Learning Objectives
1. Describe the three major layers of the heart wall and how they relate to the pericardium; identify the three layers of arteries and veins.
2. Identify and describe the function of the chambers of the heart, the major vessels that enter and exit each chamber, and the atrioventricular valves, semilunar valves, chordae tendineae, and papillary muscles.
3. Trace the path of blood through both sides of the heart; identify the coronary arteries supplying the myocardium; define collateral circulation.
4. Compare and contrast the structure and function of arteries, capillaries, and veins.
5. Describe the path of an electrical impulse through the conduction system of the heart; describe the purpose of this electrical activity.
6. Explain the events associated with S1 and S2 heart sounds, indicating where each of these sounds is best heard.
7. Describe cardiac output, including the factors involved in its regulation.
8. Differentiate between systolic and diastolic blood pressure, explaining the actions occurring during each. Identify major factors that affect the regulation of blood pressure.
9. State changes in the cardiovascular system caused by aging. Discuss nursing implications for each.
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IMPORTANT TERMINOLOGY |
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afterload |
diastole |
preload |
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aorta |
endocardium |
pulmonic valve |
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aortic valve |
epicardium |
pulse |
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apex |
ischemia |
pulse pressure |
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atria |
microcirculation |
semilunar valve |
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baroreceptor |
mitral valve |
septum |
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bicuspid valve |
myocardium |
systole |
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collateral circulation |
pericardial fluid |
tricuspid valve |
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coronary sinus |
pericardium |
ventricle |
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Acronyms |
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AV |
LCA |
S2 |
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BP |
LCX |
SA node |
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CO |
LMCA |
sBP |
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CO2 |
MI |
SV |
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dBP |
O2 |
SVC |
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HR |
PDA |
SVR |
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IVC |
RCA |
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LAD |
S: |
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The cardiovascular (heart and blood vessel) system is designed for transportation and communication throughout the body. In approximately 1 minute, a drop of blood travels through the right side of the heart, the lungs, the left side of the heart, and the systemic circulation, completing its circuit by returning to the right side of the heart. In this brief time, cells located at the tips of the toes and fingers receive oxygen (O2) from the lungs and nutrients from the intestines. They simultaneously send carbon dioxide (CO2) and other wastes to be excreted. Thus, the cardiovascular system is vital in maintaining homeostasis within the body.
Key Concept In an average lifetime of 65 to 70 years, the human heart will beat approximately 2.5 billion times (about 100,000 times each day). The embryonic heart beat is detectable about 23 days after conception.
Structure and Function
The cardiovascular system is composed of the heart and blood vessels. Its functions include pumping blood and transporting gases, nutrients, hormones, and wastes (Box 22-1).
BOX 22-1.
Functions of the Cardiovascular System
Functions of the Heart
Pumping Action
♦ Pumps blood to body and lungs
♦ Receives blood from body and lungs
♦ Influences blood pressure
Functions of the Blood Vessels
Transportation
♦ Provides channels through which blood and lymph travel
♦ Provides areas (capillaries) where transfer of gases, nutrients, fluids, electrolytes, and wastes can occur
HEART
The human heart is a strong, muscular pump about three-quarters the size of a clenched fist. It weighs less than 1 pound (approximately 250-350 g). It lies in the thoracic cavity in the mediastinal space (a mass of tissues and organs lying behind the sternum, between the lungs, and in front of the vertebral column). The heart is shaped like an irregular and slightly flattened cone. The inferior (lower) point is the apex, which is formed by the tip of the left ventricle. The apical pulse is counted here. The wide superior (top) margin, called the base, lies opposite the apex and is formed mostly by the left atrium.
The heart wall has three layers: endocardium, myocardium, and epicardium (Fig. 22-1).
• The endocardium (inner heart) is a membrane lining the heart’s interior wall.
• Thick, strong muscles make up the myocardium (myo = muscle), the middle and thickest layer. Cardiac muscle is a unique type of involuntary muscle with lightly striated cells; this type of muscle is found only in the heart.
