Biomedical Engineering Reference
In-Depth Information
the aorta (the aortic valve is open). The last phase (isovolumic relaxation) is also classified
by no change in left ventricular volume. During this phase, the mitral and the aortic valves
are closed and the pressure reduces from approximately 80 mmHg to 1 mmHg. The reduc-
tion in pressure occurs because the left ventricle myocytes are relaxing. The pressure-
volume loop forms a closed cycle, in which the work can be quantified by the area enclosed
within the cycle. The right ventricular pressure-volume relationship is very similar to the
left ventricle pressure-volume curve, except that the pressures are approximately 5 to
6 times lower than the left ventricle (the right ventricle curve is not shown). The reader is
cautioned that the pressure-volume relationship changes drastically under different patho-
logical conditions, where the systolic/diastolic pressure is not the “normal” 120 mmHg
over 80 mmHg. Therefore, under a disease condition, the heart may work more than nor-
mal, potentially leading to other pathological conditions.
4.2 CAR DIAC CONDUCTION SYSTEM/ELECTROCARDI OGRAM
The cardiac conduction system is composed of a specialized structure that generates
impulses to excite the heart and a specialized conduction system to induce ventricular
myocytes contraction at the same instant in time only after atrial contraction completes
( Figure 4.5 ). The sino-atrial (SA) node is the specialized structure that generates impulses
within the heart. It is located within the right atrium wall immediately below the opening
of the superior vena cava. The SA node is a group of specialized muscle cells that have lit-
tle to no contractility associated with them, but they are directly connected to neighboring
cardiac myocytes within the atrial syncytium.
The muscle cells that compose the SA node are surrounded by extracellular fluid with
an abnormally high sodium concentration. Unlike other cardiac myocytes, the cells that
compose the SA node have some cell membrane sodium channels that are always open in
addition to the normal fast sodium channels, which open in response to the action poten-
tial. Due to the presence of continually open sodium channels and the continuous leak of
sodium ions into the SA node cells, there is a slow increase in the SA node cells membrane
FIGURE 4.5 Sino-atrial node and the
Purkinje fiber system of the heart, show-
ing the approximate times when the
action potential signal reaches various
locations within the conducting system.
This conducting system allows for a
rapid transmission of the cardiac action
potential to each cardiac myocyte.
AV Node
t = 0.12 sec
SA Node
t = 0 sec
AV Bundle
Penetrating portion
t = 0.13 sec
Purkinje fibers
t = 0.16 sec
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