| The Heart and the Circulatory System |
The Human circulation system
Fig. 1: Schematic diagram of the
flow of blood through the circulatory system
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1. Blood rich in carbon dioxide is pumped from the
heart into the lungs through the pulmonary arteries. (Arteries are blood
vessels carrying blood away from the heart; veins are blood vessels carrying
blood to the heart.)
2. In the lungs, CO2 in the blood is exchanged for O2.
3. The oxygen-rich blood is carried back to the heart through the pulmonary
veins.
4. This oxygen-rich blood is then pumped from the heart to the many
tissues and organs of the body, through the systemic arteries.
5. In the tissues, the arteries narrow to tiny capillaries. Here, O2
in the blood is exchanged for CO2.
6. The capillaries widen into the systemic veins, which carry the carbon-dioxide-rich
blood back to the heart.
The Anatomy of the Heart
The human heart is a muscular pump. While most of
the hollow organs of the body do have muscular layers, the heart is almost
entirely muscle. Unlike most of the other hollow organs, whose muscle
layers are composed of smooth muscle, the heart is composed of cardiac
muscle. All muscle types function by contraction, which causes the muscle
cells to shorten. Skeletal muscle cells, which make up most of the mass
of the body, are voluntary and contract when the brain sends signals telling
them to react. The smooth muscle surrounding the other hollow organs is
involuntary, meaning it does not need to be told to contract. Cardiac
muscle is also involuntary. So functionally, cardiac muscle and smooth
muscle are similar.
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Anatomically though, cardiac muscle more closely resembles
skeletal muscle. Both skeletal muscle and cardiac muscle are striaded. Under
medium to high power magnification through the microscope, you can see small
stripes running crosswise in both types. Smooth muscle is nonstriated. Cardiac
muscle could almost be said to be a hybrid between skeletal and smooth muscle.
Cardiac muscle does have several unique features. Present in cardiac muscle
are intercalated discs, which are connections between two adjacent cardiac cells.
Intercalated discs help multiple cardiac muscle cells contract rapidly as a
unit. This is important for the heart to function properly. Cardiac muscle also
can contract more powerfully when it is stretched slightly. When the ventricles
are filled, they are stretched beyond their normal resting capacity. The result
is a more powerful contraction, ensuring that the maximum amount of blood can
be forced from the ventricles and into the arteries with each stroke. This is
most noticeable during exercise, when the heart beats rapidly.
There are four chambers in the heart – two atria
and two ventricles. The atria (one is called an atrium) are responsible for
receiving blood from the veins leading to the heart. When they contract, they
pump blood into the ventricles. However, the atria do not really have to work
that hard. Most of the blood in the atria will flow into the ventricles even
if the atria fail to contract. It is the ventricles that are the real workhorses,
for they must force the blood away from the heart with sufficient power to push
the blood all the way back to the heart (this is where the property of contracting
with more force when stretched comes into play). The muscle in the walls of
the ventricles is much thicker than the atria. The walls of the heart are really
several spirally wrapped muscle layers. This spiral arrangement results in the
blood being wrung from the ventricles during contraction. Between the atria
and the ventricles are valves, overlapping layers of tissue that allow blood
to flow only in one direction. Valves are also present between the ventricles
and the vessels leading from it.
Though the brain can cause the heart to speed up or slow drain, it does not
control the regular beating of the heart. As noted earlier, the heart is composed
of involuntary muscle. The muscle fibers of the heart are also self-excitatory.
This means they can initiate contraction themselves without receiving signals
from the brain. This has been demonstrated many times in high school classes
of the past by removing the heart of a frog or turtle, and then stimulating
it to contract. The heart continues to beat with no further outside stimulus,
sometimes for hours if bathed in the proper solution. In addition, cardiac muscle
fibers also contract for a longer period of time than do skeletal muscles. This
longer period of contraction gives the blood time to flow out of the heart chambers.
The heart has two areas that initiate impulses, the SA or sinoatrial
node, and the AV or atrioventricular node. The heart also has special
muscle fibers called Purkinje fibers that conduct impulses five times
more rapidly than surrounding cells. The Purkinje fibers form a pathway
for conduction of the impulse that ensures that the heart muscle cells
contract in the most efficient pattern. The SA node is located in the
wall of the right atrium, near the junction of the atrium and the superior
vena cava. This special region of cardiac muscle contracts on its own
about 72 times per minute. In contrast, the muscle in the rest of the
atrium contracts on its own only 40 or so times per minute. The muscle
in the ventricles contracts on its own only 20 or so times per minute.
Since the cells in the SA node contract the most times per minute, and
because cardiac muscle cells are connected to each other by intercalated
discs, the SA node is the pacemaker of the heart. When the SA node initiates
a contraction, Purkinje fibers rapidly conduct the impulse to another
site near the bottom of the right atrium and near the center of the heart.
This region is the AV node, and slows the impulse briefly. The impulse
then travels to a large bundle of Purkinje fibers called the Bundle of
His, where they move quickly to the septum that divides the two ventricles.
Here, the Purkinje fibers run in two pathways toward the posterior apex
of the heart. At the apex, the paths turn in opposite directions, one
running to the right ventricle, and one running to the left. The result
is that while the atria are contracting, the impulse is carried quickly
to the ventricles. With the AV node holding up the impulse just enough
to let the atria finish their contraction before the ventricles begin
to contract, blood can fill the ventricles. And, since the Purkinje fibers
have carried the impulse to the apex of the ventricles first, the contraction
proceeds from the bottom of the ventricles to the top where the blood
leaves the ventricles through the pulmonary arteries and the aorta.
