Why and how is blood glucose controlled in the body?

So before we understand this we need to quickly explain homeostasis. This is essentially the process by which the body maintains a constant internal environment. Our external environment changes all the time but it's important to keep things steady inside so that key processes in the body run smoothly. Glucose, along with carbon dioxide and temperature for example, needs to be tightly regulated in the body. We use glucose in aerobic respiration in order to release energy in the form of ATP.

Glucose levels in the blood are controlled by 2 major hormones, and these are insulin and glucagon. Insulin is released when glucose levels are high, and works to decrease levels towards a normal by promoting glucose uptake into target tissues like the liver, fat and skeletal muscle. Glucagon however is released when glucose levels are very low in the blood, in order to increase them up to an optimum normal level. Glucagon works to increase release of glucose from tissues where it's being stored, and also promotes the production of glucose from substrates found in proteins and fats.