Download The Circulatory System

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Although the circulatory system contributes to many aspects of life, its main job is to
move substances throughout the body. The blood delivers important chemicals such
as nutrients, oxygen, and hormones to every cell in the body and it carries waste
products, such as carbon dioxide and nitrogenous wastes away to be eliminated from
the body.
A transportation system like this is needed because most of the cells in the body are
not in contact with the substances they need, or in contact with a way to get rid of
wastes. Think of the human body like a large city. In cities, not every home is right
next to a food source or a garbage dump, so a transportation system (roads) is
needed so that materials can be moved around.
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The major organ of the circulatory system is the heart. The heart keeps the
transportation system moving by forcefully pumping blood into arteries. Without the
pumping action of the heart, the blood would just stay in one place and substances
would not get delivered to their destinations.
The heart is made mostly out of a special kind of muscle called cardiac muscle.
Cardiac muscle is special for two reasons. First, the muscle cells always work together
so that they contract together as a unit. This makes sure that the heart squeezes the
blood forward instead of just quivering. Second, heart muscle is autorhythmic. This
means that the heart muscle can contract (squeeze) even without a signal telling it to
do so. Even with no input from the brain, the heart will contract rhythmically.
The heart has four chambers that can be filled with blood, as shown in the figure. The
upper two chambers are called atria (singular: atrium). This is where blood enters the
heart. The word “atrium” means, literally, “entryway.” When the atria contract, they
force their blood into the lower two chambers, called ventricles. When the ventricles
contract, blood is forced out of the heart and into blood vessels. Blood always enters
the atrium of a heart, and is always forced into the ventricle on the same side, and
then forced out into blood vessels. Blood never moves directly from the left to the
right side or vice-versa. Also, both atria contract together, and both ventricles
contract together.
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Why have two atria and two ventricles instead of just one of each? It’s because the
heart is really two pumps in one. The left and right sides of the heart send their blood
to completely different parts of the body, and for completely different reasons.
First, the left side. The left side of the heart is involved in systemic circulation. It
sends its oxygenated blood (blood that is carrying lots of oxygen) out to all the cells of
the body. This is where cells get all of their oxygen. Oxygen-rich blood from the lungs
enters the left atrium, where it is forced into the left ventricle, and then out into
systemic circulation. Systemic circulation goes everywhere in the body except the
lungs.
Next, the right side. The right side of the heart is involved in pulmonary circulation.
The word “pulmonary” means “lungs,” so this side of the heart pumps its
deoxygenated blood (blood with no oxygen) to the lungs, where it picks up lots of
oxygen. Deoxygenated blood coming back from the body tissues enters the right
atrium, is forced into the right ventricle, and then is forced into arteries that carry it
to the lungs.
In addition to knowing the names of the chambers of the heart, you should know the
names of the great vessels. These are enormous blood vessels that deliver blood to
the atria and carry blood from the ventricles:
•Oxygenated blood enters the left atrium through the pulmonary veins (literally,
“lung veins”). There are four of them and they bring in blood that has just picked up
oxygen from the lungs.
•Deoxygenated blood enters the right atrium through the venae cavae. There are two
venae cavae – one (superior vena cava) brings in blood from the upper part of the
body while the other (inferior vena cava) brings in blood from the lower part of the
body. This blood is all coming back from delivering its oxygen to the systemic body
tissues.
•Oxygenated blood is pumped from the left ventricle into the aorta. This huge artery
carries oxygen-rich blood out towards the system body tissues.
•Deoxygenated blood is pumped from the right ventricle into the pulmonary arteries.
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Now that you know all of the parts involved, we can look at the path that blood takes
through the body. Blood moves in a sort of figure-8 pattern: oxygenated blood in the
heart goes out to the body tissues to deliver its oxygen, then it returns to the heart,
only to be sent to the lungs to get more oxygen. It then returns to the heart, where it
is sent back out to the body tissues, and so on.
Let’s say that a blood cell starts in the left ventricle. It has oxygen to deliver. This is
the pathway it will take:
•When the left ventricle contracts, the oxygenated blood is forced into the aorta on
its way to body tissues
•The blood reaches its body tissues and delivers its oxygen. It also picks up carbon
dioxide.
•The blood returns to the heart through one of the venae cavae where it enters the…
•Right atrium. The right atrium contracts and the blood is forced into the…
•Right ventricle. Remember, this blood is now deoxygenated because it gave its
oxygen to the body tissues.
