrow's suppression significantly contributes to anemia as fewer red blood cells are replenished to replace the old or damaged ones. This direct impact on the bone marrow is a key factor in chemotherapy-induced anemia.

Red blood cells have an average lifespan of about 120 days, after which they are removed from circulation and replaced by new cells produced in the bone marrow.
When chemotherapy affects the bone marrow, the rate of red blood cell production drops significantly below the rate of their natural degradation. This imbalance leads to a lower count of available red blood cells and, by extension, a reduced capacity for oxygen transport in the body.
The symptoms associated with a decreased rate of red blood cell production, such as fatigue and weakness, are primarily due to this lack of sufficient oxygen delivery to tissues. Treatment strategies might involve erythropoiesis-stimulating agents or transfusions to help manage anemia by encouraging red blood cell production or directly increasing their numbers.