Which parts of the human circulatory system are xylem and phloem most similar to?

The xylem and phloem of plants are most similar to veins, arteries, and capillaries.

In a plant, xylem are used to transport water, while the phloem are used to transport nutrients via sap. Xylem are deep into the plant, around the core of a branch, while the phloem are near the surface behind the bark. Inside the circulatory system, arteries and veins are deep in the body, beneath layers of protective flesh, whereas capillaries are found closer to the surface in many areas, such as the surface of skin and the surfaces of organs. Here, I would compare the xylem to arteries and veins, and the phloem to capillaries.

In the circulatory system, however, there is no system that distributes just water. Rather, all parts of the system distribute blood. In this sense, xylem are completely unlike the circulatory system. Even the sap that flows in a tree has no transport cells like red blood cells and white blood cells; it is a fluid containing loose sugars and nutrients.

Finally, I would comment on how the circulatory system uses the heart to pump blood through the body. Xylem and phloem use gravity, transpiration, and root pressure to move things through the plant, expending no energy in the process.