Answer:
The cardiac cycle
consist of one heart beat or one cycle of contraction and relaxation i'.e.,
takes place in the cardiac muscles. During the heart beat there is a
contraction and relaxation of atria and ventricles. The contraction phase is
referred as systole while the relaxation phase is called as diastole.
The successive
events of the cardiac cycle are briefly described as below
(i) Atrial
Systole The atria contract due to the wave of contraction, stimulated by the SA
node. The blood is forced into the ventricles as the bicuspid and tricuspid
valves are open.
(ii) Beginning
of Ventricular Systole The contraction of ventricles begin due to the wave of
contraction stimulated by AV node. This led to the closing of bicuspid and
tricuspid valve producing part of first heart sound, i.e., lub.
(iii) Complete
Ventricular Systole After ventricular contraction, the blood flows into the pulmonary
trunk and aorta as the semilunar valves open.
(d)
Beginning of the Ventricular Diastole The ventricles relax and the semilunar
valves are closed: This cause the second heart sound, i.e., dub.
(e) Complete
Ventricular Diastole The opening of tricuspid and bicuspid valves due to fall in
pressure of ventricles and blood flows from the atria into the ventricles.
Contraction of the heart does not cause this blood to flow, backward direction,
due to the fact that the pressure within the relaxed ventricles is less than
that of the atria and veins.
The duration
of cardiac cycle last for 0.8 sec,
In double
circulation, the blood passes twice through the heart during one complete
cycle.
Double
circulation is carried out by two ways
(i) Pulmonary
circulation (ii) Systemic circulation
Significance of
Double Circulation In birds and mammals, two separate circulatory pathways are
present. Oxygenated and deoxygenated blood received by the left and right atria
respectively passes on to the ventricles of the same sides. The ventricles pump
it out without mixing the oxygenated and deoxygenated blood in the heart.
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