In order to maintain homeostasis, which in this case means a stable blood glucose, certain hormones must be released in situations which alter blood glucose. This occurs most commonly after a meal (increase) or during fasting states (decrease).
Insulin and glucagon are two hormones that are released by an organ called the pancreas to control blood glucose. They are released from beta and alpha pancreatic cells respectively in parts of the pancreas called islets. Insulin increases the uptake of circulating glucose into cells, as well as the subsequent sequestering of the sugar, while glucagon does the opposite.