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Heart and Blood Vessels
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Printed: February 18, 2013
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C ONCEPT
Concept 1. Heart and Blood Vessels
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Heart and Blood Vessels
Lesson Objectives
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Describe the structure of the heart.
Outline how blood moves through the heart.
Describe the importance of valves in the heart.
Describe the coronary circulation.
Check Your Understanding
• What is the role of the cardiovascular system?
• What is the main function of the heart?
Vocabulary
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atrioventricular (AV) valves
atrium
semilunar (SL) valves
valves
ventricles
The Heart
What is the heart? How does it pump blood? The heart is divided into four chambers, or spaces: the left and right
atria, and the left and right ventricles. An atrium (singular for atria) is one of the two small, thin-walled chambers
on the top of the heart where the blood first enters. A ventricle is one of the two muscular V-shaped chambers that
pump blood out of the heart. You can remember they are called ventricles because they are shaped like a "V." The
four chambers of the heart are shown in Figure 1.1.
The atria receive the blood, and the ventricles pump the blood out of the heart. Each of the four chambers of the
heart has a specific job.
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The right atrium receives oxygen-poor blood from the body.
The right ventricle pumps oxygen-poor blood toward the lungs.
The left atrium receives oxygen-rich blood from the lungs.
The left ventricle pumps oxygen-rich blood out of the heart to the rest of the body.
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Where is the Heart?
The heart is closer to the center of the body than you may think. It is usually found in the left to middle of the
chest, with the largest part of the heart slightly to the left. It always feels like the heart is on the left side of the body
because the left ventricle is bigger and stronger than the right ventricle. The heart is also surrounded by the lungs.
FIGURE 1.1
The atria receive blood and the ventricles
pump blood out of the heart.
Blood Flow Through the Heart
Blood flows through the heart in two separate loops. You can think of them as a “left side loop” and a “right side
loop." The right side of the heart collects oxygen-poor blood from the body and pumps it into the lungs, where it
releases carbon dioxide and picks up oxygen. The left side carries the oxygen-rich blood back from the lungs into
the left side of the heart, which then pumps the oxygen-rich blood to the rest of the body.
The Heartbeat
To move blood through the heart, the cardiac muscle needs to contract in an organized way. Blood first enters the
atria, as shown in Figure 1.2. When the atria contract, blood is pushed into the ventricles. After the ventricles fill
with blood, they contract, and blood is pushed out of the heart. So how is the blood kept from flowing back on itself?
Valves in the heart keep the blood flowing in one direction. You can see some of the valves in Figure 1.2. The valves
do this by opening and closing in one direction only. Blood only moves forward through the heart. The valves stop
the blood from flowing backward. There are four valves of the heart.
• The two atrioventricular (AV) valves stop blood from moving from the ventricles to the atria.
• The two semilunar (SL) valves are found in the arteries leaving the heart, and they prevent blood flowing back
from the arteries into the ventricles.
Why does a heart beat? The “lub-dub” sound of the heartbeat is caused by the closing of the AV valves ("lub") and
SL valves ("dub"), after blood has passed through them.
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Concept 1. Heart and Blood Vessels
FIGURE 1.2
Blood flows in only one direction in the
heart. Blood enters the atria, which contract and push blood into the ventricles.
The atria relax and the ventricles fill with
blood. Finally, the ventricles contract and
push blood around the body.
Control of the Heartbeat
The heart is a made up of cardiac muscle cells. Cardiac cells are able to contract by themselves. They do not need
help from the nervous system. This is different from skeletal muscle, which needs messages from nerves to contract.
The number of times a heart contracts over a certain amount of time is called the heart rate. Exercising or getting
scared can make the heart rate increase. After the exercise is over, or the fear has passed, the heart rate returns to
normal.
Blood Circulation and Blood Vessels
The blood vessels are an important part of the cardiovascular system. They connect the heart to every cell in the
body. Arteries carry blood away from the heart, while veins return blood to the heart. The main arteries and veins of
the heart are shown in Figure 1.3.
Important Arteries and Veins
There are specific veins and arteries that are more significant than others. The pulmonary arteries carry oxygen-poor
blood away from the heart to the lungs. These are the only arteries that carry oxygen-poor blood. The aorta is the
largest artery in the body. It carries oxygen-rich blood away from the heart. Further away from the heart, the aorta
branches into smaller arteries, which eventually branch into capillaries.
The veins that return oxygen-poor blood to the heart are the superior vena cava and the inferior vena cava. The
pulmonary veins return oxygen-rich blood from the lungs to the heart. The pulmonary veins are the only veins that
carry oxygen-rich blood.
Pulmonary Circulation
Pulmonary circulation is the part of the cardiovascular system that carries oxygen-poor blood away from the heart
and brings it to the lungs. Oxygen-poor blood returns to the heart from the body and leaves the right ventricle
through the pulmonary arteries, which carry the blood to each lung. Once at the lungs, the red blood cells release
carbon dioxide and pick up oxygen when you breathe. The oxygen-rich blood then leaves the lungs through the
pulmonary veins, which return it to the left side of the heart. This completes the pulmonary cycle. The oxygenated
blood is then pumped to the body through systemic circulation, before returning again to pulmonary circulation.
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FIGURE 1.3
The right side of the heart pumps deoxygenated blood into pulmonary circulation, while the left side pumps oxygenated
blood into systemic circulation.
Systemic Circulation
Systemic circulation is the part of the cardiovascular system that carries oxygen-rich blood away from the heart,
to the body, and returns oxygen-poor blood back to the heart. Oxygen-rich blood leaves the left ventricle through
the aorta, then it travels to the body’s organs and tissues. The tissues and organs absorb the oxygen through the
capillaries. Oxygen-poor blood is collected from the tissues and organs by tiny veins, which then flow into bigger
veins, and eventually into the inferior vena cava and superior vena cava. This completes systemic circulation.
The blood releases carbon dioxide and gets more oxygen in pulmonary circulation before returning to systemic
circulation.
Lesson Summary
• The heart is divided into four chambers, the left and right atria and the left and right ventricles.
• The right side of the heart collects oxygen-poor blood from the body and pumps it into the lungs, where it
releases carbon dioxide and picks up oxygen.
• The left-side carries the oxygen-poor blood back from the lungs into the left side of the heart, which then
pumps the oxygen-poor blood to the rest of the body.
• The valves in the heart prevent blood from flowing backward into the heart.
Further Reading / Supplemental Links
• http://en.wikipedia.org/wiki/William_Harvey
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Concept 1. Heart and Blood Vessels
• http://thevirtualheart.org/anatomyindex.html
• http://en.wikipedia.org/wiki/Cardiac_cycle
Review Questions
Recall
1. Name the four chambers of the heart.
2. Where does oxygen-poor blood first enter the heart?
3. Do ventricles pump blood out of the heart, or do they pump blood into the atria?
4. Do the inferior vena cava and superior vena cava carry oxygen-poor or oxygen-rich blood?
5. To what organ or organs does systemic circulation bring blood?
6. To what organ or organs does pulmonary circulation bring blood?
Apply Concepts
7. Why can the heart be considered to be two separate pumps?
8. What is the purpose of the valves in the heart?
Critical Thinking
9. How might a hole in the heart wall between the two ventricles affect the circulation of blood?
Points to Consider
A more in-depth look at blood is next.
• How do different parts of the blood impact the cardiovascular system?
• How can diet affect how blood carries oxygen?
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