Insulin production. Artwork of a section through beta cells secreting insulin (blue spheres) into a capillary (small blood vessel, grey tube). Beta cells are found in the pancreas. Energy-producing structures (mitochondria, green) are seen within the cells, as well as the folded membrane (upper centre), called the endoplasmic reticulum, where such proteins can be synthesised and stored. The insulin in the cell is contained within secretory granules (pink, round). These migrate through the cell to release their insulin cargo into the bloodstream. Insulin is a hormone that controls blood sugar level by promoting glucose storage in the liver and muscles.