Venules are small, thin-walled vessels that carry blood from capillaries to veins, which then transport it back to the heart. Venules have thin tunica externas and are sites for diapedesis. Veins, with their thin walls and valves, ensure blood flows back to the heart despite lower pressure.
The very small, thin-walled vessels that empty into the veins are known as venules. Venules are extremely small veins, with diameters ranging from approximately 8 to 100 micrometers. They are the blood vessels that receive blood from the capillary beds and start the return journey of blood toward the heart.
The walls of venules are made up of endothelium and a very thin tunica externa composed of connective tissue fibers. They function as important sites for diapedesis, a process where white blood cells exit the bloodstream and enter the tissue fluid. The venules then join together to form veins, which are larger blood vessels with larger and irregular lumens.
Veins, in comparison to arteries, have thinner walls and larger lumens due to their role in carrying blood at lower pressure. In order to facilitate the unidirectional flow of blood back to the heart and to prevent backflow, larger veins are equipped with valves. Veins and venules are crucial components of the circulatory system, ensuring the continuous flow of blood by connecting capillaries, where exchange of nutrients and wastes takes place, with the heart.