Chapter 20 the heart Flashcards
(208 cards)
1
Q
The heart contributes to homeostasis by
A
pumping blood through blood vessels to the tissues of the body to deliver oxygen and nutrients and remove wastes.
2
Q
the cardiovascular system consists of
A
the blood, the heart, and blood vessels.
3
Q
For blood to reach body cells and exchange materials with them,
A
it must be pumped continuously by the heart through the body’s blood vessels.
4
Q
The heart beats about
A
100,000 times every day, which adds up to about 35 million beats in a year, and approximately 2.5 billion times in an average lifetime
5
Q
The right side of the heart pumps blood through
A
the lungs, enabling blood to pick up oxygen and unload carbon dioxide.
6
Q
your heart pumps more than about
A
14,000 liters (3600 gal) of blood in a day, or 5 million liters (1.3 million gal) in a year.
7
Q
cardiology
A
The scientific study of the normal heart and the diseases associated with it is known as
8
Q
the heart is relatively
A
small, roughly the same size (but not the same shape) as your closed fist
9
Q
describe the dimensions and size of the heart
A
It is about 12 cm (5 in.) in its long axis, 9 cm (3.5 in.) wide at its broadest point, and 6 cm (2.5 in.) in depth (anterior to posterior), with an average mass of 250 g (8 oz) in adult females and 300 g (10 oz) in adult males.
10
Q
The heart rests on
A
the diaphragm, near the midline of the thoracic cavity.
11
Q
The heart lies in the
A
mediastinum (mē′-dē-as-TĪ-num), an anatomical region that extends from the sternum to the vertebral column, from the first rib to the diaphragm, and between the lungs (Figure 20.1a).
12
Q
About two-thirds of the mass of the heart lies
A
to the left of the body’s midline
13
Q
You can visualize the heart as
A
a cone lying on its side
14
Q
The pointed apex is formed by the __________________and rests on the __________________
A
tip of the left ventricle (a lower chamber of the heart)
diaphragm
15
Q
The base of the heart is
A
opposite the apex and is its posterior aspect.
16
Q
The base of the heart is formed by
A
the atria (upper chambers) of the heart, mostly the left atrium (see Figure 20.3c).
17
Q
The anterior surface of the heart .
A
is deep to the sternum and ribs
18
Q
The inferior surface of the heart is the part of the heart
A
between the apex and right surface and rests mostly on the diaphragm
19
Q
The right surface of the heart faces
A
the right lung and extends from the inferior surface to the base.
20
Q
The left surface of the heart
A
faces the left lung and extends from the base to the apex.
21
Q
pericardium
A
The membrane that surrounds and protects the heart
22
Q
What is the function of the pericardium
A
It confines the heart to its position in the mediastinum, while allowing sufficient freedom of movement for vigorous and rapid contraction.
23
Q
The pericardium consists of two main parts:
A
(1) the fibrous pericardium and (2) the serous pericardium
24
Q
the fibrous pericardium
A
is superficial and composed of tough, inelastic, dense irregular connective tissue
25
What is the function of the fibrous pericardium
The fibrous pericardium prevents overstretching of the heart, provides protection, and anchors the heart in the mediastinum
26
The fibrous pericardium near the apex of the heart is
partially fused to the central tendon of the diaphragm and therefore movement of the diaphragm, as in deep breathing, facilitates the movement of blood by the heart.
27
The pericardium is
a triple-layered sac that surrounds and protects the heart.
28
Cardiopulmonary resuscitation (CPR) (kar-dē-ō-PUL-mo-nar′-ē rē-sus-i-TĀ-shun) refers to
an emergency procedure for establishing a normal heartbeat and rate of breathing
29
serous pericardium is
a deeper, thinner, more delicate mesothelial membrane than the fibrous pericardium that forms a double layer around the heart
30
The outer parietal layer of the serous pericardium
lines the inside of the fibrous pericardium.
31
The inner visceral layer of the serous pericardium
is one of the layers of the heart wall and adheres tightly to the surface of the heart.
32
The inner visceral layer of the serous pericardium,
is also called the epicardium
33
Between the parietal and visceral layers of the serous pericardium
is a thin film of a few mililiters of lubricating serous fluid. This slippery secretion of the pericardial cells, known as pericardial fluid, reduces friction between the layers of the serous pericardium as the heart moves.
34
the pericardial cavity.
