What is the primary function of erythropoietin (EPO)?

The primary function of erythropoietin (EPO) is to stimulate the production of red blood cells in the bone marrow. EPO is a glycoprotein hormone primarily produced by the kidneys in response to low oxygen levels in the blood. When oxygen levels drop, the kidneys increase EPO production, which in turn binds to specific receptors in the bone marrow, promoting the proliferation and differentiation of erythroid progenitor cells into mature red blood cells. This process enhances the blood's oxygen-carrying capacity, which is crucial for maintaining adequate tissue oxygenation.

The other options refer to processes or substances that are not the primary role of erythropoietin. For instance, while increasing white blood cell count is important for immune responses, this is not a function of EPO; it is instead related to other growth factors and hormones. Additionally, promoting iron absorption is a function associated with dietary considerations and the role of other hormones like hepcidin, rather than EPO itself. Finally, reducing inflammation is typically associated with anti-inflammatory mediators rather than a hormone that primarily regulates erythropoiesis. Therefore, the correct answer directly aligns with EPO's established role in the body.