Ans: The cyclical events that occur from the beginning of one heart beat to the
beginning of next constitute a cardiac cycle. It is divisible into three phases,
namely atrial systole, ventricular systole and cardiac diastole. The contraction
of heart is known as systole and relaxation is known as diastole. A systole and
its following diastole constitute a heart beat.
A. Atrial Systole (0.1 Sec):
When the atria are filled with blood [right atrium with deoxygenated blood and
left atrium with oxygenated blood], S.A. node (pace maker) generates an action potential which spreads over atria. Thus they contract. As a result, blood from right atrium flows into right ventricle and from the left atrium to left ventricle [during atrial systole 30% of blood reaches the ventricles and the 70% of blood flows into ventricles before atrial systole.
B. Ventricular Systole (0.3 Sec):
i. The A.V. node receives the action potentials and conduct them to walls of ventricles through Bundle of His and Parkinje fibres. Thus ventricles contract
(Now atria relaxes and are filled with blood).
ii. Thus blood flows into the aortic arches.
iii. Due to high pressure in ventricles, tricuspid valve and bicuspid valve are
closed causing the first sound of heart beat - LUB.
C. Cardiac diastole (0.4 Sec):
i. Now the ventricles relax and hence, pressure in ventricles falls. As a result,
pulmonary valve and aortic valve are closed causing second sound of heart beat
- DUP.
ii. Now blood that reached atria flows into ventricles by the pressure of blood in
atria. At this stage all the four chambers are in relaxed state.
iii. Now another cardiac cycle sets in.
In Man, the duration of Cardiac cycle is 0.8 Sec, if heart beats at the
rate of 72 times per minute.
3. Cardiac Output:
a. The volume of blood pumped by each ventricle for each heart beat is known
as stroke volume. It is about 70 ml.
b. The volume of blood pumped out by the heart from each ventricle per minute
is known as cardiac output.
Cardiac output = Stroke volume × No. of beats per minute
= 70 ml × 72 times = 5,040 ml/ min.
4. Double Circulation:
In the heart of man, oxygenated and deoxygenated blood remain separate. Right
side of the heart contains deoxygenated blood and left side contains oxygenated blood.
In one complete circuit in the body, the blood passes twice through the heart, once through the right side and the other time through the left side. This type of circulation is called double circulation.
It includes systemic circulation and pulmonary circulation.
i. Systemic Circulation: The Oxygenated blood from left ventricle goes to all
organs (except alveoli of lungs) through left systemic arch and its branches
(arteries). Deoxygenated blood from different organs (except from alveoli of
lungs) enters the right atrium through the vena cavae. This circulation is
called systemic circulation (greater circulation).
ii. Pulmonary circulation:
Deoxygenated blood from right atrium flows into right ventricle and goes to alveoli of lungs through pulmonary arch. From the alveoli of lungs, oxygenated
blood is collected and send it to left atrium by pulmonary veins. This
circulation is called pulmonary circulation (lesser circulation).
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