Insulin is continuously made and stored in special vesicles (little storage bubbles in the cells), ready to be released into the blood on certain stimuli.
The level of glucose in the blood is the main signal for releasing insulin from the pancreas. Protein, or more correctly amino acids, the break down products of protein in the food, will also trigger the release of insulin, but not in the same amounts.
When blood glucose rises, after a meal, the glucose will be taken up by the cells in the pancreas and through a complex signal pattern, insulin will be released into the blood stream. Insulin will facilitate the uptake of glucose by binding to special receptors that are located on cells all over the body. This binding will signal to the cell to bring channels (that are packed away in the cell) to the cell wall to transport the glucose into it.
Most cells in the body have these channels, and they are most abundant in muscle and fat cells. Insulin also promotes the storage of energy by signaling to the muscle cells to convert the glucose to glycogen and to the fat cells to make triglycerides (fat).
Insulin is needed for the uptake of glucose into the cells, without its signals the channels won’t reach the cell membrane and glucose remains outside, in the blood stream. Some glucose will still be taken up by muscle, when exercising, because using the muscles will have the same effect as insulin on the cells, but in much lesser extent and not enough to clear the blood stream after a full meal.