646
P A R T 8
Drugs acting on the cardiovascular system
These two branches of the autonomic nervous
system work together to help the heart meet the body’s
demands. Drugs that influence either branch can exert
autonomic effects on the heart.
Myocardial contraction
The end result of the electrical stimulation of the heart
cells is the unified contraction of the atria and ventricles,
which moves the blood throughout the vascular system.
The basic unit of the cardiac muscle is the
sarcomere
(Figure 42.4). A sarcomere is made up of two contrac
tile proteins:
actin
, a thin filament, and
myosin
, a thick
filament with small projections on it. These proteins are
anchored at the Z bands, the outer edges of each sarco
mere. These proteins readily react with each other, but
at rest they are kept apart by the protein
troponin
.
When a cardiac muscle cell is stimulated, calcium
enters the cell though channels in the cell membrane
and also from storage sites within the cell. This occurs
during phase 3 of the action potential, when the cell
is starting to repolarise. The calcium reacts with the
troponin and inactivates it. This action allows the actin
and myosin proteins to react with each other, forming
actomyosin bridges. These bridges then break quickly,
and the myosin slides along to form new bridges.
As long as calcium is present, the actomyosin bridges
continue to form. This action slides the proteins together,
shortening or contracting the sarcomere. Cardiac muscle
cells are linked together: When one cell is stimulated to
contract, they are all stimulated to contract.
The shortening of numerous sarcomeres causes the
contraction and pumping action of the heart muscle. As
the cell reaches its repolarised state, calcium is removed
from the cell by a sodium–calcium pump, and calcium
released from storage sites within the cell returns to the
storage sites. The contraction process requires energy
and oxygen for the chemical reaction that allows the
formation of the actomyosin bridges, and calcium to
allow the bridge formation to occur.
The degree of shortening (the strength of con
traction) is determined by the amount of calcium
present—the more calcium present, the more bridges
will be formed—and by the stretch of the sarcomere
before contraction begins. The further apart the actin
and myosin proteins are before the cell is stimulated, the
more bridges will be formed and the stronger the con
traction will be. This correlates with Starling’s law of
the heart. The more the cardiac muscle is stretched, the
greater is the contraction. The more blood that enters
the heart, the greater is the contraction that is needed to
empty the heart, up to a point; however, if the actin and
myosin molecules are stretched too far apart, they will
not be able to reach each other to form the actomyosin
bridges, and no contraction will occur.
■■
The heart, a hollow muscle with four chambers
comprising two upper atria and two lower ventricles,
pumps oxygenated blood to the body’s cells and also
collects waste products from the tissues.
■■
The two-step process known as the cardiac cycle
includes diastole (resting period when the veins carry
blood back to the heart) and systole (contraction
period when the heart pumps blood out to the
arteries for distribution to the body).
■■
Impulses generated in the heart—not the brain—
stimulate contraction of the heart muscle.
■■
The heart’s conduction (or stimulatory) system consists
of the sinoatrial (SA) node, the atrial bundles, the AV
node, the bundle of His, the bundle branches and the
Purkinje fibres.
ELECTROCARDIOGRAPHY
Electrocardiography is a process of recording the
patterns of electrical impulses as they move through
the heart. It is an important diagnostic tool in the care of
the person experiencing cardiac problems. The electro
cardiography machine detects the patterns of electrical
impulse generation and conduction through the heart
and translates that information into a recorded pattern,
which is displayed as a waveform on a cardiac monitor
or printout on calibrated paper. An
electrocardiogram
(ECG)
is a measure of electrical activity; it provides no
information about the mechanical activity of the heart.
The important aspect of cardiac output—the degree to
which the heart is doing its job of pumping blood out
to all of the tissues—needs to be carefully assessed by
looking at and evaluating the person.
The normal ECG waveform is made up of five main
waves: the P wave, which is formed as impulses originat
ing in the SA node or pacemaker pass through the atrial
tissues; the QRS complex, which represents depolarisa
tion of the bundle of His (Q) and the ventricles (RS); and
the T wave, which represents repolarisation of the ventri
cles (Figure 42.5).
KEY POINTS
Ca
2+
channels
Actin filaments
Myosin filaments
Z bands
Troponin
Z bands
Ca
2+
Ca
2+
Ca
2+
FIGURE 42.4
A sarcomere, the functioning unit of cardiac muscle.
