Here's a breakdown:

* Erythropoietin (EPO) is a hormone primarily produced by the kidneys (and to a lesser extent the liver).

* Erythroid progenitor cells are immature cells in the bone marrow that are destined to become red blood cells (erythrocytes).

* Bone marrow is the spongy tissue inside certain bones where blood cells are made.

When EPO binds to its receptors on erythroid progenitor cells, it triggers a cascade of events that:

1. Stimulates the production of more red blood cells by promoting cell division and differentiation of erythroid progenitor cells.

2. Increases the rate of hemoglobin synthesis within developing red blood cells.

This ultimately leads to an increase in the oxygen-carrying capacity of the blood.