The smallest branches of arteries are called arterioles (Fig. 11-11), a term often used when the tunica media has only 2-3 layers of smooth muscle. (Arterioles with only 1 or 2 layers of smooth muscle fibers are sometimes called "metarterioles".)

• Examine arterioles in the CT on slides 92 and 45.

Capillaries (Fig. 11-15) usually have narrow lumens, often no more than the diameter of
erythrocytes.

Sinusoids have much larger lumens, but are present in only certain organs, such as bone marrow, where you examined them previously.

• Both capillaries and sinusoids lack muscular and adventitial layers. Fig. 11-16 compares the three major types of capillaries.
• Examine capillaries (Fig. 11-15) in skeletal muscle (slide 8) and in the CT of mesentery (slide
  116). Look carefully for pericytes.

Examine the electron micrographs of the two common types of capillaries (Figs. 11-17 and 11-18), noting particularly the pinocytotic vesicles often present and the fenestrations in one type.

What is the functional significance of the differences in capillary endothelium?

Veins and venules