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.
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