The blood is conducted or carried through the body by tubular structures
known as blood vessels. Since at no time does the whole blood ever leave a blood vessel of
some sort, we refer to this system as a closed system.
a. General Construction. The blood vessels in general are tubular
and have a three-layered wall.
(1) Intima. The lumen (hollow central cavity) is lined by a layer of
smooth epithelium known as the intima.
(2) Media. A middle layer of smooth muscle tissue is called the media.
(3) Adventitia. The adventitia is the outer layer of fibrous connective
tissue that holds everything together.
b. Types of Blood Vessels. See figure 9-1 for a diagram of the
human circulatory system. We recognize three types of blood vessels:
(1) The arteries carry blood away from the chambers of the heart.
(2) The veins carry blood to the chambers of the heart.
(3) Capillaries are extremely thin-walled vessels having only the intimal
layer through which exchanges can take place between the blood and the tissue cells.
c. Relationships. Arteries and veins are largest where they are
closest to the heart. Away from the heart, they branch into smaller and smaller and more
numerous vessels. The branching continues until the smallest arteries (arterioles) empty
into the capillaries. The capillaries in turn are drained by the venules of the venous
system.
d. Valves. Within the heart and the veins are structures known as
valves. Valves function to insure that the blood flows in only one direction.
Primary Content Providers: The U. S. Army, The
U.S. Navy
Ancillary Content and Online Version: David L. Heiserman
Publisher: SweetHaven Publishing Services
