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The pancreas has both endocrine and exocrine functions. The pancreas' exocrine functions involve the secretion of digestive enzymes into the small intestine. The endocrine functions of the pancreas are performed by clusters of endocrine cells called the pancreatic islets. There are two types of cells in the pancreas: alpha and beta. Alpha cells secrete glucagon, which breaks down glycogen into glucose. Beta cells secrete insulin, which causes glucose to be taken up by muscle and fat cells and causes the building of fats and proteins.