Blood vessels are the channels or conduits through which blood
is distributed to body tissues. The vessels make up two closed systems
of tubes that begin and end at the heart. One system, the pulmonary
vessels, transports blood from the right ventricle to the lungs and
back to the left atrium. The other system, the systemic vessels, carries
blood from the left ventricle to the tissues in all parts of the body
and then returns the blood to the right atrium. Based on their structure
and function, blood vessels are classified as either arteries, capillaries,
or veins.
Arteries
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Arteries carry blood away from the heart.
Pulmonary arteries transport blood that has a low oxygen content
from the right ventricle to the lungs. Systemic arteries transport
oxygenated blood from the left ventricle to the
body tissues. Blood
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is pumped from the ventricles into large elastic arteries that branch
repeatedly into smaller and smaller arteries until the branching results
in microscopic arteries called arterioles. The arterioles play a key
role in regulating blood flow into the tissue capillaries. About 10
percent of the total blood volume is in the systemic arterial system
at any given time.
The wall of an artery consists of three layers. The innermost layer,
the tunica intima (also called tunica interna), is simple squamous
epithelium surrounded by a connective tissue basement membrane with
elastic fibers. The middle layer, the tunica media, is primarily
smooth muscle and is usually the thickest layer. It not only provides
support for the vessel but also changes vessel diameter to regulate
blood flow and blood pressure. The outermost layer, which attaches
the vessel to the surrounding tissue, is the tunica externa or tunica
adventitia. This layer is connective tissue with varying amounts
of elastic and collagenous fibers. The connective tissue in this
layer is quite dense where it is adjacent to the tunic media, but
it changes to loose connective tissue near the periphery of the
vessel.
Capillaries
Capillaries, the smallest and most numerous of the blood vessels,
form the connection between the vessels that carry blood away from
the heart (arteries) and the vessels that return blood to the heart
(veins). The primary function of capillaries is the exchange of materials
between the blood and tissue cells.
Capillary distribution varies with the metabolic activity of body
tissues. Tissues such as skeletal muscle, liver, and kidney have
extensive capillary networks because they are metabolically active
and require an abundant supply of oxygen and nutrients. Other tissues,
such as connective tissue, have a less abundant supply of capillaries.
The epidermis of the skin and the lens and cornea of the eye completely
lack a capillary network. About 5 percent of the total blood volume
is in the systemic capillaries at any given time. Another 10 percent
is in the lungs.
Smooth muscle cells in the arterioles where they branch to form
capillaries regulate blood flow from the arterioles into the capillaries.
Veins
Veins carry blood toward the heart. After blood passes through
the capillaries, it enters the smallest veins, called venules. From
the venules, it flows into progressively larger and larger veins
until it reaches the heart. In the pulmonary circuit, the pulmonary
veins transport blood from the lungs to the left atrium of the heart.
This blood has a high oxygen content because it has just been oxygenated
in the lungs. Systemic veins transport blood from the body tissue
to the right atrium of the heart. This blood has a reduced oxygen
content because the oxygen has been used for metabolic activities
in the tissue cells.
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The walls of veins have the same three layers
as the arteries. Although all the layers are present, there
is less smooth muscle and connective tissue. This makes the
walls of veins thinner than those of arteries, which
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is related to the fact that blood in the veins has less pressure than
in the arteries. Because the walls of the veins are thinner and less
rigid than arteries, veins can hold more blood. Almost 70 percent
of the total blood volume is in the veins at any given time. Medium
and large veins have venous valves, similar to the semilunar valves
associated with the heart, that help keep the blood flowing toward
the heart. Venous valves are especially important in the arms and
legs, where they prevent the backflow of blood in response to the
pull of gravity.
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