•When the right ventricle contracts, the blood is forced into the pulmonary arteries.
Eventually it reaches the…
•Lungs. Here, the carbon dioxide is eliminated and the blood cells pick up new
oxygen. The blood continues on to the…
•Pulmonary veins, which dumps the blood into the…
•Left atrium. When the left atrium contracts, the blood is delivered back to the left
ventricle.
And the cycle continues. Forever.
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Now let’s look at the vessels that carry the blood. The main “pipes” that blood travels
through are called arteries and veins.
Arteries carry blood away from the heart. This blood might be going out to the body
tissues (systemic circulation, from the left ventricle) or the lungs (pulmonary
circulation, from the right ventricle). No oxygen or other substances is every
exchanged with cells in the arteries. The walls are too thick.
Veins carry blood back to the heart. The blood might be coming from the body
tissues or the lungs, but no materials are ever exchanged here either.
Out in the body tissues, blood moves from arteries into veins through very, very small
blood vessels called capillaries. Capillaries have very thin walls, so materials are
exchanged as the blood moves through them. In fact, the capillaries are the only
place in the whole circulatory system where oxygen, carbon dioxide, and other
materials can enter or leave the blood. Every tissue in the body, including the lungs,
has lots of capillaries going through it.
If you look at the micrograph above, you might notice some differences between
arteries and veins. First, the walls of the arteries (called “tunics”) are thicker than the
walls of the veins. This is because the artery walls have lots of elastic fibers so they
can stretch under the immense pressure of blood being forced through them. You can
feel this stretch, called a pulse, at various places in the body where arteries are close
to the surface, such as in your wrist. Second, the hollow space in the arteries (called
the “lumen”) is narrower than it is in veins. This is because veins need to be wide so
that less blood touches the venous walls. Less touching of the walls means less
friction, so the blood in the veins can move with very little pressure.
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Although most people don’t think about the blood as an “organ,” it really is. It is
composed of both living cells and non-living fluid.
There are three kinds of blood cells:
•Erythrocytes are the red blood cells. There are billions of them in your body right
now and their red color is why blood is red. Erythrocytes are disc-shaped with a
“collapsed” center, so they look kind of like doughnuts. The main job of the red blood
cells is to carry oxygen, and they carry a lot of it. In fact, the cells are actually dead
bags of oxygen – dead so that they don’t use any of the oxygen they carry.
•Leukocytes are white blood cells. There are several types of leukocytes, but they are
all involved in protecting the body against infection or damage from cancer.
Leukocytes are generally larger than red blood cells and have different shapes,
depending on their specific jobs.
•Platelets are small fragments of cells that are involved in stopping blood loss.
Platelets become “sticky” when a blood vessel gets damaged (like when you cut
yourself). When platelets are sticky, they stick to each other, and to the damaged
edges of the blood vessel. The sticky mass of platelets basically seals off the rip in the
blood vessel until it can be repaired.
The liquid part of the blood, the plasma, is made mostly out of water. There are
important things dissolved in the water, though. Proteins that help blood clot and
defend the body against infection are dissolved in the plasma, as are hormones,
nutrients, and some dissolved gases (like oxygen and carbon dioxide).
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There are many diseases that can affect the circulatory system.
One of the biggies is coronary artery disease. The muscles of the heart need oxygen
and nutrients just like every other tissue in the body. They get their blood through
four arteries called coronary arteries. If one or more of the coronary arteries starts to
get clogged up, the heart muscle doesn’t get enough blood, and it can die. This can
lead to heart attack. Some factors that can cause the coronary arteries to become
clogged include a diet high in fat (particularly cholesterol), lack of exercise, a family
history of heart disease, and smoking cigarettes.
Arteriosclerosis is a condition that affects everybody eventually. As we age, the
arteries become less “stretchy.” This makes them more vulnerable to damage from
the pressure of the blood pushing against the artery walls and makes the heart have
to work harder. The harder the heart has to work, the sooner it wears out.
Blood pressure is the pressure that blood exerts against the walls of the blood
vessels, particularly the arteries. If this pressure is too high, or if the arteries have to
stretch too much, the arteries might be damaged. In addition, the heart has to work
harder than usual. Symptoms of a low blood pressure (hypotension) include dizziness,
fatigue, and confusion. High blood pressure (hypertension) generally does not have
any major symptoms, but it is still causing damage. This is why the doctor probably
checks your blood pressure every time you see her.
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