What is blood glucose (sugar)?

Blood glucose, also known as blood sugar, is an energy source for muscles and other tissue cells, which comes from food and your liver. Sugar enters the bloodstream, gets absorbed, then with insulin’s help, enters the cells. If your blood glucose levels are low (like from lack of eating), stored glycogen in the liver is broken down into glucose so your glucose level stays within a normal range.

Source: http://www.mayoclinic.org