Structure of the Heart  The heart is divided vertically into two
cavities by a muscular wall called the septum. The left cavity
pumps blood throughout the body, while the right cavity pumps
blood only to the lungs. Each cavity is in turn divided
horizontally into two chambers, making a total of four chambers
altogether. The two upper chambers are known as the atria, and
the two lower chambers as the ventricles. The atria receive
blood flowing back to the heart, while the ventricles hold blood
that is to be pumped out of the heart.   Inside the right atrium
of the heart sits a small bundle of muscle fibres and nerves.
This is the sinus or sinoatrial node, which acts as the heart’s
natural pacemaker. Each electrical impulse emitted by the sinus
node sparks off a chain reaction within the heart that begins
with the contraction of the atria. As the right atrium
contracts, the blood in this chamber (i.e. oxygen-poor blood
which has been returned to the heart) is pushed through the
tricuspid valve into the right ve