TABLE Characteristics of Pancreatic Hormones18.4 Hormone Molecular Weight (daltons) Structure Insulin 5,700–6,000 Two protein chains linked by disulfide bridges: chain, 21 amino acids;  chain, 30 amino acids Glucagon 3,500 Linear polypeptide: 29 amino acids Somatostatin 1,638 Cyclic polypeptide: 14 amino acids VIP 3,300 Linear polypeptide: 28 amino acids Pancreatic 4,200 Linear polypeptide polypeptide: 36 amino acids contribute to the availability of circulating glucose in stress reactions.
The blood supply to the pancreas provides a cascading perfusion of the islets and acini.
Several arterioles enter the periphery of the islets and branch into fenestrated capillaries. In humans, the capillaries first perfuse the A and D cells, peripherally, before the blood reaches the B cells, centrally. Larger vessels that travel in septa that penetrate the central portion of the islet are also accompanied by A and D cells, so that blood reaching the B cells has always first perfused the A and D cells.
Large efferent capillaries leave the islet and branch into the capillary networks that surround the acini of the exocrine pancreas. This cascading flow resembles the portal systems of other endocrine glands (pituitary, adrenal).
Secretions of the islet cells have regulatory effects on the acinar cells:  Insulin, the vasoactive intestinal peptide (VIP), and CCK stimulate exocrine secretion.
 Glucagon, pancreatic polypeptide (PP), and somatostatin inhibit exocrine secretion.