Download The Heart Chambers and Valves Of the cardiac chambers, only the

The Heart Chambers and Valves
Of the cardiac chambers, only the main parts of the atria have a smooth
wall. In the auricles (parts of the atria) and especially in the ventricles, muscular
ridges (trabeculae carneae) protrude into the chambers. A single layer of
epithelium (endocardium) covers all the chambers. The four valves of the heart
are anchored in dense fibrous connective tissue rings that lie nearly in a plane
(Fig. 5.5). Together with the connective tissue between them they form a unit,
the so-called cardiac skeleton, to which the atria are separated from the
ventricles above and below the skeleton.
The cusps of the valves between the atria and ventricles (atrioventricular
valves) arise as double layers of endocardium from the cardiac skeleton (Fig.
5.5).The free ends of the valve cusps are attached by tendinous threads
(chordae tendineae) to the papillary muscles. These cone-shaped processes
on the inner ventricular walls, together with the chordae tendineae, prevent the
cusps from flapping back during ventricular contractions (Figs. 10a, b). A valve
with three cusps (tricuspid valve) is situated between the right atrium and the
right ventricle. A valve with two cusps (bicuspid valve, mitral valve) separates
the left atrium and left ventricle (Fig. 5.6).
The semilunar valves are situated at the entrances to the pulmonary artery
and the aorta. They prevent the blood from flowing back after a completed
ventricular contraction (Fig. 3). The pulmonary and aortic valves consist of three
pockets of duplicated endocardium that project into the lumen with their inferior
surfaces directed toward the heart. When the margins of the semilunar valves
are tightly apposed, the corresponding exit valve is closed. As the pressure in
the ventricles increases, the margins of the semilunar valves draw apart, and the
exit valve opens.
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Cardiovascular System Lecture No; 2
Dr. Abdul-Majeed Alsaffar
Wall of the Heart.
The heart wall consists of three layers of different thickness and structure (Fig.
5.7):
 an inner serous coat (endocardium)
 the actual cardiac muscle (myocardium)
 an outer serous coat (epicardium)
Between the epicardium and the inner side of the pericardium lies a thin serous
cavity, scantily filled with fluid, which allows the frictionless movement of the
heart in the pericardial sac. The myocardium consists of striated cardiac
muscle and is about 0.7 cm thick in the right ventricle. The wall of the left
ventricle about 1.4 cm in thickness, because of its higher pressure and the
consequently increased load, averages.
Differences between Cardiac and Skeletal Muscle
The cardiac-muscle cells of the myocardium are arranged in layers that are
tightly bound together and completely encircle the blood-filled chambers. When
the walls of a chamber contract, they come together like a squeezing fist and
exert pressure on the blood they enclose. Cardiac muscle combines properties
of both skeletal and smooth muscle. The cells are striated (Figure11) as the
result of an arrangement of thick myosin and thin actin filaments similar to that
of skeletal muscle. Cardiac-muscle cells are considerably shorter than skeletal
muscle fibers, however, and have several branching processes. Adjacent cells
are joined end to end at structures called intercalated disks, within which are
desmosomes that hold the cells together and to which the myofibrils are
attached. Adjacent to the intercalated disks are gap junctions, similar to those
in many smooth muscles. Approximately 1 percent of the cardiac-muscle cells
does not function in contraction, but have specialized features that are essential
for normal heart excitation. These cells constitute a network known as the
conducting system of the heart and are in contact with the other cardiac-muscle
cells via gap junctions. The conducting system initiates the heartbeat and helps
Cardiovascular System Lecture No; 2
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Dr. Abdul-Majeed Alsaffar
spread the impulse rapidly throughout the heart. A single adequate stimulus for
action potential in one myocyte results in the rapid spread of excitation to all
myocytes via gap junctions. This is known as the all-or-none electrical response
of the heart.
One final point about the cardiac-muscle cells in the atria, are certain cells
when they become stretched, secrete the family of peptide hormones
collectively called atrial natriuretic factor. (ANF):During heart failure almost always excessive increase in both right left atrial
pressure causing their wall stretch, the circulating levels of ANF in the blood
increase from fivefold to tenfold in sever heart failure. The ANF in turn has a
direct effect on kidneys to increase greatly their excretion of salt and water.
Therefore, ANF plays a natural role to help prevent extreme congestive
symptoms during cardiac failure.
Fig.11 a−c Muscle tissue in longitudinal section
a Smooth muscle tissue
b Striated (skeletal) muscle
c Cardiac muscle
The Coronary Vessels
The coronary arteries supply the heart muscle exclusively (Fig. 5.9). They arise
from the aorta immediately above the aortic valve and send their major branches
over the myocardium, their terminal branches entering the cardiac muscle from
the outside.
After leaving the aorta, the right coronary artery runs in the coronary sulcus,
at first under the right auricle, then around the right cardiac border toward the
diaphragmatic surface of the heart. It ends in the posterior interventricular
branch, which runs to the apex of the heart (Fig. 5.9). The left coronary artery,
after a short course, divides into two braches the anterior inter ventricular
branch, which runs over the anterior surface, and the circumflex branch, which
runs posteriorly. The veins collect the blood in the small, middle, and great
cardiac veins, which collect in the coronary sinus and drain into the right atrium.
If the coronary arteries narrow (arteriosclerosis), the affected cardiac muscle
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Cardiovascular System Lecture No; 2
Dr. Abdul-Majeed Alsaffar
suffers from lack of oxygen and can die (cardiac infarct) if the vessel is totally
occluded.
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Cardiovascular System Lecture No; 2
Dr. Abdul-Majeed Alsaffar
References
1. Textbook of Medical Physiology 11th, Edition .by Guyton A.C.
2. Human Physiology The Basis of Medicine 2nd, Edition. by Gillian
Pocock and Christopher D.R.
3. Human Physiology: The Mechanism of Body Function 10th Edition. by
Vander, et al
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Cardiovascular System Lecture No; 2
Dr. Abdul-Majeed Alsaffar