Final answer:
Postcapillary venules are the smallest type of veins that connect with capillaries, allowing for the transfer of substances between blood and tissues.
Step-by-step explanation:
Postcapillary venules are the smallest type of veins. These venules connect with multiple capillaries that exit from a capillary bed, facilitating the transfer of substances between blood and tissues. Postcapillary venules, typically measuring 8-100 micrometers in diameter, have walls consisting of an endothelium layer, a thin middle layer with a few muscle cells and elastic fibers, and an outer layer of connective tissue. Like capillaries, they are key locations for diapedesis, where white blood cells move through the vessel walls into the tissue fluid.