An athlete travels to the Colorado Rockies to train for an upcoming marathon. He runs 15 miles every other day and while breathing O2 deficient air. His blood O2 levels become lowered as a result. His kidney cells sense the reduction in O2 and secrete a hormone, erythropoietin into the blood stream. The hormone circulates to the bone marrow where it acts on stem cells. The stem cells differentiate into mature red blood cells which can carry additional oxygen.

1. What is the stimulus?
2. What is the efferent pathway?
3. What is the effector?
4. Is this a negative or positive feedback mechanism? Explain your answer.