The kidneys pick up on the lack of oxygen, making it the sensor. They are also the control, for they act to balance out the oxygen levels. Unfortunately, the kidneys do not help the situation. They release a chemical that tells the bone marrow to produce more red blood cells despite the fact it does nothing to fix the actual problem: low iron levels. The membranes of the veins stiffen up in response to the large number of red blood cells, trying to ensure that they pass through. The kidney still detects low oxygen and continues mass producing red blood cells. The red blood cells can get caught and build up until a blood clot is created, further harming the body.