The smallest branches of arteries are called arterioles. The walls of arterioles are thin, the tunica media has only 1-3 layers of smooth muscle, and the tunica adventitia is a thin layer of CT. Arterioles control blood flow to capillaries and resist blood flow, making them major regulators of systemic blood pressure.

• Examine arterioles in the CT on these slides (sample 1, sample 2) and in the image below. Compare the arterioles to their companion venules and small veins, noting the irregular, collapsed lumens of the veins and their relatively thinner walls.

Capillaries usually have small lumens, often no more than the diameter of erythrocytes. They are composed of a lining endothelium and basement membrane, and lack the typical tunics seen in other vessels. They function to exchange oxygen, nutrients, and wastes with cells.

Capillaries can be classified as one of three types:

• Continuous - endothelium and basement membrane are continuous, lack fenestrations (pores), and contain pinocytotic vesicles.
• Fenestrated - endothelium contains pores.
• Sinusoidal - endothelium contains very large pores and basement membrane may be discontinuous.

• Examine capillaries in this section of skeletal muscle of the tongue and CT of mesentery. Also, look carefully for pericytes. These unique cells modulate capillary functions and also serve as progenitor cells to provide new cells for repairing damaged vessels.

• Finally, examine these electron micrographs of the two common types of capillaries, noting the pinocytotic vesicles often present in continuous capillaries and the fenestrations seen in the very thin endothelium found within capillaries of the kidney where blood is filtered to produce urine.
   

Veins and venules