Biomedical Engineering Reference
In-Depth Information
Chapter 6
Heart Wall
The heart lodges in the mediastinum. It is usually situated in the middle of the thorax
with its largest part in the left side; its apex points toward in the inferior and left
direction. However, sometimes it resides on the right thoracic region (dextrocardia
situs inversus totalis).
The heart is made of 2 synchronized left and right pumps with a common
wall, the septum between the left and right atria at its upper part and left and
right ventricles at its lower part. These twin pumps in series are composed of
2 chambers — upstream atrium and downstream ventricle — to adapt to the pressure
difference between the low-pressure, upstream, venous compartment and high-
pressure, downstream, arterial bed.
Under sedentary conditions, atria are minor contributors to blood pumping.
During exercise, atrial contraction rises to increase the amount of blood that fills
the ventricles (
atrial kick
). This blood-pumping reserve falls during aging as well as
during atrial fibrillation.
Arteries leave the cardiac pump, without relation with the blood gas content;
veins run toward atria. The left ventricle propels blood through the
systemic
circulation
that carries arterial oxygenated blood under high arterial pressure; the
aorta is the arterial trunk that receive blood from the left ventricle during the ejection
phase of its contraction. The right ventricle expels blood through the
pulmonary
circulation
that transport arterial deoxygenated blood with a lower arterial pressure;
the pulmonary arterial trunk corresponds to the exit of the right ventricle.
Cardiac muscle cells, or cardiomyocytes (CMC; Chap.
5
), are joined both
directly and indirectly via fibers of the extracellular matrix to form muscular
bundles (myofibers) connected to neighboring coronary vessels and nerves (Vol. 6 -
Chaps. 2. Anatomy of the Cardiovascular System and 4. Cardiovascular Physiol-
ogy). These myofibers form muscular layers within the heart wall thickness that
are characterized by their orientation (Sect.
5.9
), which influences spreading of the
electrochemical command (cardiomyocyte depolarization) wave and cardiac wall
deformation associated with cardiomyocyte contraction.
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