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The
Cardiovascular System
In 1628, William Harvey demonstrated
that the heart is a pump that pumps bright red blood through arteries
away from the heart at high pressure and that darker (bluish) blood returns
to the heart through veins at lower pressure. He correctly hypothesized
the existence of blood vessels (capillaries) that connect arteries to
veins. Since Harvey's early experiments, we have greatly expanded our
knowledge of cardiovascular anatomy and physiology.
The cardiovascular or circulatory
system is composed of the heart, the blood vessels and the
blood. During the lifetime of an individual human, the heart contracts
nearly
3 billion times, never stopping to rest except for a fraction of a second
between beats. The heart is one of the first organs to from during development.
Within three weeks of conception, the heart of the developing embryo starts
to basically function while the embryo is only a few millimeters long.
The heart develops so early because the circulatory system is the critical
transport system for the body carrying nutrients and oxygens to tissues
and removing carbon dioxide and wastes to the lungs and filtering organs.
There are three basic components
to the circulatory (or cardiovascular) system:
1. The heart which serves
as a pump that creates pressure to keep blood flowing. Blood flows down
a pressure gradient from higher to lower pressure.
2. The blood vessels
which are passageways through which blood travels and is distributed to
various parts of the body.
3. Blood which is the
transport medium in which the materials being carried are suspended or
dissolved.
Blood flows through two distinct
circuits; the pulmonary circuit and the systemic circuit.
In the Pulmonary Circuit,
blood that is high in carbon dioxide and low in oxygen flows from the
right heart to the lungs. In the capillaries of the lungs, blood takes
on oxygen and off-loads carbon dioxide. Oxygenated blood then flows from
the lungs to the left heart.
In the Systemic Circuit,
oxygenated blood flows from the left heart to the systemic tissues (meaning
all cells/systems of the body except the lungs). Systemic capillaries
are the site of exchange of nutrients and wastes. The blood off-loads
oxygen to the tissues and picks up carbon dioxide wastes. Deoxygenated
blood then flows from the systemic tissues to the right heart, completing
the circuit.
The heart is a pump that continually
moves blood between these two circuits. The average heart rate is 70 beats
per minute = about 100,000 times/day = about 2.5 billion times/average
lifetime.
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The
Flow of Blood through Blood Vessels
Blood vessels
form a tubular network that allows blood to travels from the heart
to the tissues and back to the heart again. Blood that leaves the
heart passes into arteries. Large arteries branch into progressively
smaller arteries that function to deliver blood to various regions
of the body. Small arteries branch into even smaller vessels called
arterioles, which function to regulate the flow of blood
into different tissues. Arterioles branch into capillaries,
the smallest of all blood vessels. Capillaries are the sites of
nutrient and waste exchange between the blood and body cells. Capillaries
are microscopic vessels that join the arterial system with the venous
system. Blood coming out of the capillaries passes into vessels
of increasing diameter as it flows back toward the heart. Capillaries
join to form venules, which then merge to form small veins.
Small veins unite to form large veins that eventually deliver blood
back to the heart.
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Arteries
Arteries serve as (1)
efficient conduits for the movement of blood and (2) pressure reservoirs
that keep blood moving during diastole. Arteries have a large internal
diameter and thus offer little resistance to the flow of blood.
Arteries also contain an elastic layer in their walls. The elastic
qualities of this layer are due to the presence of a stretchable
protein fiber called elastin. Arteries also have a smooth
muscular layer that functions to regulate the flow of blood through
the artery. Contraction of the smooth muscle decreases the internal
diameter of the vessel in a process called vasoconstriction.
Relaxation of the smooth muscle increases the internal diameter
of the vessel in a process called vasodilation.
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Arterioles
Arterioles serve as (1)
the major determinant of blood pressure and (2) as the major determinant
of blood flow to the individual organs. Arterioles have a much smaller
diameter than arteries and thus provide significant resistance to
the flow of blood. This resistance creates pressure in the circulatory
system. Pressure is required to provide adequate flow of blood to
all parts of the body. Blood flow to individual organs can be regulated
by controlling the diameter of the arterioles.
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Capillaries
Capillaries are the smallest
and most numerous of blood vessels. Capillaries function as the
site of exchange of nutrients and wastes between blood and tissues.
The anatomy of capillaries is well suited to the task of efficient
exchange. Capillary walls are composed of a single layer of cells.
The thin nature of the walls facilitates efficient diffusion of
oxygen and carbon dioxide. Most capillaries also have pores between
cells that allow for bulk transport of fluid and dissolved substances
from the blood into the tissues and visa versa.
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| Although capillaries are
extremely numerous (40 billion in the body), collectively they hold
only about 5% of the total blood volume at any one time. This is because
most capillaries are closed most of the time. Precapillary sphincters,
which are bands of smooth muscle that wrap around arterioles, control
the amount of blood flowing in a particular capillary bed. Contraction
of the sphincter shuts off blood flow to a capillary bed, while relaxation
of the sphincter allows blood to flow. |
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Veins
Veins are larger and
more stretchable than arteries, thus they can hold more blood. In
fact, the veins act somewhat like a blood reservoir, containing
60% of the total blood volume at rest. As physical activity increases,
the veins undergo vasoconstriction, driving more blood back to the
heart and increasing circulation. Also, the return of venous blood
to the heart is aided by one-way valves that insure unidirectional
flow of blood.
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Return
of Venous Blood to the Heart
By the time blood has
passed from the capillaries into the venous system the pressure
has dropped significantly. The average blood pressure in the venous
system is very low as compared to the arterial system. The low venous
pressure is barely adequate to drive blood back to the heart, particulary
from the legs. Other mechanisms are needed to aid in the return
of blood to the heart. The flow of venous blood back to the heart
is increased by (1) the sympathetic nervous system, (2) the skeletal
muscle pump, and (3) the respiratory pump.
Veins are innervated
by sympathetic motor neurons. Sympathetic input causes vasoconstriction,
which increases pressure, which drives blood back to the heart.
When the body needs to mobilize more blood for physical activity,
the sympathetic nervous system induces vasoconstriction of veins.
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The figure to the right
illustrates the action of the skeletal muscle pump. Veins
pass between skeletal muscles.The contraction of skeletal muscle
squeezes the vein, thus increasing blood pressure in that section
of the vein. Pressure causes the upstream valve (furthest from the
heart) to close and the downstream valve (the one closest to the
heart) to open. Repeated cycles of contraction and relaxation, as
occurs in the leg muscles while walking, effectively pumps blood
back to the heart.
While the contraction
of skeletal muscle in the legs drives venous blood out of the lower
limbs, the act of breathing helps to drive venous blood out of the
abdominal cavity. As air is inspired, the diaphragm descends and
abdominal pressure increases. The increasing pressure squeezes veins
and moves blood back toward the heart. The rhythmic movement of
venous blood caused by the act of breathing is called the respiratory
pump.
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