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Blood
Blood is the fluid of life, transporting oxygen from the lungs to body tissue and carbon
dioxide from body tissue to the lungs
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Blood is the fluid of health,
transporting disease-fighting substances to the tissue and waste to the kidneys
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Red blood cells and white blood cells are responsible for
nourishing and cleansing the body
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The vessels make up two closed systems of tubes that begin and end at the heart
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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
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Arteries
Arteries carry blood away from the heart
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Systemic arteries transport
oxygenated blood from the left ventricle to the body tissues
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The arterioles play a key
role in regulating blood flow into the tissue capillaries
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The wall of an artery consists of three layers
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The middle layer, the tunica media, is
primarily smooth muscle and is usually the thickest layer
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The
outermost layer, which attaches the vessel to the surrounding tissue, is the tunica externa
or tunica adventitia
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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
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The primary function of capillaries is the exchange of materials
between the blood and tissue cells
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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
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The epidermis of the
skin and the lens and cornea of the eye completely lack a capillary network
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Another 10 percent
is in the lungs
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Veins
Veins carry blood toward the heart
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From the venules, it flows into progressively larger and larger
veins until it reaches the heart
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This blood has a high oxygen content because it
has just been oxygenated in the lungs
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This blood has a reduced oxygen content because the oxygen has
been used for metabolic activities in the tissue cells
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Although all the layers are
present, there is less smooth muscle and connective tissue
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Because the walls of the veins are thinner and less rigid than
arteries, veins can hold more blood
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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
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Physiology of Circulation
Roles of Capillaries

In addition to forming the connection between the arteries and veins, capillaries have a vital
role in the exchange of gases, nutrients, and metabolic waste products between the blood and
the tissue cells
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Oxygen and carbon dioxide move across the capillary wall by diffusion
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The net result of the capillary microcirculation created by hydrostatic
and osmotic pressure is that substances leave the blood at one end of the capillary and return
at the other end
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It can be felt where an artery is close to the surface and rests on something firm
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Systolic pressure is due to ventricular contraction
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Pulse pressure is the difference between systolic
pressure and diastolic pressure
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Four major factors interact to
affect blood pressure: cardiac output, blood volume, peripheral resistance, and viscosity
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Arterial blood pressure is maintained within normal ranges by changes in cardiac output and
peripheral resistance
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Circulatory Pathways
The blood vessels of the body are functionally divided into two distinctive
circuits: pulmonary circuit and systemic circuit
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The left ventricle is the pump for the
systemic circuit, which provides the blood supply for the tissue cells of the body
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Then it returns the oxygen-rich blood to the
left atrium
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It carries
oxygen and nutrients to the cells and picks up carbon dioxide and waste products
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From the tissue capillaries, the deoxygenated blood
returns through a system of veins to the right atrium of the heart
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The
brachiocephalic, left common carotid, and left subclavian arteries branch from the aortic arch
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The
subclavian arteries provide the blood supply for the upper extremity
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Lumbar arteries provide blood for

the muscles and spinal cord
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The internal iliac artery supplies the pelvic viscera
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The aorta
ascends from the left ventricle, curves posteriorly and to the left, then descends through the
thorax and abdomen
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The descending aorta is further subdivided into the thoracic
aorta and abdominal aorta
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The capillaries, where the gaseous exchange occurs, merge
into venules and these converge to form larger and larger veins until the blood reaches either
the superior vena cava or inferior vena cava, which drain into the right atrium
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The foetus obtains its oxygen and nutrients from the mother and also depends on
maternal circulation to carry away the carbon dioxide and waste products
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The ductus venosus allows blood to bypass the immature liver in foetal circulation
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