Pancreas. Illustration of the secretory cells of the human pancreas, showing an islet of Langerhans (purple) surrounded by pancreatic acini (yellow). The pancreatic acini produce digestive enzymes (green spots) that pass through ducts (lower left, for example) into the small intestine. The cells of the islets of Langerhans secrete hormones (blue & pale green spots) directly into the bloodstream, and are surrounded by a rich network of blood vessels (red). These hormones include insulin (blue spots, produced by beta cells), which lowers blood sugar levels, and glucagon (pale green spots, produced by alpha cells), which raises blood sugar.