Fig. 4.

An overview of the circulation is shown at top. In human subjects, blood is sampled from a peripheral vein, an arterialized hand vein, or an artery. Since the tissues of the splanchnic bed (including pancreas) consume and release hormones and metabolites, the content of the blood from these vessels does not reflect the content in portal vein blood. Glucagon and insulin are secreted by the pancreas into the portal vein (via pancreatic veins) and extracted from the portal vein by the liver. Data from a dog model show that arterial glucagon underestimates the glucagon concentration perfusing the liver (bottom left). Conversely, arterial epinephrine overestimates the epinephrine concentration perfusing the liver (bottom right). Data are means ± SE.