Insuloacinar Portal System

The pancreas is part of the splanchnic circulation, where the celiac and superior mesenteric arteries supply blood by and venous drainage empties into the portal vein. The superior mesenteric artery supplies the head of the pancreas, while the celiac supplies the body and tail via the splenic artery.14 The microcirculation of the pancreas usually divides into that of the acini and duct and the islets, but remains interconnected (see also Chapter 4). Wharton first described these microvascular connections between the islets and exocrine pancreas half a century ago.24 Years later, this microvascular connection of capillary networks in the islets and acinar tissue came to be referred as the "insulo-acinar portal circulation."25

The hypothesis is that the islets of Langerhans were scattered throughout the exocrine pancreas and that this microcirculation helped coordinate hormonal and digestive function of the pancreas. As blood passes through the islets, it becomes enriched with a high concentration of islet hormones (see Chapter 4 and Chapter 6). These hormones then diffuse through highly permeable capillaries into the acinar tissue to regulate its function. Over the past two decades, the insulo-acinar portal system has been confirmed in rats, guinea pigs, dogs, and man. 26,27 Furthermore, the microvascular architecture within the islets appears to be organized to drain blood from a- and 6-cell areas to p-cell areas of the islets. In addition, there are three types of efferent vessels from the islets to acinar tissue:

1. Insulo-acinar portal vessels that radiate from the islets to join the capillary network

2. Emissary venules of the islets that lead directly into the systemic circulation

3. Insulo-ductal portal vessels that drain into the periductal capillary network

Therefore, the functional unit of the pancreas consists of the islets and acinar and ductal tissues, which are all connected by an insulo-acinar portal system.

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