Fundamentals of Anatomy and Physiology

The Cardiovascular Circulatory System 333

Artery

Lumen

Vein^

TUNICA INTIMA

Endothelium

Valve

TUNICA MEDIA

Smooth muscle

TUNICA ADVENTITIA

Capillary

Squamous

epithelial cells

(endothelium)

Vasa vasorum

(^) ®
Learning
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Figure 14- 7 The three layers of the walls of arteries and
veins.
the organ. The junction of two or more blood vessels is
called an anastomosis (ah-nas-toh-MOH-sis).
Arterioles (ar-TEE-ree-olz) are small arteries that de-
liver blood to capillaries.
Capillaries (CAP-ih-lair-eez) are microscopic ves-
sels made of simple squamous epithelial cells, one cell
layer thick, called endothelium. They are found in close
proximity to nearly every cell of the body. They con-nect
arterioles with venules. Their primary critical func-tion is
to permit the exchange of nutrients and oxygen and waste
and carbon dioxide between the blood and the tissue cells
of the body. Their unique wall structure of a single cell
layer allows this to occur by diffusion. A -substance in the

Blood Flow through the Heart

membrane of just one cell to reach the tissue cell and vice

versa. This vital exchange occurs only through cap-illary

walls.

Venules (VIN-yoolz) are small vessels that connect

capillaries to veins. They collect blood from capillaries and

drain it into veins.

Veins are made of the same three coats or tunics as

arteries but have less elastic tissue and smooth muscle but

more white fibrous connective tissue in the outer layer or

adventitia (see Figure 14-7). They also are capable of

distention to adapt to variations of blood volume and blood

pressure. Veins also contain valves that ensure blood flow

in one direction, toward the heart.