The pancreas and the control of blood sugar levels:
When you digest a meal, large amount of glucose pass in your blood. You need to control this because too much glucose and it would be removed in the urine and too little glucose and your cells wouldn't be able to respire.
Your pancreas is the organ that controls this. It secretes two hormones depending on your blood sugar level. If it's too high after you've just eaten, it secretes insulin and this turns the glucose into glycogen which is then stored in your liver.
When your blood sugar is too low, your pancreas secretes glucagon which turns the glycogen in your liver, back into glucose.