cardiac cycle, double circulation and ECG ncert
Channel: ISHTA’s BIO ACADEMY
Date: 2022-12-30

How does the heart function? Let us take a look. To begin with, all the
four chambers of heart are in a relaxed state, i.e., they are in joint diastole.
As the tricuspid and bicuspid valves are open, blood from the pulmonary
veins and vena cava flows into the left and the right ventricle respectively
through the left and right atria. The semilunar valves are closed at this
stage. The SAN now generates an action potential which stimulates both
the atria to undergo a simultaneous contraction – the atrial systole. This
increases the flow of blood into the ventricles by about 30 per cent. The
action potential is conducted to the ventricular side by the AVN and AV
bundle from where the bundle of His transmits it through the entire
ventricular musculature. This causes the ventricular muscles to contract,
(ventricular systole), the atria undergoes relaxation (diastole), coinciding
with the ventricular systole. Ventricular systole increases the ventricular During each cardiac cycle two prominent sounds are produced which
can be easily heard through a stethoscope. The first heart sound (lub) is
associated with the closure of the tricuspid and bicuspid valves whereas
the second heart sound (dub) is associated with the closure of the
semilunar valves. These sounds are of clinical diagnostic significance.During each cardiac cycle two prominent sounds are produced which
can be easily heard through a stethoscope. The first heart sound (lub) is
associated with the closure of the tricuspid and bicuspid valves whereas
the second heart sound (dub) is associated with the closure of the
semilunar valves. These sounds are of clinical diagnostic significance.You are probably familiar with this scene from a typical hospital television
show: A patient is hooked up to a monitoring machine that shows voltage
traces on a screen and makes the sound “... pip... pip... pip.....
peeeeeeeeeeeeeeeeeeeeee” as the patient goes into cardiac arrest. This type
of machine (electro-cardiograph) is used to obtain an electrocardiogram
(ECG). ECG is a graphical representation of the electrical activity of the
heart during a cardiac cycle. To obtain a standard ECG (as shown in the
2015-16(19/01/2015)
286 BIOLOGY
Figure 18.3), a patient is connected to the
machine with three electrical leads (one to each
wrist and to the left ankle) that continuously
monitor the heart activity. For a detailed
evaluation of the heart’s function, multiple
leads are attached to the chest region. Here,
we will talk only about a standard ECG.
Each peak in the ECG is identified with a
letter from P to T that corresponds to a specific
electrical activity of the heart.
The P-wave represents the electrical
excitation (or depolarisation) of the atria,
which leads to the contraction of both the atria.
The QRS complex represents the depolarisation of the ventricles,
which initiates the ventricular contraction. The contraction starts shortly
after Q and marks the beginning of the systole.
The T-wave represents the return of the ventricles from excited to
normal state (repolarisation). The end of the T-wave marks the end of
systole.
Obviously, by counting the number of QRS complexes that occur in a
given time period, one can determine the heart beat rate of an individual.
Since the ECGs obtained from different individuals have roughly the same
shape for a given lead configuration, any deviation from this shape
indicates a possible abnormality or disease. Hence, it is of a great clinical
significance.