Venules collect blood from capillary networks and gradually merge to form veins.

Examine the examples of venules (Fig. 11-20) and small veins (Fig. 11-21) in CT (slides 45, 8, 116).

• Compare the size, wall thickness, and other structural features of venules and arterioles.

What differences are evident between arterioles and venules?

Why are vessels of the microvasculature frequently difficult to classify histologically?

Examine examples of medium size veins (Fig. 11-21c,d) in slide 92 and identify the 3 layers or tunics seen earlier in arteries. Examine the structure of valves in veins (Figs. 11-21 and 11-22).

What is the function of the valves?

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.

Examine a section of a large vein (slide 16; Fig. 11-22) and note how the thickness of the wall and its various layers compare to the wall of the neighboring artery.

The lymphatic vessels are next.