In humans and other vertebrates, the pancreas is a glandular structure. One of its main functions is associated with the endocrine system. Its endocrine function is the production of hormones, e.g. insulin and glucagon. The cells in the pancreas that carry out endocrine functions are the islets of Langerhans. The two major cells of the islets are the alpha cells and the beta cells. The alpha cells produce glucagon whereas the beta cells make insulin. These hormones are essential in regulating carbohydrate and fat metabolism. Insulin, in particular, regulates the metabolism of carbohydrates and fats by promoting the uptake of glucose from the bloodstream into the skeletal muscles and fat tissues to be stored as glycogen for later use in glycogenolysis. Glucagon, in turn, acts by stimulating the production of sugar. Thus, glucagon works in opposite to insulin in a way that it raises the concentration of glucose in the bloodstream. It causes the stored glycogen in the liver to be converted into glucose that will be released into the bloodstream. Glucagon and insulin, thus, works in a feedback system, regulating the level of glucose in the bloodstream.