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

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

Veins conduct blood away from body tissues. Veins generally have a larger diameter than arteries, though the width of their walls are thinner than arteries. Like arteries, veins also have three distinct layers: the tunica intima, the tunica media and the tunica adventitia. The vasa vasorum supply both elastic and muscular arteries.

Examine examples of medium size veins  and identify the 3 layers or tunics seen earlier in arteries. Examine the structure of valves in veins.

What are 2 major histological differences between veins and arteries?

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.

Arterioles and capillaries