Islet Blood Flow

The pancreatic islets have a rich blood supply compared with the surrounding exocrine tissue. Approximately 10% of the pancreatic blood flow is directed to the islets, which make up only approximately 1% to 2% of the pancreatic mass. The microvasculature of the pancreatic islets exhibits a characteristic feature, which has been carefully studied in the rat.14 It has been shown that the arterioles nourishing an islet enter into the central B-cell mass, where fenestrated and highly permeable capillaries are formed (see Fig. 78-1). The capillaries then pass out from the central B-cell mass to the mantle zone, where they either empty into venules or anastomose with the capillaries of the exocrine parenchyma. Careful studies have also presented evidence that within the central B-cell mass, approximately 8 to 10 B cells are distributed around a central capillary.67 This microvascular organization makes it likely that the nourishing blood passes first through the central B-cell mass before reaching the peripherally located A, D, and F cells.

This anatomy of the islet vessels is probably of importance for the function of the islet because blood-borne substances first reach the B cells before they can affect the other cells. Also, insulin secreted from the B cells reaches the other cells in high concentrations and, therefore, affects the secretion of glucagon and somatostatin in an inhibitory direction. In contrast, because of the flow direction within the islet, the products from the A and D cells probably do not reach the B cells; therefore, it seems unlikely that the stimulatory action of glucagon and the inhibitory action of somatostatin with regard to insulin secretion are exerted by local endocrine effects within the islets. The islet microvasculature anatomy also suggests that anastomoses between the islet and exocrine capillaries result in an insular acinar portal system, which suggests that the peptides secreted from the islet cells reach the acinar cells in high concentrations.14

The regulation of the islet blood flow has not been studied in such a great detail as that of islet hormone secretion. It seems, however, that the blood flow in the islets is controlled differently from that in the exocrine tissue. For example, glucose, vagal nerve activation, and IAPP all increase the relative fraction of islet blood flow versus total pancreatic blood flow.68,69

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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