Food enters the body. The food is broken down. Sugars from the food enter the bloodstream. Blood sugar must be within a particular range to avoid damaging many organs. Sometimes the brain tells the pancreas the blood sugar (glucose) is too low. The pancreas releases glucagon to change the blood sugar to a better level. Sometimes the brain tells the pancreas the blood sugar (glucose) is too high. The pancreas releases insulin to change the blood sugar to a better level. 
How does the body control its blood sugar level?