the endocrine system regulates many body conditions with feedback loops each feedback loop has the following components stimulus a change in a body condition production cell an endocrine cell that produces a hormone after being affected by stimulus hormone the signaling chemical target cell a cell receptive to the hormone action what the cell does when affected by the hormone response the overall change in controlled body condition as a result of the feedback loop production of erythropoietin or EPO is part of hormonal regulation of blood oxygen low blood oxygen stimulates kidney cells to secrete EPO EPO targets precursor erythrocytes in the hemopoietic tissue and causes them to differentiate and proliferate increased numbers of mature red blood cells are released into the blood increasing its oxygen carrying capacity blood oxygen level is restored to normal