hand. Can you see the swellings where you have pushed blood against the
valves? Discuss with your teacher about reasons.
Think and discuss
Artery walls are very strong and elastic. why?
Why we compare arteries like tree which divides into smaller and smaller branches.
The lumen size is bigger in vein when compared with artery. Why?
The cardiac cycle
The human heart starts beating around 21st day during the
embryonic development (refer reproduction chapter). If it stops
beating, it results in the death of a person.
One contraction and one relaxation of atria and ventricles
is called one cardiac cycle.
1. We start with the imagination that all the four chambers of
the heart are in relaxed state (diastole).
2. Blood from venacava and pulmonary veins enters the right
and left atria respectively.
3. Now the atria contract, forcing the blood to enter into the
ventricles.
4. When the ventricles are filled with blood they start
contracting and atria start relaxing. The aperture between
the atria and ventricles is closed by valves. On ventricular
contraction due to pressure the blood moves into the aorta
and pulmonary artery. When the valves are closed forcibly,
we can listen to the first sharp sound of the heart lub.
5. When the ventricles start relaxing the pressure in the
ventricles is reduced. The blood which has entered the
arteries tries to come back into the ventricles. The valves
which are present in the blood vessels are closed to prevent
backward flow of blood into the ventricles. Now we can
listen to a dull sound of the heart dub. The atria are filled up
with blood and are ready to pump the blood into the
ventricles.
The sequential events in the heart which are cyclically
repeated are called cardiac cycle. The cardiac cycle includes an
active phase systole and a resting phase the diastole of atria and
ventricles.
The cardiac cycle