Final answer:
Capillaries are the smallest of blood vessels and are responsible for the exchange of fluids and substances between the blood and tissue cells.
Step-by-step explanation:
Small, porous veins that exchange fluid with surrounding tissue cells are known as capillaries. These are the smallest of blood vessels and play a critical role in the cardiovascular system by connecting arterioles and venules. Capillaries form a capillary bed, a network that provides a large surface area for the physical exchange of substances, including oxygen and nutrients, between the blood and the cells, as well as the surrounding interstitial fluid. Fenestrated capillaries have pores that increase their permeability, allowing for the transfer of larger molecules.