Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
0.1 seconds, passes it on to a bunch of conducting fibres
called the Purkyne tissue, which runs down the septum
between the ventricles. This transmits the excitation wave
very rapidly down to the base of the septum, from where it
spreads outwards and upwards through the ventricle walls.
As it does so, it causes the cardiac muscle in these walls to
contract, from the bottom up, so squeezing blood upwards
and into the arteries.
In a healthy heart, therefore, the atria contract and
then the ventricles contract from the bottom upwards.
Sometimes, this coordination of contraction goes wrong.
The excitation wave becomes chaotic, passing through the
ventricular muscle in all directions, feeding back on itself
and re-stimulating areas it has just left. Small sections
of the cardiac muscle contract while other sections are
relaxing. The result is fibrillation, in which the heart wall
simply flutters rather than contracting as a whole and then
relaxing as a whole. Fibrillation is almost always fatal,
unless treated instantly. Fibrillation may be started by an
electric shock or by damage to large areas of muscle in the
walls of the heart.
Electrocardiograms (ECGs)
It is relatively easy to detect and record the waves of
excitation flowing through heart muscle. Electrodes can
be placed on the skin over opposite sides of the heart, and
the electrical potentials generated recorded with time. The
result, which is essentially a graph of voltage against time,
is an electrocardiogram (ECG) (Figure 8.30). You do not
need to know about electrocardiograms, but you may find
it interesting to relate what is shown in Figure 8.30 to what
you know about the spread of electrical activity through
the heart during a heart beat.
The part labelled P represents the wave of excitation
sweeping over the atrial walls. The parts labelled Q, R and
S represent the wave of excitation in the ventricle walls.
The T section indicates the recovery of the ventricle walls.
P
R
T
178
Q
0.2 s
S
Figure 8.30 A normal ECG.
Summary
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Blood is carried away from the heart in arteries, passes
through tissues in capillaries, and is returned to the
heart in veins. Blood pressure drops gradually as it
passes along this system.
Arteries have thick, elastic walls, to allow them to
withstand high blood pressures and to smooth out the
pulsed blood flow. Arterioles are small arteries that help
to reduce blood pressure and control the amount of
blood flow to different tissues. Capillaries are only just
wide enough to allow the passage of red blood cells, and
have very thin walls to allow efficient and rapid transfer
of materials between blood and cells. Veins have thinner
walls than arteries and possess valves to help blood at
low pressure flow back to the heart.
Blood plasma leaks from capillaries to form tissue fluid.
This is collected into lymphatics as lymph, and returned
to the blood in the subclavian veins. Tissue fluid and
lymph are almost identical in composition; both of them
contain fewer plasma protein molecules than blood
plasma, as these are too large to pass through the pores
in the capillary walls.
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Red blood cells are relatively small cells. They have a
biconcave shape and no nucleus. Their cytoplasm is full
of haemoglobin. White blood cells include phagocytes
and lymphocytes. They all have nuclei, and are either
spherical or irregular in shape. Red blood cells carry
oxygen in combination with haemoglobin.
Haemoglobin picks up oxygen at high partial pressures
of oxygen in the lungs, and releases it at low partial
pressures of oxygen in respiring tissues. A graph showing
the percentage saturation of haemoglobin at different
partial pressures (concentrations) of oxygen is known as
a dissociation curve.
At high carbon dioxide concentrations, the dissociation
curve shifts downwards and to the right, showing that
haemoglobin releases oxygen more easily when carbon
dioxide concentration is high. This is known as the
Bohr effect.