Veins and Venules

Venules collect blood from capillary networks and gradually merge to form veins to carry blood from organs and tissues back to the heart. Venules are also the main vessels from which immune cells exit the circulation (i.e. undergo diapedesis) and enter the CT at sites of injury or infection. Check out this TEM image to the right where an immune cell can be seen squeezing between endothelial cells.

• Examine examples of venules and small veins in these three slides (sample 1, sample 2, sample 3).
  Compare the size, lumen shape, and wall thickness (relative amounts of smooth muscle in the tunica media and CT in the tunica adventitia) of venules and arterioles. Veins and arteries often travel together and can be seen near each other in histological specimens (as seen in the images to the right), helping you to make these comparisons.
• Examine examples of medium size veins in this slide of CT. Medium veins provide channels for blood flow from organs and into large veins for return to the heart. Identify the 3 layers or tunics seen earlier in arteries. The tunica media of medium veins typically has 3-5 layers of smooth muscle, far fewer than the 20-40 muscle layers found in medium arteries.
• Finally, study these sections of large veins (sample 1 and sample 2), which function to return blood to the heart. Note how the thickness of the wall and its various layers compare to the wall of the neighboring artery. Large veins usually have 5 or more layers of smooth muscle in their tunica media and are unique in that they often have longitudinal bundles of smooth muscle in their thick tunica adventitia.

Veins contain valves to prevent backflow of blood that’s flowing against gravity. These are difficult to find in histological specimens because they are thin and delicate, making it hard to catch them in sections.

Clinical note: Weakness in the walls or valves of veins can lead to abnormally dilated or varicose veins, which most commonly occur in the lower legs where backflow of blood is particularly common due to the pull of gravity.
 

The lymphatic vessels are next.