Erythropoiesis is the process that ultimately forms more red blood cells in the bloodstream. Red blood cells are called erythrocytes. The hormone erythropoietin is secreted and this causes a cellular cascade of reactions to occur that ultimately produces additional red blood cells. Most of this process occurs in the bone marrow in humans. The reason that this process increases at higher altitudes is because it is triggered by decreasing levels of oxygen (O2) in the bloodstream. Decreased levels of oxygen in the blood is the result of less oxygen being inhaled from the air through the lungs. Since the partial pressure of oxygen in the atmosphere decreases as elevation increases, this is the reason why erythropoiesis occurs more often at higher altitudes.