The space that contains the few milliliters of pericardial fluid
35
Inflammation of the pericardium is called
pericarditis
36
The most common type, acute pericarditis,
begins suddenly and has no known cause in most cases but is sometimes linked to a viral infection.
37
Chronic pericarditis
begins gradually and is long-lasting
38
The wall of the heart consists of three layers (Figure 20.2a):
the epicardium (external layer), the myocardium (middle layer), and the endocardium (inner layer).
39
The epicardium is composed of two
tissue layers.
40
The outermost layer of the epicardium is called the visceral layer of the serous pericardium. This layer is
Thin and transparent and made of mesothelium
41
Beneath the mesothelium is
a variable layer of delicate fibroelastic tissue and adipose tissue. the adipose tissue predominates and becomes thickest over the ventricular surfaces, where it houses the major coronary and cardiac vessels of the heart.
42
The epicardium imparts
a smooth, slippery texture to the outermost surface of the heart.
43
The epicardium contains
blood vessels, lymphatics, and nerves that supply the myocardium.
44
The middle myocardium (mī′-ō-KAR-dē-um; myo- = muscle) is responsible for
the pumping action of the heart and is composed of cardiac muscle tissue.
45
The myocardium makes up
about 95 percent of the heart wall
46
The cardiac muscle fibers are organized in
bundles that swirl diagonally around the heart and generate the strong pumping actions of the heart
47
Although it is striated like skeletal muscle, recall that
cardiac muscle is involuntary like smooth muscle.
48
Myocarditis (mī-ō-kar-DĪ-tis) is
an inflammation of the myocardium that usually occurs as a complication of a viral infection, rheumatic fever, or exposure to radiation or certain chemicals or medications
49
Endocarditis (en′-dō-kar-DĪ-tis) refers to
an inflammation of the endocardium and typically involves the heart valves. Most cases are caused by bacteria (bacterial endocarditis).
50
The innermost endocardium (en′-dō-KAR-dē-um; endo- = within) is
a thin layer of endothelium overlying a thin layer of connective tissue.
51
the endocaardium
provides a smooth lining for the chambers of the heart and covers the valves of the heart.
52
The endocardium is continuous with
the endothelial lining of the large blood vessels attached to the heart.
53
The heart has
four chambers.
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atria
The two superior receiving chambers
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ventricles (= little bellies).
the two inferior pumping chambers
56
The paired atria
receive blood from blood vessels returning blood to the heart, called veins,
57
the ventricles
eject the blood from the heart into blood vessels called arteries
58
auricle
a wrinkled pouchlike structure On the anterior surface of each atrium
59
What is the function of the auricles on each atrium
Each auricle slightly increases the capacity of an atrium so that it can hold a greater volume of blood
60
sulci (SUL-sī),
a series of grooves on the surface of the heart that contain coronary blood vessels and a variable amount of fat
61
Each sulcus (SUL-kus; singular)
marks the external boundary between two chambers of the heart.
62
The deep coronary sulcus (coron- = resembling a crown)
encircles most of the heart and marks the external boundary between the superior atria and inferior ventricles.
63
The anterior interventricular sulcus (in′-ter-ven-TRIK-ū-lar)
is a shallow groove on the anterior surface of the heart that marks the external boundary between the right and left ventricles on the anterior aspect of the heart.
64
the anterior ventricular sulcus continues around to the posterior surface of the heart as
the posterior interventricular sulcus, which marks the external boundary between the ventricles on the posterior aspect of the heart
65
Sulci are
grooves that contain blood vessels and fat and that mark the external boundaries between the various chambers.
66
The right atrium
forms the right surface of the heart and receives blood from three veins: the superior vena cava, inferior vena cava, and coronary sinus (Figure 20.4a).
67
Veins always carry blood
toward the heart.
68
The inside of the posterior wall of the right atrium
is smooth;
69
the inside of the anterior wall of the right atrium
is rough due to the presence of muscular ridges called pectinate muscles
70
The pectinate muscles
extend into the auricle
71
Between the right atrium and left atrium is
a thin partition called the interatrial septum
72
A prominent feature of the interatrial septum is an oval depression called the
fossa ovalis
73
fossa ovalis,
the remnant of the foramen ovale, an opening in the interatrial septum of the fetal heart that normally closes soon after birth
74
Blood passes from the right atrium into the right ventricle through
a valve that is called the right atrioventricular (tricuspid) valve
75
why is the right atrioventricular valve also called the tricuspid valve
because it consists of three cusps or leaflets
76
The valves of the heart are composed of
dense connective tissue covered by endocardium.
