The heart has four chambers, two upper ones called the atria and
two lower ones called the ventricles. The whole heart works
electrically by a very small surge of current which is controlled by the
pacemaker (the sinus node). The pacemaker lies in the right atrium
and generates rhythmical electrical impulses which pass through both
atria causing them to contract. There is a junction box isolating the
atria from the ventricles (the AV node). The pathway through th
node is like a maze which delays the current before it reaches the
ventricles via ‘wires’ (the Purkinje system) which cause both
ventricles to contract simultaneously pumping the blood out of the
heart. Thus a single impulse can cause contraction of both sets of
chambers in the correct sequence.
At rest the pacemaker fires off between 50 and 100 impulses a
minute causing our pulse. When we exercise, the pacemaker
speeds up its rate of discharge, because it responds to nervous and
chemical influences in the body which speed up the heart. causing the body to react more aggressively and the heart
to beat
The chemicals are called catecholamines one of which i
adrenaline. Adrenaline is also released when we are frightened
more rapidly and powerfully in order to protect ourselves. Other
chemicals, from the thyroid gland, also increase the rate of
heart-beat. Fever increases the heart rate, e.g., with influenza.
occur with nausea or vomiting. Fainting occurs when the heart
When the heart slows it is due to a direct nervous control and can
slows too much. 
HARMLESS PALPITATIONS
Sinus tachycardia
Sinus tachycardia is when the heart beats normally but
rapidly, e.g., with exercise, emotion, blood loss, anaemia or
pregnancy.
Ectopics or extra beats
The extra beat is the most common abnormality. This is felt
as a ‘thud’ in the chest or a missed beat, although it sometimes
may not be noticed at all. Most people have extra beats and they I can be made more evident with strong tea. coffee, red wines and some
spirits. The electric signal which caused the extra beat arises from an area
away from the normal pacemaker, either the atria, AV node or ventricles,
and are named accordingly. An electrocardiogram can determine from
where they arise. They produce a heart-beat which comes earlier than
expected-hence the sensation of palpitations. Until recently these beats
were thought to be dangerous but now it is known their effect on the
heart’s performance is minimal.