Cambridge International A Level Biology Revision Guide

Chapter 8: Transport in mammals
The left ventricle, however, has to develop sufficient
force to supply blood to all the rest of the body organs.
For most organs, most of the time, the high pressures
that the left ventricle is capable of producing would be too
great and, as we have seen, arterioles play an important
role in reducing this pressure before blood flows into
the capillaries. However, during vigorous exercise, when
muscles are working hard, the arterioles supplying blood
to them dilate, increasing blood flow to them. The left
ventricle must be able to develop enough force to ensure
that there is still sufficient blood reaching other organs,
especially the brain. Kidneys require high-pressure
blood all the time, to carry out ultrafiltration (page 307).
Therefore, the thickness of the muscular wall of the left
ventricle is much greater than that of the right.
Figure 8.28 shows the pressure changes in the left side
of the heart and the aorta during two consecutive cardiac
cycles. You can see that the pressure developed in the left
ventricle is much greater than that in the left atrium.
Control of heart beat
Cardiac muscle differs from the muscle in all other areas
of the body in that it is myogenic. This means that it
naturally contracts and relaxes; it does not need to receive
impulses from a nerve to make it contract. If cardiac
muscle cells are cultured in a warm, oxygenated solution
containing nutrients, they contract and relax rhythmically,
all by themselves.
However, the individual heart muscle cells cannot be
allowed to contract at their own natural rhythms. If they
did, parts of the heart would contract out of sequence with
other parts. The cardiac cycle would become disordered,
and the heart would stop working as a pump. The heart
has its own built-in controlling and coordinating system
which prevents this happening.
The cardiac cycle is initiated in a specialised patch of
muscle in the wall of the right atrium, called the sinoatrial
node. It is often called the SAN for short, or pacemaker.
The muscle cells of the SAN set the rhythm for all the other
cardiac muscle cells. Their natural rhythm of contraction
is slightly faster than that of the rest of the heart muscle.
Each time the muscles of the SAN contract, they set up a
wave of electrical activity which spreads out rapidly over
the whole of the atrial walls. The cardiac muscle in the atrial
walls responds to this excitation wave by contracting, at the
same rhythm as the SAN. Thus, all the muscle in both atria
contracts almost simultaneously.
As we have seen, the muscles of the ventricles do not
contract until after the muscles of the atria. (You can
imagine what would happen if they all contracted at once.)
This delay is caused by a feature of the heart that briefly
delays the excitation wave in its passage from the atria to
the ventricles. There is a band of fibres between the atria
and ventricles which does not conduct the excitation wave.
Thus, as the wave spreads out from the SAN over the atrial
walls, it cannot pass into the ventricle walls. The only
route through is via a patch of conducting fibres, situated
in the septum, known as the atrioventricular node, or
AVN (Figure 8.29). The AVN picks up the excitation wave
as it spreads across the atria and, after a delay of about
177
1 Each cardiac cycle begins in the right atrium.
There is a small patch of muscle tissue in the
right atrium wall, called the sinoatrial node
(SAN), which automatically contracts and relaxes
all the time. It doesn’t need a nerve impulse to
start it off, so it is said to be myogenic – that is,
‘started by the muscle’.
The pacemaker’s rate can be adjusted by nerves
transmitting impulses to the pacemaker from
the brain.
5 The ventricles then relax.
Then the muscle in the
SAN contracts again, and
the whole sequence runs
through once more.
4 The excitation wave moves swiftly down
through the septum of the heart, along fibres
known as Purkyne tissue. Once the excitation
wave arrives at the base of the ventricles it
sweeps upwards, through the ventricle walls.
The ventricles contract.
Figure 8.29 How electrical excitation waves move through the heart.
2 As the muscle in the SAN contracts, it
produces an electrical excitation wave
which sweeps through all of the
muscle in the atria of the heart. This
excitation wave makes the muscle in
the atrial walls contract.
3 The excitation wave sweeps onwards
and reaches another patch of cells,
called the atrioventricular node
(AVN). This node is the only way in
which the electrical impulse can get
down to the ventricles. The AVN
delays the impulse for a fraction of a
second, before it travels down into
the ventricles. This delay means that
the ventricles receive the signal to
contract after the atria receive
the signal.