77
Blood flows into the right atrium through
the superior vena cava, inferior vena cava, and coronary sinus and into the left atrium through four pulmonary veins.
78
The right ventricle is
about 4–5 mm (0.16–0.2 in.) in average thickness and forms most of the anterior surface of the heart.
79
trabeculae carneae
a series of ridges formed by raised bundles of cardiac muscle fibers
80
The cusps of the right atrioventricular valve are connected to
tendonlike cords, called the chordae tendineae
81
The chordae tendineae
are connected to cone-shaped trabeculae carneae called papillary muscles
82
Internally, the right ventricle is separated from the left ventricle by
a partition called the interventricular septum.
83
Blood passes from the right ventricle through
the pulmonary valve into a large artery called the pulmonary trunk, which divides into right and left pulmonary arteries and carries blood to the lungs.
84
Arteries always take blood
away from the heart (a mnemonic to help you: artery = away).
85
The left atrium
is about the same thickness as the right atrium and forms most of the base of the heart
86
The left atrium recieves
receives blood from the lungs through four pulmonary veins.
87
Like the right atrium, the inside of the left atrium has
a smooth posterior wall
88
Because pectinate muscles are confined to the auricle of the left atrium,
the anterior wall of the left atrium also is smooth.
89
Blood passes from the left atrium into the left ventricle through
the left atrioventricular (bicuspid or mitral) valve (bi- = two)
90
The left ventricle is
the thickest chamber of the heart, averaging 10–15 mm (0.4–0.6 in.), and forms the apex of the heart
91
Like the right ventricle, the left ventricle
contains trabeculae carneae and has chordae tendineae that anchor the cusps of the bicuspid valve to papillary muscles
92
Blood passes from the left ventricle through
the aortic valve into the ascending aorta
93
Some of the blood in the aorta flows into the
coronary arteries, which branch from the ascending aorta and carry blood to the heart wall. The remainder of the blood passes into the aortic arch and descending aorta
94
Branches of the aortic arch and descending aorta
carry blood throughout the body
95
During fetal life,
a temporary blood vessel, called the ductus arteriosus, shunts blood from the pulmonary trunk into the aorta. Hence, only a small amount of blood enters the nonfunctioning fetal lungs
96
The ductus arteriosus normally
closes shortly after birth, leaving a remnant known as the ligamentum arteriosum (lig′-a-MEN-tum ar-ter-ē-Ō-sum), which connects the aortic arch and pulmonary trunk
97
The thickness of the myocardium of the four chambers
varies according to each chamber’s function
98
The thin-walled atria deliver blood
under less pressure into the adjacent ventricles.
99
Because the ventricles pump blood under higher pressure over greater distances
, their walls are thicker
100
Although the right and left ventricles act as two separate pumps that simultaneously eject equal volumes of blood,
the right side has a much smaller workload. It pumps blood a shorter distance to the lungs at lower pressure, and the resistance to blood flow is small.
101
The left ventricle pumps blood
a greater distance to all other parts of the body at higher pressure, and the resistance to blood flow is larger.Therefore, the left ventricle works much harder than the right ventricle to maintain the same rate of blood flow.
102
The anatomy of the two ventricles confirms this functional difference—
the muscular wall of the left ventricle is considerably thicker than the wall of the right ventricle
103
the perimeter of the lumen (space) of the left ventricle is
roughly circular, in contrast to that of the right ventricle, which is somewhat crescent-shaped.
104
In addition to cardiac muscle tissue, the heart wall also contains dense connective tissue that forms
the fibrous skeleton of the heart
105
Essentially, the fibrous skeleton consists of
four dense connective tissue rings that surround the valves of the heart, fuse with one another, and merge with the interventricular septum
106
In addition to forming a structural foundation for the heart valves, the fibrous skeleton prevents
overstretching of the valves as blood passes through them.
107
the fibrous skeleton of the heart
serves as a point of insertion for bundles of cardiac muscle fibers and acts as an electrical insulator between the atria and ventricles.
108
As each chamber of the heart contracts,
it pushes a volume of blood into a ventricle or out of the heart into an artery.
109
Valves open and close in response to
pressure changes as the heart contracts and relaxes
110
Each of the four valves helps ensure the one-way flow of blood by
opening to let blood through and then closing to prevent its backflow
111
atrioventricular (AV) valves
The valves located between an atrium and a ventricle
112
When an AV valve is open,
the rounded ends of the cusps project into the ventricle.