• The epicardium (epi = upon) is the thin, outer layer of the cardiac wall (also called the visceral layer of the serous pericardium).
The pericardium (pericardial sac) is a sac that surrounds and protects the heart. It is also made up of three layers:
• The epicardium portion of the heart wall, which also makes up the pericardium’s visceral layer, adheres to the heart’s surface.
• The parietal layer is the inner serous pericardium.
• The space between the visceral and parietal layers is called the pericardial space or cavity. It houses a small amount of serous fluid called pericardial fluid that acts as a lubricant and reduces friction between the layers as the heart contracts and relaxes.
FIGURE 22-1 · Heart and great vessels (anterior view). The heart is enclosed in the pericardial sac, the innermost layer of which is the epicardium.
• The outermost layer is called the fibrous pericardium (composed of dense fibrous connective tissue). This tissue anchors the heart in the mediastinum and prevents overfilling.
Heart Chambers and Valves
A complete muscular wall called the septum divides the heart into right and left sides. The two sides are completely separated, with no communication from right to left. Each side is a separate pump.
Special Considerations :LIFESPAN
Congenital Heart Defects
If a child is born with a hole in the cardiac septum, surgical correction is usually necessary.
Chambers
The interior of the heart is divided into four chambers (see Fig. 22-1).
Atria. The two upper chambers are the right and left atria (singular: atrium). These thin-walled, low-pressure chambers are the receiving centers for blood.
Ventricles. The two lower chambers are right and left ventricles. Ventricles are high-pressure chambers; they pump blood out of the heart. The left ventricle must contract with sufficient force to send blood to the entire body; therefore, its muscle walls are thickest and its internal pressures the highest. The right ventricle needs only to pump blood into the low-pressure lungs and therefore is a thinner-walled chamber.
Key Concept The left ventricle contains the thickest layer of muscles in the heart. It must pump strongly enough to send blood out to the entire body The right ventricle also has thick muscles; the muscles in the atria are thinner than those of either ventricle.
The left lung is smaller than the right lung, to make room for the heart (see Fig. 25-3).
Valves
As each heart chamber contracts, it pushes blood either into a ventricle or out of the heart to the lungs or body. The cardiac valves are one-way flaps of tissue that open and close in response to pressure changes within the chambers. These unidirectional (one-way) valves allow blood to flow in one direction only, preventing backflow.
Atrioventricular Valves. The atrioventricular (AV) valves lie between the atria and ventricles (Fig. 22-2). The valve between the right atrium and the right ventricle is called the tricuspid valve, because it is formed of three flaps (cusps) of tissue. The valve between the left atrium and the left ventricle is called the mitral valve (bicuspid valve) because it has only two flaps of tissue. The tissue flaps of these valves attach to tendon-like strands called chordae tendineae. These strands are anchored to papillary muscles located on the inner surface of the ventricles.
FIGURE 22-2 · Heart valves.
Blood flows from the atria to the ventricles through open AV valves when ventricular pressure is lower than atrial pressure. During this time, the papillary muscles and chordae tendineae relax. As the ventricles contract, increased pressure causes the AV valves to close. The papillary muscles also contract at this time, tightening the chordae tendineae, to prevent the valve cusps from everting (turning inside-out). If the AV valves, chordae tendineae, or papillary muscles become damaged, backflow of blood (regurgitation) into the atria can occur with ventricular contraction.
Overflow Valves. Each ventricle empties through a valve with three crescent-shaped (half-moon) cusps. These valves are called the semilunar valves. The pulmonary semilunar valve separates the right ventricle from the pulmonary artery and is called the pulmonic valve. The aortic (semilunar) valve separates the left ventricle from the aorta, the body’s largest artery. Increased ventricular pressure, as when the ventricles contract, opens the semilunar valves. As the ventricles relax, blood begins to flow backward toward the ventricles. Blood fills the semilunar cusps and causes the valves to close. Therefore, semilunar valves prevent backflow from their respective arteries into their ventricles.
