Final answer:
A venule is a small blood vessel that connects capillaries to the larger veins, and it plays a crucial role in the movement of white blood cells into tissues. In contrast, a vein carries blood back to the heart and features thin walls with valves to prevent backflow due to low pressure.
Step-by-step explanation:
Venules and Veins
A venule is a very small vein, typically ranging from 8 to 100 micrometers in diameter. They are key in connecting the capillaries to larger veins, ensuring blood return to the heart. The structure of a venule includes an endothelial lining, a thin middle layer comprised of some muscle cells and elastic fibers, and a very thin outer layer known as the tunica externa. These structures allow for diapedesis, which is a critical process wherein white blood cells move through the vessel walls to reach tissue fluid.
On the other hand, a vein is responsible for transporting blood towards the heart, and is characterized by thin walls, large and irregular lumens, and, in larger veins, the presence of valves. These valves prevent the backflow of blood, aiding the circulation despite the lower pressure in veins compared to arteries.