113
When the ventricles are relaxed,
the papillary muscles are relaxed, the chordae tendineae are slack, and blood moves from a higher pressure in the atria to a lower pressure in the ventricles through open AV valves
114
When the ventricles contract,
the pressure of the blood drives the cusps upward until their edges meet and close the opening (Figure 20.6b, e). At the same time, the papillary muscles contract, which pulls on and tightens the chordae tendineae. This prevents the valve cusps from everting (opening into the atria) in response to the high ventricular pressure.
115
If the AV valves or chordae tendineae are damaged,
blood may regurgitate (flow back) into the atria when the ventricles contract.
116
Heart valves function
prevent the backflow of blood.
117
the aortic and pulmonary valves are known as
the semilunar (SL) valves (sem-ē-LOO-nar; semi- = half; -lunar = moon-shaped) because they are made up of three crescent moon–shaped cusps
118
In the semilunar valves
Each cusp attaches to the arterial wall by its convex outer margin
119
The SL valves allow
ejection of blood from the heart into arteries but prevent backflow of blood into the ventricles.
120
in the semilunar valves
The free borders of the cusps project into the lumen of the artery
121
When the ventricles contract in semilunar valves,
pressure builds up within the chambers. The semilunar valves open when pressure in the ventricles exceeds the pressure in the arteries, permitting ejection of blood from the ventricles into the pulmonary trunk and aorta
122
As the ventricles relax in semilunar valves
blood starts to flow back toward the heart. This backflowing blood fills the valve cusps, which causes the free edges of the semilunar valves to contact each other tightly and close the opening between the ventricle and artery
123
Surprisingly, there are no valves guarding
the junctions between the venae cavae and the right atrium or the pulmonary veins and the left atrium.
124
As the atria contract,
a small amount of blood does flow backward from the atria into these vessels. However, backflow is minimized by a different mechanism: as the atrial muscle contracts, it compresses and nearly collapses the weak walls of the venous entry points. Furthermore, the weak pumping pressure of the atria is not strong enough to overcome the hydrostatic pressure in the veins.
125
In postnatal (after birth) circulation,
the heart pumps blood into two closed circuits with each beat—systemic circulation and pulmonary circulation
126
the systemic circulation and the pulmonary circulation are arranged
are arranged in series: The output of one becomes the input of the other, as would happen if you attached two garden hoses
127
The left side of the heart is the pump for
systemic circulation; it receives bright red oxygenated (oxygen-rich) blood from the lungs.
128
The left ventricle ejects blood into
the aorta
129
After The left ventricle ejects blood into the aorta. From the aorta, the blood
divides into separate streams, entering progressively smaller systemic arteries that carry it to all organs throughout the body—except for the pulmonary alveoli (air sacs) of the lungs, which are supplied by the pulmonary circulation
130
In systemic tissues, arteries give rise to ,
smaller-diameter arterioles
131
arterioles, finally lead into
extensive beds of systemic capillaries
132
Exchange of nutrients and gases occurs
across the thin capillary walls, Blood unloads O2 (oxygen) and picks up CO2 (carbon dioxide).
133
In most cases, blood flows through only one capillary and then
enters a systemic venule
134
Venules carry
deoxygenated (oxygen-poor) blood away from tissues and merge to form larger systemic veins. Ultimately the blood flows back to the right atrium.
135
The right side of the heart pumps
deoxygenated blood into the pulmonary circulation to the pulmonary alveoli of the lungs
136
The right side of the heart is the pump for
pulmonary circulation; it receives all of the dark-red deoxygenated blood returning from the systemic circulation.
137
Blood ejected from the right ventricle flows into
the pulmonary trunk
138
The pulmonary trunk branches into
pulmonary arteries that carry blood to the right and left lungs.
139
In pulmonary capillaries, around pulmonary alveoli,
blood unloads CO2, which is exhaled, and picks up O2 from inhaled air. The freshly oxygenated blood then flows into pulmonary veins and returns to the left atrium.
140
Nutrients are not able to diffuse quickly enough from blood in the chambers of the heart to
supply all layers of cells that make up the heart wall.
141
since Nutrients are not able to diffuse quickly enough from blood in the chambers of the heart to supply all layers of cells that make up the heart wall.
the myocardium has its own network of blood vessels, the coronary circulation or cardiac circulation (coron- = crown).
