What type of system does the kidney contain with two capillary beds in series?

The correct answer is the portal system because this type of system is characterized by two capillary beds connected in series by a vein. In the context of the kidney, the specific portal system involved is known as the renal portal system. This system includes the glomerular capillaries where filtration of blood occurs, and the peritubular capillaries that surround the nephron and facilitate the reabsorption and secretion of various substances.

In a portal system, blood flows from one capillary bed to another before returning to the heart, allowing for specialized processing of substances, which is essential for the kidney's function in filtering blood and regulating electrolytes and waste products. The presence of two capillary beds in tandem allows for increased efficiency and regulation of renal function.

Other systems mentioned, such as the arterial and venous systems, do not specifically imply the arrangement of two capillary beds in series. The lymphatic system is distinct and primarily involved in the transport of lymph, not blood filtration or renal functions. Therefore, the distinction of a portal system is crucial to understanding how the kidney efficiently manages blood filtration and homeostasis.