Final answer:
The portal blood supply with two capillary beds in series occurs in the hepatic portal system of the liver and the renal portal system of the kidneys. These systems allow for efficient processing of substances in the blood, including nutrients and toxins in the liver, and filtration and reabsorption in the kidneys.
Step-by-step explanation:
The portal blood supply with two capillary beds in series is most commonly associated with the hepatic and renal portal systems. In the liver, blood flows through capillaries in the gastrointestinal tract before reaching the hepatic portal vein and then passing through another set of capillaries within the liver itself. This two-tiered flow allows for nutrients and toxins to be processed efficiently.
The other example is found in the kidneys, where blood first passes through a capillary network in the glomerulus, then through the efferent arteriole before reaching a second capillary network of peritubular capillaries and the vasa recta. This arrangement is critical in the filtration and reabsorption processes of the renal system.
Specifically, the National Cancer Institute describes how the portal vein supplies the liver with substances filtered out of the blood, whereas Figure 25.10 shows a clear representation of the two capillary beds within the nephron of the kidney.