Cambridge International A Level Biology Revision Guide

Chapter 8: Transport in mammals
The heart
The heart of an adult human has a mass of around 300 g,
and is about the size of your fist (Figure 8.21). It is a bag
made of muscle and filled with blood. Figure 8.22 shows
the appearance of a human heart, looking at it from the
front of the body.
The muscle of which the heart is made is called cardiac
muscle. Although you do not need to know the structure of
cardiac muscle, you may find it interesting, and it is shown
in Figure 8.23. It is made of interconnecting cells, whose
cell surface membranes are very tightly joined together.
This close contact between the muscle cells allows waves of
electrical excitation to pass easily between them, which is a
very important feature of cardiac muscle, as you will
see later.
vena cava
from head
right
pulmonary
artery
right
pulmonary
vein
right atrium
right ventricle
vena cava from
lower regions of
body
aorta
left
pulmonary
artery
left atrium
left
pulmonary
vein
left
ventricle
coronary
arteries
Figure 8.22 Diagram of the external structure of a human
heart, seen from the front.
Figure 8.22 also shows the blood vessels that carry
blood into and out of the heart. The large, arching blood
vessel is the largest artery, the aorta, with branches
leading upwards towards the head, and the main flow
doubling back downwards to the rest of the body. The
other blood vessel leaving the heart is the pulmonary
artery. This, too, branches very quickly after leaving the
heart, into two arteries taking blood to the right and
left lungs. Running vertically on the right-hand side
of the heart are the two large veins, the venae cavae,
one bringing blood downwards from the head and the
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Figure 8.21 A human heart. The blood vessels in the
photograph lie immediately below the surface of the heart
and have been injected with gelatine containing a dye. The
cardiac muscle was treated to make it transparent to a depth
of 2 millimetres to allow the blood vessels to seen.
Figure 8.23 You do not need to know this structure, but you
may like to compare it with striated muscle, shown in Figures
15.25 and 15.26.