There are two sources from which glucose can enter the blood:
FROM THE GUT: After we eat, food is broken down in the upper intestine and absorbed, partly as glucose. Then it enters the blood directly, raising the blood glucose.
FROM THE LIVER: If you think about it, the body must have the ability to make its own glucose. If it didn't, every time an individual starved (fasted) for a prolonged period, the cells would use up the pre-existing glucose that was present in the blood, and the level of glucose in the blood would drop. Eventually, there would be no glucose left in the blood and levels would be zero. This process would happen very quickly in fact, and within a few hours the individual would be in trouble from a lack of glucose. As mentioned earlier, the brain is absolutely dependent on glucose for energy, i.e., it cannot use any other fuel. In fact, our brains could never survive this lack of glucose.
Therefore the human body has a mechanism to protect itself: as glucose is used, the liver makes more of it. The amount of glucose in the blood is thus held constant, a process which is necessary for life. Several factors influence and coordinate the liver's glucose production, including current blood glucose level, how much insulin is available, and the levels of other hormones and nutrients in the blood.