The Heart - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 123

muscle. The major difference between cardiac muscle tissue and skeletal muscle is that car-
diac muscle contains voltage-gated calcium channels which help to elongate the depolariza-
tion phase of the cardiac action potential. This elongation allows the atria to contact as one
and then the ventricles to contract as one, instead of having a wave of contractions through
the cardiac tissue. The work that the heart undergoes to contract can be depicted by the
pressure-volume relationship of each heart chamber. The area within this closed-loop circuit
describes the work of heart contraction.
4.2 The cardiac action potential is generated in the sino-atrial (SA) node, which is a highly spe-
cialized structure that is surrounded by extracellular fluid with an abnormally high sodium
concentration. This sodium continually enters the cells that comprise the SA node and
approximately every 60 seconds generates an action potential. The cells of the SA node are
directly connected with neighboring cardiac myocytes, to allow for the coupling of the atria
with the SA node. There are three pathways along the right atrium wall, which help to pass
the action potential signal from the SA node to the atrioventricular (AV) node. The AV node
is responsible for initiating ventricular contraction, but the AV node slows down the conduc-
tion speed of the cardiac action potential so that the atria can finish contracting prior to ven-
tricle contraction. The action potential passes from the AV node to the Purkinje fiber system,
which rapidly transmits the action potential to the apex of the heart and then penetrates into
the ventricular muscle mass. This causes the bottom portion of the ventricle to contract,
pushing the blood toward the valves located superior to the ventricle. The electrocardiogram
(ECG) is a common way to measure the electrical potential within the cardiac tissue. The
ECG signal is composed of three waves: the P wave, which is associated with atrial contrac-
tion; the QRS complex, which is associated with ventricular contraction; and the T wave,
which is associated with ventricular repolarization. Changes in the intervals between these
waves, the lengths of the waves, and the electrical potential of these waves can help physi-
cians diagnose various cardiac conditions.
4.3 The cardiac cycle describes all of the events that occur during one heart beat and is divided
into two phases, diastole and systole. Diastole is characterized by cardiac myocytes that are
not actively contracting, whereas systole is characterized by active cardiac myocyte contrac-
tion. During diastole, there is a rapid reduction in the cardiac pressure and there is a rise in
the blood volume for the chambers within the heart. During systole, there is a rapid increase
in cardiac pressure accompanied by a rapid decrease in the blood volume.
4.4 During active contraction, the entire heart moves in three-dimensional space, which can be
modeled by changes in the forces/pressures within the heart. A.V. Hill modeled the heart
contraction as a three-component model that is comprised of the active contraction mecha-
nism (actin-myosin), a viscoelastic relaxation, and a parallel resistive element. Hill's model
can be used to calculate the force generated during contraction. The solid mechanics of the
heart are also very important to understand when modeling heart motion. Assuming that
the heart is a perfect spherical shell with homogeneous material properties, the radial stress
and the circumferential stress generated within the cardiac tissue can be described by

p o b 3 ð r 3

2 a 3

p i a 3 ð b 3

2 r 3

Þ Þ

σ r ð r Þ 5

Þ 1 r 3 ð a 3

2 b 3

r 3 ð a 3

2 b 3

Þ