142
The coronary arteries branch from
the ascending aorta and encircle the heart as a crown encircles the head
143
While the heart is contracting,
little blood flows in the coronary arteries because they are squeezed shut.
144
When the heart relaxes
the high pressure of blood in the aorta propels blood through the coronary arteries, into capillaries, and then into coronary veins
145
stenosis
A narrowing of a heart valve opening that restricts blood flow is known as
146
insufficiency (in′-su-FISH-en-sē) or incompetence
failure of a valve to close completely
147
In mitral (left atrioventricular) stenosis,
scar formation or a congenital defect causes narrowing of the left atrioventricular valve
148
One cause of mitral (left atrioventricular) insufficiency, in which there is backflow of blood from the left ventricle into the left atrium, is
mitral (left atrioventricular) valve prolapse (MVP).
149
In MVP
one or both cusps of the left atrioventricular valve protrude into the left atrium during ventricular contraction.
150
In aortic stenosis
the aortic valve is narrowed
151
in aortic insufficiency
there is backflow of blood from the aorta into the left ventricle.
152
Two coronary arteries, the left and right coronary arteries,
branch from the ascending aorta and supply oxygenated blood to the myocardium
153
The left coronary artery
passes inferior to the left auricle and divides into the anterior interventricular and circumflex arteries.
154
The anterior interventricular artery or left anterior descending (LAD) artery
is in the anterior interventricular sulcus and supplies oxygenated blood to the walls of both ventricles.
155
The circumflex artery (SER-kum-fleks)
lies in the coronary sulcus and distributes oxygenated blood to the walls of the left ventricle and left atrium.
156
The right coronary artery supplies
small branches (atrial branches) to the right atrium It continues inferior to the right auricle and ultimately divides into the posterior interventricular and marginal branches.
157
The inferior (posterior) interventricular artery
follows the posterior interventricular sulcus and supplies the walls of the two ventricles with oxygenated blood.
158
The marginal branch beyond the coronary sulcus
runs along the right margin of the heart and transports oxygenated blood to the wall of the right ventricle.
159
The left and right coronary arteries
deliver blood to the heart
160
; the coronary veins
drain blood from the heart into the coronary sinus.
161
Most parts of the body
receive blood from branches of more than one artery, and where two or more arteries supply the same region, they usually connect.
162
where two or more arteries supply the same region, they usually connect. These connections, are called
anastomoses
163
anastomoses (a-nas′-tō-MŌ-sēs), provide
alternate routes, called collateral circulation, for blood to reach a particular organ or tissue.
164
The myocardium contains many
anastomoses that connect branches of a given coronary artery or extend between branches of different coronary arteries. They provide detours for arterial blood if a main route becomes obstructed.
165
After blood passes through the arteries of the coronary circulation,
it flows into capillaries, where it delivers oxygen and nutrients to the heart muscle and collects carbon dioxide and waste, and then moves into coronary veins
166
Most of the deoxygenated blood from the myocardium
drains into a large vascular sinus in the coronary sulcus on the posterior surface of the heart, called the coronary sinus
167
vascular sinus
a thin-walled vein that has no smooth muscle to alter its diameter
168
The deoxygenated blood in the coronary sinus
empties into the right atrium.
169
What veins flow into the coronary sinus
Great cardiac vein
Middle cardiac vein
small cardiac vein
anterior cardiac veins
170
Describe the location and function of the great cardiac vein
in the anterior interventricular sulcus, which drains the areas of the heart supplied by the left coronary artery (left and right ventricles and left atrium)
171
Describe the location and function of the middle cardiac vein
in the posterior interventricular sulcus, which drains the areas supplied by the inferior interventricular artery of the right coronary artery (left and right ventricles)
172
Describe the location and function of the small cardiac vein
in the coronary sulcus, which drains the right atrium and right ventricle
173
Describe the function of the anterior cardiac veins
drain the right ventricle and open directly into the right atrium
174
When blockage of a coronary artery deprives the heart muscle of oxygen,
reperfusion (re′-per-FYŪ-zhun), the reestablishment of blood flow, may damage the tissue further
175
why When blockage of a coronary artery deprives the heart muscle of oxygen, does reperfusion (re′-per-FYŪ-zhun), the reestablishment of blood flow, damage the tissue further
the formation of oxygen free radicals from the reintroduced oxygen.
176
free radicals are
molecules that have an unpaired electron
177
free radicals
cause chain reactions that lead to cellular damage and death.
