FIGURE 21.

Portal vessel systems. A: a typical circulatory system (top) consists of arteries that originate from the aorta and bifurcate into smaller arteriole branches until they reach the capillary bed. From there, blood begins its return to the heart via small venules, which feed into larger veins that ultimately drain into the vena cava and then the heart. In a portal system (bottom), blood from the first arterial system collects into a first capillary bed, which drains into portal vessels (yellow) that branch out again into a second capillary bed, before returning to the main circulatory system through the vena cava. B: the hepatic portal system is the largest portal system in the circulation. Blood flows from the aorta into mesenteric arteries, which feed into individual capillary beds in the stomach and intestine (the “first” capillary bed). Venules and veins that arise from these capillary beds drain into the portal vein, which delivers blood to the liver sinusoids (the “second” capillary bed). These ultimately collect to form the hepatic veins that drain into the vena cava. C: the hypophysial-hypothalamic circulation, which joins vasculature of the hypothalamus with vasculature of the anterior pituitary, represents another example of a portal system. D: the renal vascular system is conceptually similar to a portal system. Blood enters the glomerular tuft (the “first” capillary bed) via the afferent arteriole, and exits through the efferent arteriole, which feeds into the vasa recta (the “second” capillary bed) that supplies other portions of the nephron. [A is from the University of Cumberland Comparative Anatomy website, with permission; B is from Socratic Anatomy and Physiology (covered under creative commons license); C is from Servier Medical Arts, with permission; D is from Aird (4), with permission from Circ Res.]