Insulin is a hormone which is produced by the pancreas in order to regulate blood sugar levels by storing excess glucose away as glycogen for use later when the body is in need of that glucose. It is usually released when blood sugar levels quickly rise to a high level, which is usually after meals.
The pancreas, which is an organ in the digestive system, is the organ that produces and releases insulin when needed. The reason why the pancreas is needed to produce and release insulin is because if it didn’t you would have diabetes and would need an insulin pump to be able to store away excess glucose. Lack of insulin is a cause of diabetes.
Of course, once the glucose has been stored away as glycogen, it needs to be released. When blood sugar levels are low, the pancreas produces glucagon, which causes the stored glycogen to be converted back into glucose which can be used by the body to increase blood sugar levels. It works in the opposite way to that of insulin.