Human heart which has the size of a clenched fist is a
specialised organ that pumps blood to all parts of the
It is made up of cardiac muscles.
It is divided into 2 parts (left and right) and has 4
chambers (2 atria and 2 ventricles).
The wall of the atria is slightly thinner than the wall of the
ventricles because atria functioned as collection chambers
while the ventricles pumps blood away from the heart.
The wall of the left ventricle is thicker than the right
ventricle because it has to pump blood under high
pressure to all parts of the body while the right ventricle
only pumps blood to the lungs for re-oxygenated.
Coronary arteries supply the heart muscles with food
(glucose) and oxygen.
The heart has valves that only allow blood to flow in one
Tricuspid valve /Atrioventricular valve
Between right atrium and right ventricle
Bicuspid valve / Mitral valve
Between left atrium and left ventricle
Semilunar valve / Pulmonic valve
In the wall of pulmonary artery and aorta
Chordae tendineae anchor the valve flaps to papillary muscles in the ventricular wall.
(Source: How the body works by DR Peter Abraham)
PUMPING OF THE HEART
Cardiac muscle is myogenic which means it contracts and relaxes
without the need of receiving stimulation by the nerve impulses.
The contraction of the heart is initiated by pacemaker.
SINO-ATRIAL NODE (SA Node)
Cluster of cells situated within the wall of right atrium. SA Node acts
as heart pacemaker by contracting at a faster rate than any other
cardiac muscle cells.
It generates electrical impulses for each contraction which then spread
rapidly to the wall of both atria, causing the contraction of the atria.
ATRIOVENTRICULAR NODE (AV Node)
AV Node is located within the floor of the right atrium.
It passes electrical impulses originated from the SA Node to the wall of
ventricles to contract.
Cardiac muscle is made up of 3 layers: the epicardium (outer
layer), the myocardium (middle layer) and the endocardium
Myocardial layer responsible for the contraction of the heart.
Thus, myocardial layer of the left ventricle is much thicker
because the ventricle pumps blood to all parts of the body
under high pressure.
Sequence of heart contraction:
The SA Node generated electrical impulses.
2. The electrical impulses spread rapidly to the wall of both
atria causing the contraction of the atria. These allow the
blood to be pump into the ventricles.
3. The impulses are then relayed to the AV Node down to the
bundle of His, Purkinje fibres and the apex of the heart.
4. The electrical impulses spread to the ventricles allowing
the ventricular contractions begins from the apex and
upwards to pump blood away from the heart.
Electrical impulses from the SAN is being delayed by 0.1s upon reaching AVN,
thus allowing ventricles to fill up with blood.
CARDIAC CYCLE (a series of changes within the heart
that causes it to pump blood)
1. Blood enters the atria. At the same time, the tricuspid valve and
bicuspid valve open as the atrial pressure exceeds the ventricular
pressure. The resting phase is called diastole.
2. As diastole ends and the atrial systole begins, more blood is
pushed to fill the ventricles. Hence, the both atria contract.
3. During the ventricular systole (contract), the ventricular pressure
increase causing the tricuspid valve and bicuspid valve to close
and semilunar valve to open. Hence, pushing the blood into the
aorta and pulmonary artery. At this time, the atria relax.
4. The ventricles relax (ventricular diastole) and its pressure will
soon decrease causing the semilunar valve to close while blood
starting to fill the atria.
ELECTROCARDIOGRAM (ECG) – use to detect the sequence of the
Atrial contraction (atrial depolarization) happens and excitation travels through
the AV node (atrial excitation)
Delay of electrical impulses from the AVN and depolarization of the atria
Ventricular contraction occurs (ventricular excitation)
Resting period of the ventricle (ventricular repolarization)
ST segment (beginning of ventricular