178
To counter the effects of oxygen free radicals,
body cells produce enzymes that convert free radicals to less reactive substances.
179
What enzymes are produced to combat the effets of free radicals
superoxide dismutase (dis-MŪ-tās) and catalase (KAT-a-lās).
180
nutrients such as vitamin E, vitamin C, beta-carotene, zinc, and selenium serve as antioxidants, which
remove oxygen free radicals from circulation.
181
Partial obstruction of blood flow in the coronary arteries may cause
myocardial ischemia
182
myocardial ischemia (is-KĒ-mē-a; ische- = to obstruct; -emia = in the blood),
a condition of reduced blood flow to the myocardium.
183
Usually, ischemia causes
hypoxia (hī-POKS-ē-a = reduced oxygen supply), which may weaken cells without killing them.
184
Silent myocardial ischemia, ischemic episodes without pain, is particularly dangerous because
the person has no forewarning of an impending heart attack.
185
A complete obstruction to blood flow in a coronary artery may result in a
myocardial infarction (MI) (in-FARK-shun), commonly called a heart attack.
186
Infarction is
the death of an area of tissue because of interrupted blood supply
187
Compared with skeletal muscle fibers (cells), cardiac muscle fibers are
shorter in length and less circular in transverse section
188
cardiac muscle fibers
exhibit branching, which gives individual cardiac muscle fibers a “stair-step” appearance
189
in cardiac muscle fibers
Usually one centrally located nucleus is present, although an occasional cell may have two nuclei.
190
The ends of cardiac muscle fibers connect to neighboring fibers by
irregular transverse thickenings of the sarcolemma called intercalated discs (in-TER-ka-lāt-ed; intercalat- = to insert between)
191
intercalated discs contain
desmosomes, which hold the fibers together, and gap junctions, which allow muscle action potentials to conduct from one muscle fiber to its neighbors.
192
Gap junctions allow
the entire myocardium of the atria or the ventricles to contract as a single, coordinated unit.
193
Mitochondria are
larger and more numerous in cardiac muscle fibers than in skeletal muscle fibers.
194
In a cardiac muscle fiber, mitochondria take up
25% of the cytosolic space; in a skeletal muscle fiber only 2% of the cytosolic space is occupied by mitochondria
195
Cardiac muscle fibers have the same arrangement of ___________________as skeletal muscle fibers
actin and myosin, and the same bands, zones, and Z discs,
196
The T (transverse) tubules of cardiac muscle are
wider but less abundant than those of skeletal muscle; the one T tubule per sarcomere is located at the Z disc
197
The sarcoplasmic reticulum of cardiac muscle fibers is
somewhat smaller than the SR of skeletal muscle fibers. As a result, cardiac muscle has a smaller intracellular reserve of Ca2+.
198
autorhythmic fibers
a network of specialized cardiac muscle fibers (cells) and The source of electrical activity in the heart
199
autorythmic fibers are
self excitable
200
Autorhythmic fibers repeatedly
generate action potentials that trigger heart contractions.
201
During embryonic development, only about 1% of the cardiac muscle fibers
become autorhythmic fibers
202
auto rythmic fibers have two main functions
They act as a natural pacemaker, setting the rhythm of electrical excitation that causes contraction of the heart.
They form the cardiac conduction system, a network of specialized cardiac muscle fibers that provide a path for each cycle of cardiac excitation to progress through the heart.
203
The conduction system ensures that
cardiac chambers become stimulated to contract in a coordinated manner, which makes the heart an effective pump.
204
Cardiac muscle fibers connect to neighboring fibers by
intercalated discs, which contain desmosomes and gap junctions.
205
Where does the cardiac conduction system begin
Cardiac excitation normally begins in the sinuatrial (SA) node, located in the right atrial wall just inferior and lateral to the opening of the superior vena cava
206
SA node cells do not have a stable resting potential. Rather,
they repeatedly depolarize to threshold spontaneously. The spontaneous depolarization is a pacemaker potential.
207
What is the first step in the cardiac conduction system
When the pacemaker potential reaches threshold, it triggers an action potential (Figure 20.10b). Each action potential from the SA node propagates throughout both atria via gap junctions in the intercalated discs of atrial muscle fibers. Following the action potential, the two atria contract at the same time.
208
in the cardiac conduction system what happens after the two atria contract
By conducting along atrial muscle fibers, the action potential reaches the atrioventricular (AV) node, located in the interatrial septum, just anterior to the opening of the coronary sinus
