Glucose and glucagon

All activity of the body requires energy, mediated by a chemical called ATP, which is the only substance in the body cells can use directly.  ATP (Adenosine-5'-triphosphate) is often called the "molecular unit of currency" of intracellular energy transfer. It transports chemical energy within cells for metabolism.

Outside our cells, a fairly complex process takes the food we eat and converts the food into glucose, which then becomes the fuel for our bodies at exercise or rest.  Our brain uses glucose, all our muscles use glucose.  The glucose is supplied to all our organs via our blood.

The glucose in the blood gets pulled into the cell and via ATP becomes usable energy [this is highly simplified, but does for our purposes]. 

It is thus quite normal for the blood to have glucose in it.  The normal blood glucose is about 90 mg of glucose per 100 cm3 of blood.  When we need more energy, more glucose is pumped into the blood, when we need less, the liver converts any excess glucose created by our digestive systems or in the blood into glycogen and stores it ready for use when we do need it.  The liver has what are called alpha and beta cells in the ‘islets of Langerhan’ that secrete two hormones that help regulate the supply. 

If the glucose is too high insulin is secreted by the beta cells in the liver and this converts the glucose to glycogen. 

If it is too low alpha cells secrete Glucagon and the liver converts the glucagons to glucose.