Biomedical Engineering Reference
In-Depth Information
small), which eventually branch into approximately 160,000 arterioles, which branch into
35 billion capillaries, which converge into approximately 10,000,000 venules, which con-
verge into 50 veins, which lead to 1 inferior vena cava and 1 superior vena cava.
5.2 VENOUS SYSTEM PHYSIOLOGY
A vein is defined as any blood vessel that transports blood to the heart (note that this
does not mean that the blood within a vein is deoxygenated; i.e., the pulmonary veins
carry oxygenated blood to the heart).Veins also perform the critical task of acting as a res-
ervoir for blood within the circulatory system (remember that approximately 60% of the
total blood volume is housed within the venous system). In contrast to the arterial circula-
tion, venous transport is carried out under much lower pressures (typically less than
5 mmHg). Due to the low pressure in the venous system, the walls of blood vessels in the
venous circulation are thinner than the arterial walls, but they are still relatively muscular.
Contraction and dilation of muscles within the venous wall alters the amount of blood
that can be held in the venous reservoir, but this does not regulate blood velocity as signif-
icantly as it does in the arterial circulation; blood flows much slower and in response to
skeletal muscle contraction (as we will see later).
Similar to the arterial wall, the venous wall is composed of three layers, the tunica
intima, the tunica media, and the tunica adventitia (
Figure 5.3
). These layers have the
same anatomy as the arterial layers described previously, with the following differences.
The endothelial layer within the tunica intima is typically smooth because veins do not
experience significant dilation or contraction as often as arteries. The tunica media is rela-
tively thin, as compared with the arterial wall and it is composed of smooth muscle cells
and collagen. This layer is still responsible for controlling blood vessel diameter. The
tunica adventitia is the thickest layer and is composed of collagen, elastin, and smooth
muscle cells. This layer is responsible for vessel anchoring to nearby tissues.
FIGURE 5.3
Anatomical structure of large
elastic veins and medium-sized veins, respec-
tively. The tunica adventitia is composed of
elastic fibers and helps to anchor the blood ves-
sel to the surrounding tissue. This is the thick-
est layer in veins. The tunica media is
composed of smooth muscle cells and is smal-
ler in veins as compared to arteries because
they are under a lower pressure. The internal
layer, the tunica intima, is composed of endo-
thelial cells that are in contact with blood.
Tunica adventitia
Tunica media
Endothelial cells
Tunica intima
Tunica adventitia
Tunica media
Endothelial cells
Tunica intima
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