How does the body respond to decreasing blood glucose?

Blood glucose levels start to fall in the blood. Blood flowing through the pancreas is low in glucose, and this is sensed by cells in the pancreatic tissue. In response, it produces glucagon, (which is a hormone). Glucagon causes the breakdown of glyocgen in the liver and muscles, to produce glucose. This is called glycogenolysis. 

This homeostatic mechanism results is activated in response to low blood glucose.
An easy way to remember it is: "gluca-gone": the glucose has gone, so it releases glucagon to raise the blood glucose.