calcium concentration in your blood is regulated by your parathyroid gland. if it falls below 4.3 meq/l, the parathyroid gland recognizes it and signals to the signals to the kidney to prevent it from being released in urine as well as causes bone to break down and release calcium into the blood. if it gets above 5.3 meq/l, the kidneys excrete more calcium and your bone absorbs additional calcium. using the homeostatic model that we learned in class, what is the control center?