Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
174
other bringing it upwards from the rest of the body. The
pulmonary veins bring blood back to the heart from the
left and right lungs.
On the surface of the heart, the coronary arteries can
be seen (Figure 8.21). These branch from the aorta, and
deliver oxygenated blood to the walls of the heart itself.
If the heart is cut open vertically (Figures 8.24 and
8.25) it can be seen to contain four chambers. The two
chambers on the left of the heart are completely separated
from those on the right by a wall of muscle called the
septum. Blood cannot pass through this septum; the only
way for blood to get from one side of the heart to the other
is for it to leave the heart, circulate around either the lungs
or the rest of the body, and then return to the heart.
The upper chamber on each side of the heart is called
an atrium (plural: atria), or sometimes an auricle. The
two atria receive blood from the veins. You can see from
Figure 8.25 that blood from the venae cavae flows into the
right atrium, while blood from the pulmonary veins flows
into the left atrium.
The lower chambers are ventricles. Blood flows into
the ventricles from the atria, and is then squeezed out into
the arteries. Blood from the left ventricle flows into the
aorta, while blood from the right ventricle flows into the
pulmonary arteries.
atrium (the
space above
the cusp)
tendon
a cusp of one
atrioventricular
valve
papilllary
muscle
ventricle
wall of
ventricle
Figure 8.24 Section through part of the left side of the heart.
pulmonary artery
aorta
direction of
blood flow
left atrium
vena cava
from head
pulmonary vein
pulmonary (semilunar)
valve
right
atrium
aortic (semilunar)
valve
bicuspid
valve
papillary
muscle
thick muscular wall
of left ventricle
vena cava from
lower regions of
the body
tricuspid
valve
right
ventricle
tendon
septum
Figure 8.25 Diagrammatic section through a heart.