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Fig. 2: Opened Heart
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The contraction of the heart and its anatomy cause the distinctive
sounds heard when listening to the heart with a stethoscope. The "lub-dub"
sound is the sound of the valves in the heart closing. When the atria end their
contraction and the ventricles begin to contract, the blood is forced back against
the valves between the atria and the ventricles, causing the valves to close.
This is the "lub" sound, and signals the beginning of ventricular
contraction , known as systole. The "dub" is the sound of the valves
closing between the ventricles and their arteries, and signals the beginning
of ventricular relaxation, known as diastole.
A physician listening carefully to the heart can detect if the valves are closing
completely or not. Instead of a distinctive valve sound, the physician may hear
a swishing sound if they are letting blood flow backward. When the swishing
is heard tells the physician where the leaky valve is located.
The Pulmonary and Systemic Circuits and the Blood Supply
to the Heart.
Fig. 4: Stethoscope placement (shaded area) for hearing heart sounds
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The heart is responsible for pumping the blood to
every cell in the body. It is also responsible for pumping blood to the
lungs, where the blood gives up carbon dioxide and takes on oxygen. The
heart is able to pump blood to both regions efficiently because there are
really two separate circulatory circuits with the heart as the common link.
Some authors even refer to the heart as two separate hearts--a right heart
in the pulmonary circuit and left heart in the systemic circuit. In the
pulmonary circuit, blood leaves the heart through the pulmonary arteries,
goes to the lungs, and returns to the heart through the pulmonary veins.
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In the systemic circuit, blood leaves the heart through the aorta, goes
to all the organs of the body through the systemic arteries, and then returns
to the heart through the systemic veins. Thus there are two circuits. Arteries
always carry blood away from the heart and veins always carry blood toward
the heart. Most of the time, arteries carry oxygenated blood and veins carry
deoxygenated blood. There are exceptions. The pulmonary arteries leaving
the right ventricle for the lungs carry deoxygenated blood and the pulmonary
veins carry oxygenated blood. If you are confused as to which way the blood
flows through the heart, try this saying "When it leaves the right,
it comes right back, but when it leaves the left, it's left." The blood
does not have to travel as far when going from the heart to the lungs as
it does from the heart to the toes. It makes sense that the heart would
be larger on one side than on the other. When you look at a heart, you see
that the right side of the heart is distinctly smaller than the left side,
and the left ventricle is the largest of the four chambers.
While you might think the heart would have no problem getting enough oxygen-rich
blood, the heart is no different from any other organ. It must have its
own source of oxygenated blood. The heart is supplied by its own set of
blood vessels. These are the coronary arteries. There are two main ones
with two major branches each. They arise from the aorta right after it leaves
the heart. The coronary arteries eventually branch into capillary beds that
course throughout the heart walls and supply the heart muscle with oxygenated
blood. The coronary veins return blood from the heart muscle, but instead
of emptying into another larger vein, they empty directly into the right
atrium. |
 Fig.
5: Arterial an Venous System |
The Blood Vessels
Fig. 6: Blood vessel anatomy
We need to briefly discuss the anatomy of the vessels. There are three types
of vessels – arteries and veins and capillaries. Arteries, veins, and
capillaries are not anatomically the same. They are not just tubes through which
the blood flows. Both arteries and veins have layers of smooth muscle surrounding
them. Arteries have a much thicker layer, and many more elastic fibers as well.
The largest artery, the aorta leaving the heart, also has cardiac muscle fibers
in its walls for the first few inches of its length immediately leaving the
heart. Arteries have to expand to accept the blood being forced into them from
the heart, and then squeeze this blood on to the veins when the heart relaxes.
Arteries have the property of elasticity, meaning that they can expand to accept
a volume of blood, then contract and squeeze back to their original size after
the pressure is released. A good way to think of them is like a balloon. When
you blow into the balloon, it inflates to hold the air. When you release the
opening, the balloon squeezes the air back out. It is the elasticity of the
arteries that maintains the pressure on the blood when the heart relaxes, and
keeps it flowing forward. if the arteries did not have this property, your blood
pressure would be more like 120/0, instead of the 120/80 that is more normal.
Arteries branch into arterioles as they get smaller. Arterioles eventually become
capillaries, which are very thin and branching.
Fig. 7: Cappillary bed
Capillaries are really more like a web than a branched tube. It is in the capillaries
that the exchange between the blood and the cells of the body takes place. Here
the blood gives up its carbon dioxide and takes on oxygen. In the special capillaries
of the kidneys, the blood gives up many waste products in the formation of urine.
Capillary beds are also the sites where white blood cells are able to leave
the blood and defend the body against harmful invaders. Capillaries are so small
that when you look at blood flowing through them under a microscope, the cells
have to pass through in single file. As the capillaries begin to thicken and
merge, they become venules. Venules eventually become veins and head back to
the heart. Veins do not have as many elastic fibers as arteries. Veins do have
valves, which keep the blood from pooling and flowing back to the legs under
the influence of gravity. When these valves break down, as often happens in
older or inactive people, the blood does flow back and pool in the legs. The
result is varicose veins, which often appear as large purplish tubes in the
lower legs.