Blood cells concentration

Can anyone explain why blood cell concentration increases on decresing oxygen concentration?

The reason that blood cell production increases when lowering oxygen levels is because of a process called erythropoiesis. Erythropoiesis is the process by which red blood cells are produced. The body needs oxygen to create energy, and red blood cells are responsible for transporting oxygen to the body’s tissues. When oxygen levels are low, the body signals the bone marrow to produce more red blood cells. The bone marrow then releases more erythropoietin, a hormone that stimulates the production of red blood cells.
Example: If someone is living at a high altitude, they will have less oxygen in their bloodstream because oxygen molecules are further apart at high altitudes. The body will adjust by producing more red blood cells in order to transport more oxygen to the tissues.