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P A R T 8
Drugs acting on the cardiovascular system
volume in the atria is greater than the pressure in the
ventricles, blood flows through the atrioventricular
(AV) valves into the ventricles. The valve on the right
side of the heart is called the tricuspid valve because it is
composed of three leaflets or cusps. The valve on the left
side of the heart, called the mitral or bicuspid valve, is
composed of two leaflets or cusps (see Figure 42.1). Just
before the ventricles are stimulated to contract, the atria
contract, pushing about one more tablespoon of blood
into each ventricle. The much more powerful ventricles
then contract, pumping blood out to the lungs through
the pulmonary valve or out to the aorta through the
aortic valve and into the systemic circulation. The con
traction of the ventricles is referred to as
systole
. Each
period of systole followed by a period of diastole is
called a
cardiac cycle
. The heart’s series of one-way
valves keeps the blood flowing in the correct direction:
•
Deoxygenated blood enters
the right atrium, flows
through the tricuspid valve to the right ventricle, and
flows through the pulmonary valve to pulmonary
arteries and the lungs.
•
Oxygenated blood from the lungs returns
through
the pulmonary veins to the left atrium, flows
through the mitral valve into the left ventricle, and
then flows through the aortic valve to the aorta
and the rest of the body.
The AV valves close very tightly when the ventri
cles contract, preventing blood from flowing backwards
into the atria, thereby keeping blood moving forwards
through the system. The pulmonary and aortic valves
open with the pressure of ventricular contraction and
close tightly during diastole, keeping blood from flowing
backwards into the ventricles. These valves operate much
like one-way automatic doors: you can go through in the
intended direction, but if you try to go the wrong way,
the doors close and stop your movement. The proper
functioning of the cardiac valves is important in main
taining the functioning of the cardiovascular system.
Cardiac conduction
Each cycle of cardiac contraction and relaxation is con
trolled by impulses that arise spontaneously in certain
pacemaker cells of the
sinoatrial (SA) node
of the heart.
These impulses are conducted from the pacemaker cells
by a specialised conducting system that activates all of
the parts of the heart muscle almost simultaneously.
These continuous, rhythmic contractions are controlled
by the heart itself; the brain does not stimulate the heart
to beat. This safety feature allows the heart to beat as
long as it has enough nutrients and oxygen to survive,
regardless of the status of the rest of the body. This
property protects the vital cardiovascular function in
many disease states; it is the same property that allows
the heart to continue functioning in a person who is
“brain dead”.
The conduction system of the heart consists of the
SA node, atrial bundles, AV node, bundle of His, bundle
branches and Purkinje fibres (Figure 42.2). The SA node,
which is located near the top of the right atrium, acts
as the pacemaker of the heart. Atrial bundles conduct
the impulse through the atrial muscle. The AV node,
which is located near the bottom of the right atrium,
slows the impulse and allows the delay needed for ven
tricular filling. The AV node then sends the impulse
from the atria into the ventricles by way of the bundle
of His, which enters the septum and then divides into
three bundle branches. These bundle branches, which
conduct the impulses through the ventricles, break into
a fine network of conducting fibres called the Purkinje
fibres, which deliver the impulse to the ventricular cells.
Automaticity
The cells of the impulse-forming and conducting system
are rather primitive, uncomplicated cells called pale or
P cells. Because of their simple cell membrane, these cells
possess a special property that differentiates them from
other cells: they can generate action potentials or electri
cal impulses without being excited to do so by external
stimuli. This property is called
automaticity
.
All cardiac cells possess some degree of automaticity.
During diastole or rest, these cells undergo a spontane
ous depolarisation because they decrease the flow of
potassium ions out of the cell and probably leak sodium
into the cell, causing an action potential. This action
potential is basically the same as the action potential of
the neuron (see Chapter 19). The action potential of the
cardiac muscle cell consists of five phases:
• Phase 0 occurs when the cell reaches a point of
stimulation. The sodium gates open along the cell
membrane and sodium rushes into the cell, resulting
in a positive flow of electrons into the cell—an
electrical potential. This is called depolarisation.
SA node
AV node
Right bundle
branch
Left bundle
branch
Purkinje
fibres
Bundle of His
Right
ventricle
Left
atrium
Right
atrium
Left
ventricle
FIGURE 42.2
The conducting system of the heart. Impulses originating
in the sinoatrial (SA) node are transmitted through the atrial bundles
to the atrioventricular (AV) node and down the bundle of His and
the bundle branches by way of the Purkinje fibres through the
ventricles.
